[PATCH 0/2] sigaltstack: return only one flag

[PATCH 0/2] sigaltstack: return only one flag

[Stas Sergeev]

Currently sigaltstack() can return multiple flags, for example
SS_DISABLE|SS_AUTODISARM. This confuses libraries (including asan
runtime) and contradicts the man page.

Patch 1 fixes this problem by ignoring any flag passed with SS_DISABLE.
Patch 2 adds a test-case for that scenario.

Signed-off-by: Stas Sergeev <stsp2@yandex.ru>
CC: "Eric W. Biederman" <ebiederm@xmission.com>
CC: Kees Cook <keescook@chromium.org>
CC: Jens Axboe <axboe@kernel.dk>
CC: Peter Zijlstra <peterz@infradead.org>
CC: Marco Elver <elver@google.com>
CC: Thomas Gleixner <tglx@linutronix.de>
CC: Alexey Gladkov <legion@kernel.org>
CC: Andrew Lutomirski <luto@mit.edu>
CC: Shuah Khan <shuah@kernel.org>
CC: "Chang S. Bae" <chang.seok.bae@intel.com>
CC: Borislav Petkov <bp@suse.de>
CC: Thomas Gleixner <tglx@linutronix.de>
CC: Len Brown <len.brown@intel.com>
CC: linux-kselftest@vger.kernel.org
CC: linux-kernel@vger.kernel.org

Stas Sergeev (2):
  sigaltstack: ignore flags if SS_DISABLE is set
  selftests: sigaltstack: add new SS_DISABLE test

 kernel/signal.c                           |  1 +
 tools/testing/selftests/sigaltstack/sas.c | 48 ++++++++++++-----------
 2 files changed, 26 insertions(+), 23 deletions(-)

-- 
2.34.1

[PATCH 1/2] sigaltstack: ignore flags if SS_DISABLE is set

[Stas Sergeev]

ss_flags combo of SS_AUTODISARM|SS_DISABLE can be used to check
the support of SS_AUTODISARM. We need to remove the like flags and
only keep SS_DISABLE because many libraries (eg asan runtime)
check if SAS is disabled by just checking "ss_flags == SS_DISABLE".

Also man page mandates that only 1 flag can be returned, so
returning SS_AUTODISARM|SS_DISABLE should be disallowed.

Signed-off-by: Stas Sergeev <stsp2@yandex.ru>
CC: "Eric W. Biederman" <ebiederm@xmission.com>
CC: Kees Cook <keescook@chromium.org>
CC: Jens Axboe <axboe@kernel.dk>
CC: Peter Zijlstra <peterz@infradead.org>
CC: Marco Elver <elver@google.com>
CC: Thomas Gleixner <tglx@linutronix.de>
CC: Alexey Gladkov <legion@kernel.org>
CC: Andrew Lutomirski <luto@mit.edu>
CC: linux-kernel@vger.kernel.org
---
 kernel/signal.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/signal.c b/kernel/signal.c
index 38602738866e..40634a500317 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -4209,6 +4209,7 @@ do_sigaltstack (const stack_t *ss, stack_t *oss, unsigned long sp,
 		if (ss_mode == SS_DISABLE) {
 			ss_size = 0;
 			ss_sp = NULL;
+			ss_flags = SS_DISABLE;
 		} else {
 			if (unlikely(ss_size < min_ss_size))
 				ret = -ENOMEM;
-- 
2.34.1

[PATCH 2/2] selftests: sigaltstack: add new SS_DISABLE test

[Stas Sergeev]

This test makes sure that when SS_DISABLE is used, all other flags
are ignored.

Also remove unneeded exit()s after ksft_exit_fail_msg(). That function
exits by itself.

Signed-off-by: Stas Sergeev <stsp2@yandex.ru>
CC: Shuah Khan <shuah@kernel.org>
CC: "Chang S. Bae" <chang.seok.bae@intel.com>
CC: Borislav Petkov <bp@suse.de>
CC: Thomas Gleixner <tglx@linutronix.de>
CC: Len Brown <len.brown@intel.com>
CC: linux-kselftest@vger.kernel.org
CC: linux-kernel@vger.kernel.org
---
 tools/testing/selftests/sigaltstack/sas.c | 48 ++++++++++++-----------
 1 file changed, 25 insertions(+), 23 deletions(-)

