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selftests/x86: Add a syscall_arg_fault_64 test for negative GSBASE 

If the kernel erroneously allows WRGSBASE and user code writes a
negative value, paranoid_entry will get confused. Check for this by
writing a negative value to GSBASE and doing SYSENTER with TF set. A
successful run looks like:

    [RUN]	SYSENTER with TF, invalid state, and GSBASE < 0
    [SKIP]	Illegal instruction

A failed run causes a kernel hang, and I believe it's because we
double-fault and then get a never ending series of page faults and,
when we exhaust the double fault stack we double fault again,
starting the process over.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/f4f71efc91b9eae5e3dae21c9aee1c70cf5f370e.1590620529.git.luto@kernel.org
This commit is contained in:
Andy Lutomirski 2020-05-27 16:02:36 -07:00 committed by Borislav Petkov
parent 5e7ec8578f
commit a5d25e01c8
1 changed files with 26 additions and 0 deletions
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26
tools/testing/selftests/x86/syscall_arg_fault.c
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@ -72,6 +72,7 @@ static void sigsegv_or_sigbus(int sig, siginfo_t *info, void *ctx_void)
if (ax != -EFAULT && ax != -ENOSYS) {
printf("[FAIL]\tAX had the wrong value: 0x%lx\n",
(unsigned long)ax);
printf("\tIP = 0x%lx\n", (unsigned long)ctx->uc_mcontext.gregs[REG_IP]);
n_errs++;
} else {
printf("[OK]\tSeems okay\n");
@ -226,5 +227,30 @@ int main()
}
set_eflags(get_eflags() & ~X86_EFLAGS_TF);
#ifdef __x86_64__
printf("[RUN]\tSYSENTER with TF, invalid state, and GSBASE < 0\n");
if (sigsetjmp(jmpbuf, 1) == 0) {
sigtrap_consecutive_syscalls = 0;
asm volatile ("wrgsbase %%rax\n\t"
:: "a" (0xffffffffffff0000UL));
set_eflags(get_eflags() | X86_EFLAGS_TF);
asm volatile (
"movl $-1, %%eax\n\t"
"movl $-1, %%ebx\n\t"
"movl $-1, %%ecx\n\t"
"movl $-1, %%edx\n\t"
"movl $-1, %%esi\n\t"
"movl $-1, %%edi\n\t"
"movl $-1, %%ebp\n\t"
"movl $-1, %%esp\n\t"
"sysenter"
: : : "memory", "flags");
}
set_eflags(get_eflags() & ~X86_EFLAGS_TF);
#endif
return 0;
}