[musl] [PATCH] Zero the leading stack canary byte

[jvoisin <julien.voisin@dustri.org>]

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