SliceHeader Literals in Go create a GC Race and Flawed Escape-Analysis. Exploitation with unsafe.Pointer on Real-World Code

In this fourth part, we will explore a very common, but unsafe code pattern: creating reflect.SliceHeader and reflect.StringHeader objects from scratch instead of deriving them by cast.

We will see three problems that come from this: an implicit read-only property, a GC race that can lead to a data confusion and information leak, and an escape-analysis error that leads to dangling pointers.

Then, we will look at an improved version of the cast, and introduce a linter that can find some of the occurances of this specific, unsafe code pattern.

Garbage Collection

First, let's quickly go through garbage collection. Go offers memory management to the programmer. It automatically allocates memory for object instances or values, such as integers, slices, or structs. It also keeps track of whether those objects are still in use, and frees the memory when they aren't anymore.

The Go garbage collector runs in the background as its own Goroutine. In fact it's several Goroutines. The garbage collector can be triggered manually by calling runtime.GC(), but usually it runs automatically when the heap doubles its size. This size threshold can be adjusted with the GOGC environment variable. It is set to a percentage. The default is 100, meaning the heap has to grow by 100% to trigger the garbage collection. Setting it to 200 for example would mean that the collection is only started when the heap has grown to three times the previous size. On top of the size condition there is also a timing condition: as long as the process is not suspended, the garbage collector will run at least once every two minutes.

Go uses a Mark-and-Sweep garbage collector. This type of garbage collection consists of two phases:

1. Mark: by recursively following all references, starting from variables in scope, reachable heap objects are marked
2. Sweep: objects that are not marked are freed

These steps can be seen in the following figure:

The light blue boxes in the heap are objects that are reachable (through the references shown by the arrows). The white objects are unreachable and will be freed in the sweep phase.

Explicit casting using unsafe pointers

Now, we will look at the most common usage pattern for unsafe.Pointer in real-world open-source Go code: casting a slice of some type or string into a slice of some other type. Let's say we wanted to convert a string to a []byte slice in-place, that is reusing the string memory instead of copying it into a new slice allocation.

A frequent pattern to do this looks like this:

func unsafeStringToBytes(s *string) []byte {
    sh := (*reflect.StringHeader)(unsafe.Pointer(s))
    sliceHeader := &reflect.SliceHeader{
        Data: sh.Data,
        Len:  sh.Len,
        Cap:  sh.Len,
    }
    return *(*[]byte)(unsafe.Pointer(sliceHeader))
}

The function gets a *string pointer (sometimes it will also be a direct string) and returns a []byte slice. Let's look at what it does, line by line.

First, a reflect.StringHeader is created from the string. The StringHeader is Go's internal representation of a string. It is very similar to the reflect.SliceHeader that we saw in the previous posts of this series:

type StringHeader struct {
    Data uintptr
    Len  int
}

The only difference is that there is no Cap field. In fact, strings in Go are by most means just a read-only []byte slice. There are some differences, for example that range will iterate over runes instead of bytes, where a rune is a Unicode code point. Because strings in Go are encoded in UTF-8, a Unicode code point might need multiple bytes (like the German umlaut ä), and in that case range will read multiple bytes in one iteration. But in other ways, like the length, strings behave like []byte slices. For example, len("ä") is 2, even if the string has only one character. You can read more on this topic in the Go strings documentation.

When we have a variable of type string in Go, it points to a reflect.StringHeader structure, which in turn has a pointer to the underlying byte-array holding the string data in its Data field. To get the StringHeader, we cast it from an unsafe.Pointer which in turn is created by casting the string pointer. If the function would have received a string instead of *string, we would have needed to do unsafe.Pointer(&s) here, but the rest would stay the same.

Now, a *reflect.SliceHeader is created from scratch, by a composite literal. The Data and Len fields are just copied from the StringHeader, and Cap is set to the same value as Len.

Lastly, we cast the *reflect.SliceHeader into a *[]byte, again using an intermediate unsafe.Pointer object. The *[]byte is dereferenced and returned.

Thus the function is casting a *string into a []byte object.

First problem: implicit read-only slices

Remember that the Go documentation said that strings are read-only []byte slices? Well, that could turn into a problem here! The []byte object returned by the function is not read-only anymore, so the compiler will not complain if we modify its contents:

func main() {
    s := "Hello"
    b := unsafeStringToBytes(&s)

    b[1] = "a" // this will crash

    fmt.Println(b)
}

The reason strings are read-only is because when we create a string like in the example above, the actual string literal (the Hello data) is placed in a special section in the binary file produced by the compiler. When the program is run, this section is probably mapped into a read-only memory page. Therefore, the Data field in the StringHeader and SliceHeader structures will contain an address inside that read-only page.

