selftests/x86/fsgsbase: Test PTRACE_PEEKUSER for GSBASE with invalid LDT GS

This tests commit:

  8ab49526 ("x86/fsgsbase/64: Fix NULL deref in 86_fsgsbase_read_task")

Unpatched kernels will OOPS.
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/c618ae86d1f757e01b1a8e79869f553cb88acf9a.1598461151.git.luto@kernel.org

tools/testing/selftests/x86/fsgsbase.c

@@ -443,6 +443,68 @@ static void test_unexpected_base(void)
 
 #define USER_REGS_OFFSET(r) offsetof(struct user_regs_struct, r)
 
+static void test_ptrace_write_gs_read_base(void)
+{
+	int status;
+	pid_t child = fork();
+
+	if (child < 0)
+		err(1, "fork");
+
+	if (child == 0) {
+		printf("[RUN]\tPTRACE_POKE GS, read GSBASE back\n");
+
+		printf("[RUN]\tARCH_SET_GS to 1\n");
+		if (syscall(SYS_arch_prctl, ARCH_SET_GS, 1) != 0)
+			err(1, "ARCH_SET_GS");
+
+		if (ptrace(PTRACE_TRACEME, 0, NULL, NULL) != 0)
+			err(1, "PTRACE_TRACEME");
+
+		raise(SIGTRAP);
+		_exit(0);
+	}
+
+	wait(&status);
+
+	if (WSTOPSIG(status) == SIGTRAP) {
+		unsigned long base;
+		unsigned long gs_offset = USER_REGS_OFFSET(gs);
+		unsigned long base_offset = USER_REGS_OFFSET(gs_base);
+
+		/* Read the initial base.  It should be 1. */
+		base = ptrace(PTRACE_PEEKUSER, child, base_offset, NULL);
+		if (base == 1) {
+			printf("[OK]\tGSBASE started at 1\n");
+		} else {
+			nerrs++;
+			printf("[FAIL]\tGSBASE started at 0x%lx\n", base);
+		}
+
+		printf("[RUN]\tSet GS = 0x7, read GSBASE\n");
+
+		/* Poke an LDT selector into GS. */
+		if (ptrace(PTRACE_POKEUSER, child, gs_offset, 0x7) != 0)
+			err(1, "PTRACE_POKEUSER");
+
+		/* And read the base. */
+		base = ptrace(PTRACE_PEEKUSER, child, base_offset, NULL);
+
+		if (base == 0 || base == 1) {
+			printf("[OK]\tGSBASE reads as 0x%lx with invalid GS\n", base);
+		} else {
+			nerrs++;
+			printf("[FAIL]\tGSBASE=0x%lx (should be 0 or 1)\n", base);
+		}
+	}
+
+	ptrace(PTRACE_CONT, child, NULL, NULL);
+
+	wait(&status);
+	if (!WIFEXITED(status))
+		printf("[WARN]\tChild didn't exit cleanly.\n");
+}
+
 static void test_ptrace_write_gsbase(void)
 {
 	int status;
@@ -529,6 +591,9 @@ int main()
 	shared_scratch = mmap(NULL, 4096, PROT_READ | PROT_WRITE,
 			      MAP_ANONYMOUS | MAP_SHARED, -1, 0);
 
+	/* Do these tests before we have an LDT. */
+	test_ptrace_write_gs_read_base();
+
 	/* Probe FSGSBASE */
 	sethandler(SIGILL, sigill, 0);
 	if (sigsetjmp(jmpbuf, 1) == 0) {