linux-kernel - [PATCH AUTOSEL 6.4 4/4] tools/nolibc: ensure stack protector guard is never zero

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Message-Id: <20230709145530.512177-4-sashal@kernel.org>
Date:   Sun,  9 Jul 2023 10:55:30 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Thomas Weißschuh <linux@...ssschuh.net>,
        Willy Tarreau <w@....eu>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH AUTOSEL 6.4 4/4] tools/nolibc: ensure stack protector guard is never zero

From: Thomas Weißschuh <linux@...ssschuh.net>

[ Upstream commit 88fc7eb54ecc6db8b773341ce39ad201066fa7da ]

The all-zero pattern is one of the more probable out-of-bound writes so
add a special case to not accidentally accept it.

Also it enables the reliable detection of stack protector initialization
during testing.

Signed-off-by: Thomas Weißschuh <linux@...ssschuh.net>
Signed-off-by: Willy Tarreau <w@....eu>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 tools/include/nolibc/stackprotector.h | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/tools/include/nolibc/stackprotector.h b/tools/include/nolibc/stackprotector.h
index d119cbbbc256f..9890e86c26172 100644
--- a/tools/include/nolibc/stackprotector.h
+++ b/tools/include/nolibc/stackprotector.h
@@ -45,8 +45,9 @@ __attribute__((weak,no_stack_protector,section(".text.nolibc_stack_chk")))
 void __stack_chk_init(void)
 {
 	my_syscall3(__NR_getrandom, &__stack_chk_guard, sizeof(__stack_chk_guard), 0);
-	/* a bit more randomness in case getrandom() fails */
-	__stack_chk_guard ^= (uintptr_t) &__stack_chk_guard;
+	/* a bit more randomness in case getrandom() fails, ensure the guard is never 0 */
+	if (__stack_chk_guard != (uintptr_t) &__stack_chk_guard)
+		__stack_chk_guard ^= (uintptr_t) &__stack_chk_guard;
 }
 #endif // defined(NOLIBC_STACKPROTECTOR)
 
-- 
2.39.2