If we now change the slice with b[1] = "a", we attempt to change a read-only memory page. The operating system will prevent this and the result is a SIGSEGV segmentation fault, crashing the program.

The fact that this is a memory access violation that the compiler will not notice since we skipped its checks when we used unsafe.Pointer is unfortunate, but a careful programmer could in theory make sure that all usages of the unsafe cast function will never change the resulting slice. At all. I think this is a pretty dangerous assumption to make and sooner or later there will be a programmer adding code that changes the slice. Therefore the casting pattern above should be avoided at all costs.

But there is a second, much more subtle and dangerous problem in the code above.

Second problem: garbage collector race introduced by slice and string header literals

Rule 6 of the unsafe package documentation specifically states that "A program should not declare or allocate variables of these struct types." Why is that?

When the garbage collector runs the mark phase, it follows pointer references to recursively mark the objects referenced by the pointer. An unsafe.Pointer and the address stored in the Data field of a valid StringHeader or SliceHeader will do the same. This means that sh.Data in the unsafe function above will in fact be treated as a reference value, therefore the garbage collector will not free the underlying array.

However, plain uintptr and invalid slice or string header values are not treated as references.

Whenever the address of a value is only stored in variable of type uintptr (not additionally in any pointer types), the garbage collector will not mark the referenced object and therefore free it. The freed memory might be reused with new variables, or the memory page might simply be unmapped, or anything else might happen. Importantly, objects that are only reachable by using an address that was stored in a uintptr variable must be treated as gone.

Unsafe usage rule 2 states that only a "conversion of an unsafe.Pointer to a uintptr (but not back to Pointer)" is allowed. The part in parenthesis is important. If we create an unsafe.Pointer object from a previously stored uintptr value, that unsafe.Pointer is a potentially dangling pointer, and dereferencing is not a safe operation!

There are some cases where unsafe.Pointer objects are created from pointer arithmetic on uintptr values, but those calculations must happen in the same statement as creating the pointer. We must never store a reference to something only in a uintptr value.

Now, let's revisit the code example above.

func unsafeStringToBytes(s *string) []byte {
    sh := (*reflect.StringHeader)(unsafe.Pointer(s))
    sliceHeader := &reflect.SliceHeader{
        Data: sh.Data,
        Len:  sh.Len,
        Cap:  sh.Len,
    }

    // At this point, s is no longer used. There is a copy of the address of
    // its underlying array in sliceHeader.Data however, and since sliceHeader
    // was not created from an actual slice, the GC does not treat the address
    // as a reference. Therefore, if the GC runs here it will free s.

    return *(*[]byte)(unsafe.Pointer(sliceHeader))
}

At the point of the comment, the garbage collector can potentially run. Remember that it is triggered by heap usage growth, and runs concurrently. If the function is used within a program that uses several Goroutines, the garbage collector can essentially trigger at any point, including the one with the comment.

When it runs, it will free string s because it is no longer used. When the []byte slice is created in the next line, its Data field will contain an invalid address. It might now point to an unmapped memory page, or simply to some undefined position in the heap that might get reused later on.

PoC: Exploiting this GC race condition

To see what can happen with this, let's look at the following proof-of-concept code. First, I add the following line at the position of the comment above:

    time.Sleep(1 * time.Nanosecond)

This just makes the exploit a bit more reliable, but is not strictly needed. Next, I add a Goroutine that will just use up more and more heap and constantly drop the references to that allocated memory. This will regularly trigger the garbage collector.

func heapHeapHeap() {
    var a *[]byte
    for {
        tmp := make([]byte, 1000000, 1000000)
        a = &tmp
        _ = a
    }
}

Finally, the main Goroutine does the following:

reader := bufio.NewReader(os.Stdin)
count := 1
var firstChar byte

for {
    s, _ := reader.ReadString('\n')
    if len(s) == 0 {
        continue
    }
    firstChar = s[0]

    // HERE BE DRAGONS
    bytes := unsafeStringToBytes(&s)

    _, _ = reader.ReadString('\n')

    if len(bytes) > 0 && bytes[0] != firstChar {
        fmt.Printf("win! after %d iterations\n", count)
        os.Exit(0)
    }

    count++
}

It initializes a reader to read data from stdin. Then it repeatedly reads two lines from it in a loop. A counter is used to count how many loops are needed to succeed. The firstChar variable is set to the first char from the first line that is read.

Then, the first line (a string) is converted to a []byte slice using the unsafe casting function from above. At this point, bytes and s should be the same string. Particularly, bytes[0] should equal firstChar.

