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 <l...@kernel.org>
---
.../testing/selftests/x86/syscall_arg_fault.c | 26 +++++++++++++++++++
1 file changed, 26 insertions(+)
diff --git a/tools/testing/selftests/x86/syscall_arg_fault.c
b/tools/testing/selftests/x86/syscall_arg_fault.c
index bc0ecc2e862e..62fba40866d5 100644
--- a/tools/testing/selftests/x86/syscall_arg_fault.c
+++ b/tools/testing/selftests/x86/syscall_arg_fault.c
@@ -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;
}
--
2.25.4
