[PATCH 3/5] x86: ascii armor the x86_64 boot init stack canary

[riel]

From: Rik van Riel <riel@xxxxxxxxxx>

Use the ascii-armor canary to prevent unterminated C string overflows
from being able to successfully overwrite the canary, even if they
somehow obtain the canary value.

Inspired by execshield ascii-armor and PaX/grsecurity.

Signed-off-by: Rik van Riel <riel@xxxxxxxxxx>
---
 arch/x86/include/asm/stackprotector.h | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/x86/include/asm/stackprotector.h b/arch/x86/include/asm/stackprotector.h
index dcbd9bcce714..8abedf1d650e 100644
--- a/arch/x86/include/asm/stackprotector.h
+++ b/arch/x86/include/asm/stackprotector.h
@@ -74,6 +74,7 @@ static __always_inline void boot_init_stack_canary(void)
 	get_random_bytes(&canary, sizeof(canary));
 	tsc = rdtsc();
 	canary += tsc + (tsc << 32UL);
+	canary &= CANARY_MASK;
 
 	current->stack_canary = canary;
 #ifdef CONFIG_X86_64
-- 
2.9.3