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Add page for Linux kernel signal bug, to be referenced by error messages printed by Go 1.14.1 runtime
Ian Lance Taylor
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015879ca71
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LinuxKernelSignalVectorBug.md
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LinuxKernelSignalVectorBug.md
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# Introduction
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If you reached this page because of a message like this printed by a Go program:
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```
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runtime: note: your Linux kernel may be buggy
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runtime: note: see https://golang.org/wiki/LinuxKernelSignalVectorBug
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```
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then you are using a Linux kernel that may have a bug.
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This kernel bug may have caused memory corruption in your Go program, and may have caused the program to crash.
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If you understand why your program crashed, then you can ignore this page.
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Otherwise, this page will explain what the kernel bug is, and includes a C program that you can use to check whether your kernel has the bug.
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# Bug description
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A bug was introduced in Linux kernel version 5.2: if a signal is delivered to a thread, and delivering the signal requires faulting in pages of the thread signal stack, then AVX YMM registers may be corrupted upon returning from the signal to the program.
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If the program was executing some function that uses the YMM registers, that function can behave unpredictably.
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The bug affects programs written in any language.
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The bug only affects programs that receive signals.
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Among programs that receive signals, the bug is more likely to affect programs that use an alternate signal stack.
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The bug only affects programs that use the YMM registers.
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In Go programs in particular the bug will normally cause memory corruption, as Go programs primarily use the YMM register to implement copying one memory buffer to another.
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The bug [was reported to the Linux kernel developers](https://bugzilla.kernel.org/show_bug.cgi?id=205663).
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It was quickly fixed.
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The bug fix was not ported back to the Linux kernel 5.2 series.
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The bug was fixed in Linux kernel versions 5.3.15, 5.4.2, and 5.5 and later.
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The bug is only present if the kernel was compiled with GCC 9 or later.
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The bug is present in vanilla Linux kernel versions 5.2.x for any x, 5.3.0 through 5.3.14, and 5.4.0 and 5.4.1.
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However, many distros that are shipping those kernel versions have in fact backported the patch (which is very small).
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And, some distros are still compiling their kernel with GCC 8, in which case the kernel does not have the bug.
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In other words, even if your kernel is in the vulnerable range, there is a good chance that it is not vulnerable to the bug.
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# Bug test
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To test whether your kernel has the bug, you can run the following C program (click on "Details" to see the program).
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On a buggy kernel, it will fail almost immediately.
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On a kernel without the bug, it will run for 60 seconds and then exit with a 0 status.
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<details>
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```C
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// Build with: gcc -pthread test.c
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//
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// This demonstrates an issue where AVX state becomes corrupted when a
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// signal is delivered where the signal stack pages aren't faulted in.
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//
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// There appear to be three necessary ingredients, which are marked
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// with "!!!" below:
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//
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// 1. A thread doing AVX operations using YMM registers.
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//
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// 2. A signal where the kernel must fault in stack pages to write the
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// signal context.
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//
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// 3. Context switches. Having a single task isn't sufficient.
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#include <errno.h>
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#include <signal.h>
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#include <stdio.h>
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#include <stdlib.h>
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#include <string.h>
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#include <unistd.h>
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#include <pthread.h>
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#include <sys/mman.h>
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#include <sys/prctl.h>
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#include <sys/wait.h>
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static int sigs;
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static stack_t altstack;
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static pthread_t tid;
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static void die(const char* msg, int err) {
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if (err != 0) {
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fprintf(stderr, "%s: %s\n", msg, strerror(err));
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} else {
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fprintf(stderr, "%s\n", msg);
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}
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exit(EXIT_FAILURE);
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}
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void handler(int sig __attribute__((unused)),
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siginfo_t* info __attribute__((unused)),
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void* context __attribute__((unused))) {
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sigs++;
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}
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void* sender(void *arg) {
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int err;
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for (;;) {
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usleep(100);
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err = pthread_kill(tid, SIGWINCH);
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if (err != 0)
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die("pthread_kill", err);
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}
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return NULL;
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}
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void dump(const char *label, unsigned char *data) {
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printf("%s =", label);
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for (int i = 0; i < 32; i++)
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printf(" %02x", data[i]);
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printf("\n");
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}
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void doAVX(void) {
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unsigned char input[32];
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unsigned char output[32];
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// Set input to a known pattern.