After the conversion, the second line is read. The result from ReadString is not even used, but the important part of this is that if the garbage collector was run inside unsafeStringToBytes, then ReadString will reuse the heap space that was previously freed.

Lastly, we check if bytes[0] is actually equal to firstChar, and if it is not we have successfully created a data confusion by exploiting a garbage collector race condition. The number of loop executions needed is printed at the end.

Running this program can have two different results:

1. The garbage collector finds an incorrect address in the heap and crashed the program with a hint to a possible incorrect usage of unsafe.Pointer
2. It succeeds with the win! message

As long as the garbage collector does not trigger at the critical point in the unsafe cast function, the loop will just run forever.

The first, crashing case happens when the garbage collector triggers inside the unsafe cast function, and again within the second ReadString call in the PoC code. At that point, bytes will be a seemingly valid []byte slice, but its Data field will point to previously freed memory.

The second, succeeding case will happen if the garbage collector triggers only inside the unsafe cast function. In that case, the bytes slice will be a dangling slice pointing into the freed heap. Then, the second ReadString will reuse that heap space, and provided that we sent a different string as second line, the first byte in the bytes slice will now be a different character.

To achieve the alternating, but infinite input data I use the following Python script:

#!/usr/bin/env python3

import errno
from signal import signal, SIGPIPE, SIG_DFL
signal(SIGPIPE,SIG_DFL)

try:
    while True:
        print("AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA")
        print("BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB")
except Exception:
    pass

It simply sends alternating A and B lines, and ignores IO exceptions caused by the pipe closing abruptly when the Go program crashes (which would spam the terminal with some Python error messages).

The PoC is run like this:

$ ./exploit.py | ./main
win! after 51446 iterations

In my experiments, the program would crash with condition 1 about 10% of the time, and succeed in 20,000 to 100,000 iterations otherwise.

Why is this a problem? A threat model

Now, all of this might seem rather staged and a succeeding memory confusion after 50,000 iterations on average might not seem that often either. But in fact this is probably the most dangerous vulnerability of the ones shown in this blog series.

First, the function that contains the actual vulnerability, unsafeStringToBytes, is taken from real-world Go code. There are hundreds of times this code pattern is used in open-source Go libraries, and taking into account that they are reused across multiple projects, there are actually tens of thousands of times this is used in the 500 most starred open-source Go projects. I already submitted pull requests with a fix to the underlying Go libraries that contained this code pattern.

Unlike in part 3 of this series, we didn't gain remote code execution with this exploit PoC. We didn't even violate the read-only nature of the slice returned from unsafeStringToBytes. And the exploit is not even particularly reliable, it takes thousands of iterations until the confusion happens once and sometimes the program even just crashes. Lastly, we added the nanosecond sleep, further increasing the likelihood of the confusion to happen.

But risk is a combination of likelihood and impact, and the impact of this problem is potentially disastrous.

First, let's create a potential real-world use case for analysis, set the likelihood in perspective, and then talk about the impact. Imagine a server application written in Go. It handles incoming requests, does some internal calculations and creates an output that is sent back to the client. Of course the application holds some private state, imagine credentials to the database backend or private key data for example. Let's also say that there are 1,000 requests coming in each second. That number of requests is not low, but still in reach of big applications.

Let's ignore the case of crashing the program for now, since for this threat model assessment we can just assume that if the server application crashes, some daemon supervisor will just restart it. We can think of the loop in the exploit proof of concept as a similar thing to the requests that are coming in. Some code path will be executed for every request, and this will be kind of like a loop iteration. Furthermore, if we get a memory confusion every 50,000 requests, that will add up to a bit more than one confusion per minute. The nanosecond sleep will make the confusion more likely, but still this might occur every few minutes.

Now, if such a memory confusion happens, there might be some read-only slice that contains unexpected data. If the server application happens to use that slice for creating the response output that is sent to the user, even with some intermediate conversions in between, the application might serve unexpected memory contents back to the user. So with this vulnerability in place, a user might suddenly, and randomly, get some scrambled data instead of an HTML response, and if that user were to look into the data they might find the application secrets. This is a clear information leak vulnerability, and the fact that it is caused by widely used code makes it very dangerous.

Third problem: flawed escape analysis can lead to invalid references

On top of the two problems already discussed, there is even a third one. Go uses escape analysis to determine whether a variable should be placed on the heap or the stack. If the Go compiler determines that a variable might live longer than the function where it is declared, then that value needs to be allocated on the heap. If the compiler can see that the value is valid at most as long as the current function executes, then that value can be on the function's stack.