diff --git a/tools/testing/selftests/sigaltstack/sas.c b/tools/testing/selftests/sigaltstack/sas.c
index c53b070755b6..7be377e84aa0 100644
--- a/tools/testing/selftests/sigaltstack/sas.c
+++ b/tools/testing/selftests/sigaltstack/sas.c
@@ -119,7 +119,7 @@ int main(void)
 	ksft_print_msg("[NOTE]\tthe stack size is %lu\n", stack_size);
 
 	ksft_print_header();
-	ksft_set_plan(3);
+	ksft_set_plan(4);
 
 	sigemptyset(&act.sa_mask);
 	act.sa_flags = SA_ONSTACK | SA_SIGINFO;
@@ -129,24 +129,18 @@ int main(void)
 	sigaction(SIGUSR2, &act, NULL);
 	sstack = mmap(NULL, stack_size, PROT_READ | PROT_WRITE,
 		      MAP_PRIVATE | MAP_ANONYMOUS | MAP_STACK, -1, 0);
-	if (sstack == MAP_FAILED) {
+	if (sstack == MAP_FAILED)
 		ksft_exit_fail_msg("mmap() - %s\n", strerror(errno));
-		return EXIT_FAILURE;
-	}
 
 	err = sigaltstack(NULL, &stk);
-	if (err) {
+	if (err)
 		ksft_exit_fail_msg("sigaltstack() - %s\n", strerror(errno));
-		exit(EXIT_FAILURE);
-	}
-	if (stk.ss_flags == SS_DISABLE) {
+	if (stk.ss_flags == SS_DISABLE)
 		ksft_test_result_pass(
 				"Initial sigaltstack state was SS_DISABLE\n");
-	} else {
+	else
 		ksft_exit_fail_msg("Initial sigaltstack state was %x; "
 		       "should have been SS_DISABLE\n", stk.ss_flags);
-		return EXIT_FAILURE;
-	}
 
 	stk.ss_sp = sstack;
 	stk.ss_size = stack_size;
@@ -167,16 +161,13 @@ int main(void)
 			ksft_exit_fail_msg(
 				"sigaltstack(SS_ONSTACK | SS_AUTODISARM)  %s\n",
 					strerror(errno));
-			return EXIT_FAILURE;
 		}
 	}
 
 	ustack = mmap(NULL, stack_size, PROT_READ | PROT_WRITE,
 		      MAP_PRIVATE | MAP_ANONYMOUS | MAP_STACK, -1, 0);
-	if (ustack == MAP_FAILED) {
+	if (ustack == MAP_FAILED)
 		ksft_exit_fail_msg("mmap() - %s\n", strerror(errno));
-		return EXIT_FAILURE;
-	}
 	getcontext(&uc);
 	uc.uc_link = NULL;
 	uc.uc_stack.ss_sp = ustack;
@@ -185,17 +176,28 @@ int main(void)
 	raise(SIGUSR1);
 
 	err = sigaltstack(NULL, &stk);
-	if (err) {
+	if (err)
 		ksft_exit_fail_msg("sigaltstack() - %s\n", strerror(errno));
-		exit(EXIT_FAILURE);
-	}
-	if (stk.ss_flags != SS_AUTODISARM) {
-		ksft_exit_fail_msg("ss_flags=%x, should be SS_AUTODISARM\n",
+	if (stk.ss_flags != SS_AUTODISARM)
+		ksft_test_result_fail("ss_flags=%x, should be SS_AUTODISARM\n",
 				stk.ss_flags);
-		exit(EXIT_FAILURE);
-	}
-	ksft_test_result_pass(
+	else
+		ksft_test_result_pass(
 			"sigaltstack is still SS_AUTODISARM after signal\n");
+	/* We are done, disable SS and exit. */
+	stk.ss_flags = SS_DISABLE | SS_AUTODISARM;
+	err = sigaltstack(&stk, NULL);
+	if (err)
+		ksft_exit_fail_msg("sigaltstack() - %s\n", strerror(errno));
+	err = sigaltstack(NULL, &stk);
+	if (err)
+		ksft_exit_fail_msg("sigaltstack() - %s\n", strerror(errno));
+	if (stk.ss_flags != SS_DISABLE)
+		ksft_test_result_fail("ss_flags=%x, should be SS_DISABLE\n",
+				stk.ss_flags);
+	else
+		ksft_test_result_pass(
+			"sigaltstack disabled\n");
 
 	ksft_exit_pass();
 	return 0;
-- 
2.34.1