path: root/libc/stdlib/malloc-standard/free.c
diff options
authorEyal Itkin <>2019-12-27 18:45:20 +0200
committerWaldemar Brodkorb <>2020-02-16 12:32:21 +0100
commit886878b22424d6f95bcdeee55ada72049d21547c (patch)
treec1ca8f9ac64bb6750e8b949635ec87963b454848 /libc/stdlib/malloc-standard/free.c
parent90f24fc94897d93deb80d933d18a4f31dc6bf05a (diff)
Add Safe-Linking to fastbins
Safe-Linking is a security mechanism that protects single-linked lists (such as the fastbins) from being tampered by attackers. The mechanism makes use of randomness from ASLR (mmap_base), and when combined with chunk alignment integrity checks, it protects the pointers from being hijacked by an attacker. While Safe-Unlinking protects double-linked lists (such as the small bins), there wasn't any similar protection for attacks against single-linked lists. This solution protects against 3 common attacks: * Partial pointer override: modifies the lower bytes (Little Endian) * Full pointer override: hijacks the pointer to an attacker's location * Unaligned chunks: pointing the list to an unaligned address The design assumes an attacker doesn't know where the heap is located, and uses the ASLR randomness to "sign" the single-linked pointers. We mark the pointer as P and the location in which it is stored as L, and the calculation will be: * PROTECT(P) := (L >> PAGE_SHIFT) XOR (P) * *L = PROTECT(P) This way, the random bits from the address L (which start at the bits in the PAGE_SHIFT position), will be merged with the LSB of the stored protected pointer. This protection layer prevents an attacker from modifying the pointer into a controlled value. An additional check that the chunks are MALLOC_ALIGNed adds an important layer: * Attackers can't point to illegal (unaligned) memory addresses * Attackers must guess correctly the alignment bits On standard 32 bit Linux machines, an attacker will directly fail 7 out of 8 times, and on 64 bit machines it will fail 15 out of 16 times. The proposed solution adds 3-4 asm instructions per malloc()/free() and therefore has only minor performance implications if it has any. A similar protection was added to Chromium's version of TCMalloc in 2013, and according to their documentation the performance overhead was less than 2%. Signed-off-by: Eyal Itkin <>
Diffstat (limited to 'libc/stdlib/malloc-standard/free.c')
1 files changed, 3 insertions, 2 deletions
diff --git a/libc/stdlib/malloc-standard/free.c b/libc/stdlib/malloc-standard/free.c
index a2d765d41..f3602cf48 100644
--- a/libc/stdlib/malloc-standard/free.c
+++ b/libc/stdlib/malloc-standard/free.c
@@ -214,8 +214,9 @@ void attribute_hidden __malloc_consolidate(mstate av)
*fb = 0;
do {
- nextp = p->fd;
+ nextp = REVEAL_PTR(&p->fd, p->fd);
/* Slightly streamlined version of consolidation code in free() */
size = p->size & ~PREV_INUSE;
@@ -308,7 +309,7 @@ void free(void* mem)
fb = &(av->fastbins[fastbin_index(size)]);
- p->fd = *fb;
+ p->fd = PROTECT_PTR(&p->fd, *fb);
*fb = p;