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

diff --git a/tools/testing/selftests/x86/syscall_arg_fault.c b/tools/testing/selftests/x86/syscall_arg_fault.c
index bc0ecc2e862ef781c610d3f337e65af4f0948683..62fba40866d51bbcefeb8147081e0005776b2789 100644 (file)
--- 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;
 }