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

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;
 }