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for (int i = 0; i < sizeof input; i++)
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input[i] = i;
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// Mix our PID in so we detect cross-process leakage, though this
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// doesn't appear to be what's happening.
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pid_t pid = getpid();
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memcpy(input, &pid, sizeof pid);
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while (1) {
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for (int i = 0; i < 1000; i++) {
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// !!! Do some computation we can check using YMM registers.
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asm volatile(
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"vmovdqu %1, %%ymm0;"
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"vmovdqa %%ymm0, %%ymm1;"
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"vmovdqa %%ymm1, %%ymm2;"
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"vmovdqa %%ymm2, %%ymm3;"
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"vmovdqu %%ymm3, %0;"
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: "=m" (output)
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: "m" (input)
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: "memory", "ymm0", "ymm1", "ymm2", "ymm3");
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// Check that input == output.
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if (memcmp(input, output, sizeof input) != 0) {
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dump("input ", input);
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dump("output", output);
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die("mismatch", 0);
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}
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}
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// !!! Release the pages of the signal stack. This is necessary
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// because the error happens when copy_fpstate_to_sigframe enters
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// the failure path that handles faulting in the stack pages.
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// (mmap with MMAP_FIXED also works.)
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//
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// (We do this here to ensure it doesn't race with the signal
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// itself.)
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if (madvise(altstack.ss_sp, altstack.ss_size, MADV_DONTNEED) != 0)
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die("madvise", errno);
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}
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}
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void doTest() {
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// Create an alternate signal stack so we can release its pages.
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void *altSigstack = mmap(NULL, SIGSTKSZ, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
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if (altSigstack == MAP_FAILED)
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die("mmap failed", errno);
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altstack.ss_sp = altSigstack;
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altstack.ss_size = SIGSTKSZ;
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if (sigaltstack(&altstack, NULL) < 0)
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die("sigaltstack", errno);
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// Install SIGWINCH handler.
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struct sigaction sa = {
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.sa_sigaction = handler,
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.sa_flags = SA_ONSTACK | SA_RESTART,
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};
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sigfillset(&sa.sa_mask);
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if (sigaction(SIGWINCH, &sa, NULL) < 0)
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die("sigaction", errno);
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// Start thread to send SIGWINCH.
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int err;
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pthread_t ctid;
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tid = pthread_self();
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if ((err = pthread_create(&ctid, NULL, sender, NULL)) != 0)
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die("pthread_create sender", err);
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// Run test.
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doAVX();
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}
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void *exiter(void *arg) {
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sleep(60);
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exit(0);
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}
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int main() {
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int err;
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pthread_t ctid;
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// !!! We need several processes to cause context switches. Threads
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// probably also work. I don't know if the other tasks also need to
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// be doing AVX operations, but here we do.
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int nproc = sysconf(_SC_NPROCESSORS_ONLN);
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for (int i = 0; i < 2 * nproc; i++) {
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pid_t child = fork();
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if (child < 0) {
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die("fork failed", errno);
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} else if (child == 0) {
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// Exit if the parent dies.
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prctl(PR_SET_PDEATHSIG, SIGKILL, 0, 0, 0);
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doTest();
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}
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}
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// Exit after a while.
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if ((err = pthread_create(&ctid, NULL, exiter, NULL)) != 0)
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die("pthread_create exiter", err);
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// Wait for a failure.
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int status;
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if (wait(&status) < 0)
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die("wait", errno);
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if (status == 0)
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die("child unexpectedly exited with success", 0);
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fprintf(stderr, "child process failed\n");
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exit(1);
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}
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```
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</details>
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# What to do
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If your kernel version is in the range that may contain the bug, run the C program above to see if it fails.
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If it fails, your kernel is buggy.
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You should upgrade to a newer kernel.
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There is no workaround for this bug.
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When running a Go program, you can make the bug less likely to interfere with your program by setting the environment variable `GODEBUG=asyncpreemptoff=1`.
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However, this just makes your program less likely to suffer memory corruption (because it reduces the number of signals that your program will receive).
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The bug is still present, and memory corruption may still occur.
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# Questions?
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Ask on the mailing list golang-nuts@googlegroups.com, or on any Go forum as described at [Questions](https://golang.org/wiki/Questions).
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# Details
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To see more details on how the bug affects Go programs and how it was detected and understood, see [#35777](https://golang.org/issue/35777) and [#35326](https://golang.org/issue/35326).
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