[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 by setting the second byte
of the canary to NULL, so that off-by-one can still be detected.

This is taken from https://github.com/GrapheneOS/platform_bionic/commit/7024d880b51f03a796ff8832f1298f2f1531fd7b
---
 src/env/__stack_chk_fail.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/src/env/__stack_chk_fail.c b/src/env/__stack_chk_fail.c
index bf5a280a..be31a88a 100644
--- a/src/env/__stack_chk_fail.c
+++ b/src/env/__stack_chk_fail.c
@@ -9,6 +9,14 @@ 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;
 
+#if UINTPTR_MAX >= 0xffffffffffffffff
+	/* Sacrifice 8 bits of entropy on 64bit to prevent leaking/overwriting the
+	 * canary via string-manipulation functions. The NULL byte is on the second
+	 * byte so that off-by-ones can still be detected. Endianness is taken care
+	 * of automatically. */
+	((char *)&__stack_chk_guard)[1] = 0;
+#endif
+
 	__pthread_self()->canary = __stack_chk_guard;
 }
 
-- 
2.30.2