Basically, a value lives potentially longer than the current function if a reference to it is stored somewhere or it is returned to the calling function. When the current function passes a reference to a value into some other function, then the Go compiler will look into that function and transitively determine whether the value can be on the stack. This goes down to the point where the value might be used by some C code, e.g. a native library, in which case the compiler must assume that the value might be retained longer than the current function, and must therefore be allocated on the heap.

If a value might live longer than the current function, we say that the value escapes. Likewise, we say it does not escape if it can never outlive the function, thus can be put on the stack. The Go compiler will show the escape decisions with the flag go build -gcflags='-m'.

Now, the incorrect cast code in the example above breaks the chain of referencing that lets the compiler see that a reference to a value is retained. Recall the function:

func unsafeStringToBytes(s *string) []byte {
    sh := (*reflect.StringHeader)(unsafe.Pointer(s))
    sliceHeader := &reflect.SliceHeader{
        Data: sh.Data,
        Len:  sh.Len,
        Cap:  sh.Len,
    }

    // At this point, there is no reference to s anymore, thus s does not escape here
    // sliceHeader does not resemble a valid reference to s to the compiler
    // it only works "by accident", and even that only if the garbage 
    // collector does not run exactly here, as described in problem 2

    return *(*[]byte)(unsafe.Pointer(sliceHeader))
}

Imagine a function that uses unsafeStringToBytes like this:

func GetBytes() []byte {
    reader := bufio.NewReader(strings.NewReader("abcdefgh"))
    s, _ := reader.ReadString('\n')
    out := StringToBytes(s)
    fmt.Printf("GetBytes:%s\n", out) // expected stdout is "abcdefgh"
    return out
}

The GetBytes function creates a string "abcdefgh". It uses a reader to do so to make sure the string is actually allocated, more on this in just a bit. Then, the string is cast using the unsafe cast function, resulting in a []byte slice. The slice is printed to stdout, and in fact this produces the abcdefgh output as expected. Finally, the slice is returned (and even with the slice being copied, which means the slice header is copied, this returns a reference to the original underlying data array of s).

When the compiler runs its escape analysis, it will see that s is passed to a function StringToBytes, so it looks into that. As described above, due to the incorrect, unsafe cast there is a broken link in the referencing, and the compiler assumes that s will not escape. Transitively, s does not escape in GetBytes because it is not used after the function call. Therefore s will be allocated on the stack of GetBytes.

If we call GetBytes from the main function, the memory will have become invalid:

func main() {
    bytesResult := GetBytes()
    // expected stdout is "abcdefgh", but in reality it's undefined
    fmt.Printf("main:%s\n", bytesResult)
}

The result of GetBytes is a slice, containing a reference into the stack of GetBytes. When the slice is returned, that reference surpasses the lifetime of that stack, because after we return to main, the stack of GetBytes is destroyed. Therefore, using the result of the unsafe cast works within GetBytes, but it might not if we return the reference to even more functions.

This works with casts from slices to strings as well, and in that case we don't have to do something like the reader call above, instead even simple direct slice literals are dangerous. The reason for the reader above is that if we had created the string from a string literal, like s := "abcdefgh, then s would have been allocated neither on the heap nor on the stack. Instead, it would have been a string constant in the constant data section of the resulting binary, and therefore the reference to that data would have continued to work after returning to main. The proof of concept relies on the value to live on the stack however.

The "correct" way of in-place slice casting using the unsafe package

There is a "correct" way to cast slices without copying. Whether this is really worth it has to be decided in the special case, but we can at least propose a safer version of the vulnerable casting function above.

func saferStringToBytes(s *string) []byte {
    // create an actual slice
    bytes := make([]byte, 0, 0)

    // create the string and slice headers by casting. Obtain pointers to the 
    // headers to be able to change the slice header properties in the next step
    stringHeader := (*reflect.StringHeader)(unsafe.Pointer(s))
    sliceHeader := (*reflect.SliceHeader)(unsafe.Pointer(&bytes))

    // set the slice's length and capacity temporarily to zero (this is actually
    // unnecessary here because the slice is already initialized as zero, but if 
    // you are reusing a different slice this is important
    sliceHeader.Len = 0
    sliceHeader.Cap = 0

    // change the slice header data address
    sliceHeader.Data = stringHeader.Data

    // set the slice capacity and length to the string length
    sliceHeader.Cap = stringHeader.Len
    sliceHeader.Len = stringHeader.Len

    // use the keep alive dummy function to make sure the original string s is not 
    // freed up until this point
    runtime.KeepAlive(s)  // or runtime.KeepAlive(*s)

