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Message-Id: <20211212183440.43118-1-julien.voisin@dustri.org>
Date: Sun, 12 Dec 2021 19:34:40 +0100
From: jvoisin <julien.voisin@...tri.org>
To: musl@...ts.openwall.com
Cc: jvoisin <julien.voisin@...tri.org>
Subject: [PATCH] Zero the leading stack canary byte
This reduces entropy of the canary from 64-bit to 56-bit in exchange for
mitigating non-terminated C string overflows.
Checking the byte order should be "good enough", since I think that the stacks
on all architectures supported by musl are growing downwards. Worse case, this
can always be improved later if needed.
This is taken from https://github.com/GrapheneOS/platform_bionic/commit/7024d880b51f03a796ff8832f1298f2f1531fd7b
---
src/env/__stack_chk_fail.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)
diff --git a/src/env/__stack_chk_fail.c b/src/env/__stack_chk_fail.c
index bf5a280a..45f948fe 100644
--- a/src/env/__stack_chk_fail.c
+++ b/src/env/__stack_chk_fail.c
@@ -2,6 +2,11 @@
#include <stdint.h>
#include "pthread_impl.h"
+// Sacrifice 8 bits of entropy to mitigate non-terminated C string overflows
+static const uintptr_t canary_mask = __BYTE_ORDER == __BIG_ENDIAN ?
+ 0x00ffffffffffffffUL :
+ 0xffffffffffffff00UL ;
+
uintptr_t __stack_chk_guard;
void __init_ssp(void *entropy)
@@ -9,7 +14,7 @@ void __init_ssp(void *entropy)
if (entropy) memcpy(&__stack_chk_guard, entropy, sizeof(uintptr_t));
else __stack_chk_guard = (uintptr_t)&__stack_chk_guard * 1103515245;
- __pthread_self()->canary = __stack_chk_guard;
+ __pthread_self()->canary = __stack_chk_guard & canary_mask;
}
void __stack_chk_fail(void)
--
2.30.2
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