x86: ascii armor the x86_64 boot init stack canary

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 Daniel Micay's linux-hardened
tree.

Link: http://lkml.kernel.org/r/20170524155751.424-4-riel@redhat.com
Signed-off-by: Rik van Riel <riel@redhat.com>
Acked-by: Kees Cook <keescook@chromium.org>
Cc: Daniel Micay <danielmicay@gmail.com>
Cc: "Theodore Ts'o" <tytso@mit.edu>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/arch/x86/include/asm/stackprotector.h b/arch/x86/include/asm/stackprotector.h
index dcbd9bcce71443eb64325051faa532a68bf50c00..8abedf1d650e9648f2a9864adf9ee815b000c0af 100644 (file)
--- 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