    // return the valid bytes slice (still read-only though)
    return bytes
}

This is a rather complicated process, but here are the important parts:

1. The bytes slice that will be returned at the end is created as an actual, valid slice using the make function. It is not created by casting a plain header structure that was created as a composite literal. This ensures that Go will treat the address stored in sliceHeader.Data as if it were a "real" pointer
2. Subsequently, the sliceHeader instance is created by casting as stated in the unsafe documentation
3. sliceHeader length and capacity are explicitly set to zero while the Data address still points to the old underlying array. This is only necessary if the slice has not just been created. Decreasing the length and capacity is a safe operation, and it ensures that if the garbage collector runs just after the switch of Data it will not run past the slice end. This is explained in further detail below.
4. The StringHeader fields are copied in this order: Data, then Cap, then Len
5. Using runtime.KeepAlive, we tell the garbage collector that the original string s should not be freed up until this point. This ensures that the underlying data array will not be freed before it is referenced by the bytes slice.

When setting the Data field, the slice length and capacity should be zero as noted in point 3. This is because if the target slice has a length greater than the source slice / string, and the garbage collector triggers right after changing the Data field but before adjusting the Len and Cap fields, the slice would momentarily reach into invalid memory. When the slice is of a type containing references, such as a struct, the garbage collector must go through the slice to recursively mark the referenced objects, and if the length is set too high it will do so on invalid memory. If the length is just zero, this won't happen. However, in order to ensure the referenced objects itself are not freed it is important to still have them referenced by the original slice / string. This is ensured by the call to runtime.KeepAlive as stated in point 5.

Setting Len after Cap ensures that the slice never has a capacity lower than its length, which would be an illegal state.

It might also be okay to do the casting in a single statement, but I am not completely sure about this. The reasoning would be that a statement is evaluated atomically and the garbage collector can not run within the statement. But this might be false, and also it does not fix the third problem, the flawed escape analysis, at all! A one-statement cast would look like this:

stringHeader := (*reflect.StringHeader)(unsafe.Pointer(&s))
b := *(*[]byte)(unsafe.Pointer(&reflect.SliceHeader{
    Data: stringHeader.Data,
    Cap: stringHeader.Len,
    Len: stringHeader.Len,
}))

The takeaway of this should be that it is very very difficult to get this cast right and safe, and therefore this type of in-place cast should better not be used at all.

Introducing a static code analysis tool!

Unfortunately, the go vet -unsafeptr will not catch this common type of unsafe.Pointer misuse. I developed a Vet-style analysis pass that is able to catch it:

jlauinger / go-safer

Go Vet-style linter to find incorrect uses of reflect.SliceHeader and reflect.StringHeader

This linter / static code analysis tool will catch the following situations:

1. There is a reflect.StringHeader or reflect.SliceHeader composite literal. It might also be contained within another composite literal.
2. There is an assignment to the fields of a composite object of type reflect.StringHeader or reflect.SliceHeader, and that object is not definitely derived by cast.

The first situation is fairly easy to detect and almost always unsafe. The linter tool will catch type aliases too. That is, if you define

type MysteryType reflect.SliceHeader

and then do

source := make([]byte, 1, 1)
myHeader := &MysteryType{Len: 42, Cap: 42, Data: uintptr(unsafe.Pointer(&source))}

the linter will catch the MysteryType composite literal just as if it were a direct SliceHeader literal.

The second situation is more difficult. It analyzes all assignments and uses the same mechanism to catch type aliases for the object receiving the assignment as well. To determine whether it is a safe header derived from a cast, the pass depends on the ctrlflow pass, receiving the control flow graph for the package. It finds the function containing the assignment. Then, starting from the assignment the linter follows the graph backwards to the last assignment to the object of SliceHeader or StringHeader type, and determines if that assignment is a cast from unsafe.Pointer, which in turn itself is cast from a slice or string.

This means it will catch situations like these:

type MysteryStruct struct {
    MysteryHeader reflect.SliceHeader
}

func main() {
    myStruct := MysteryStruct{}
    myStruct.MysteryHeader.Len = 42
}

The linter will figure out that the SliceHeader instance contained within the MysteryStruct has not been set by a cast and issue a warning.

Complete POC code

You can read the full POC code in the Github repository that I created for this post series:

jlauinger / go-unsafepointer-poc

Golang example code showing dangers with unsafe.Pointer usages

Acknowledgments

This blog post was written as part of my work on my Master's thesis at the Software Technology Group at TU Darmstadt.

Title picture by Maria Letta. Thanks a lot for her excellent Free Gophers Pack!