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authorEyal Itkin <eyalit@checkpoint.com>2019-12-27 16:45:20 (GMT)
committerWaldemar Brodkorb <wbx@openadk.org>2020-02-16 11:32:21 (GMT)
commit886878b22424d6f95bcdeee55ada72049d21547c (patch)
treec1ca8f9ac64bb6750e8b949635ec87963b454848 /libc
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 <eyalit@checkpoint.com>
Diffstat (limited to 'libc')
-rw-r--r--libc/stdlib/malloc-standard/free.c5
-rw-r--r--libc/stdlib/malloc-standard/mallinfo.c3
-rw-r--r--libc/stdlib/malloc-standard/malloc.c6
-rw-r--r--libc/stdlib/malloc-standard/malloc.h12
4 files changed, 21 insertions, 5 deletions
diff --git a/libc/stdlib/malloc-standard/free.c b/libc/stdlib/malloc-standard/free.c
index a2d765d..f3602cf 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 {
+ CHECK_PTR(p);
check_inuse_chunk(p);
- 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)
set_fastchunks(av);
fb = &(av->fastbins[fastbin_index(size)]);
- p->fd = *fb;
+ p->fd = PROTECT_PTR(&p->fd, *fb);
*fb = p;
}
diff --git a/libc/stdlib/malloc-standard/mallinfo.c b/libc/stdlib/malloc-standard/mallinfo.c
index dbe4d49..9923223 100644
--- a/libc/stdlib/malloc-standard/mallinfo.c
+++ b/libc/stdlib/malloc-standard/mallinfo.c
@@ -49,7 +49,8 @@ struct mallinfo mallinfo(void)
fastavail = 0;
for (i = 0; i < NFASTBINS; ++i) {
- for (p = av->fastbins[i]; p != 0; p = p->fd) {
+ for (p = av->fastbins[i]; p != 0; p = REVEAL_PTR(&p->fd, p->fd)) {
+ CHECK_PTR(p);
++nfastblocks;
fastavail += chunksize(p);
}
diff --git a/libc/stdlib/malloc-standard/malloc.c b/libc/stdlib/malloc-standard/malloc.c
index 1a6d4dc..1f898eb 100644
--- a/libc/stdlib/malloc-standard/malloc.c
+++ b/libc/stdlib/malloc-standard/malloc.c
@@ -260,12 +260,13 @@ void __do_check_malloc_state(void)
assert(p == 0);
while (p != 0) {
+ CHECK_PTR(p);
/* each chunk claims to be inuse */
__do_check_inuse_chunk(p);
total += chunksize(p);
/* chunk belongs in this bin */
assert(fastbin_index(chunksize(p)) == i);
- p = p->fd;
+ p = REVEAL_PTR(&p->fd, p->fd);
}
}
@@ -855,7 +856,8 @@ void* malloc(size_t bytes)
if ((unsigned long)(nb) <= (unsigned long)(av->max_fast)) {
fb = &(av->fastbins[(fastbin_index(nb))]);
if ( (victim = *fb) != 0) {
- *fb = victim->fd;
+ CHECK_PTR(victim);
+ *fb = REVEAL_PTR(&victim->fd, victim->fd);
check_remalloced_chunk(victim, nb);
retval = chunk2mem(victim);
goto DONE;
diff --git a/libc/stdlib/malloc-standard/malloc.h b/libc/stdlib/malloc-standard/malloc.h
index 44120d3..30a696e 100644
--- a/libc/stdlib/malloc-standard/malloc.h
+++ b/libc/stdlib/malloc-standard/malloc.h
@@ -839,6 +839,18 @@ typedef struct malloc_chunk* mfastbinptr;
#define get_max_fast(M) \
((M)->max_fast & ~(FASTCHUNKS_BIT | ANYCHUNKS_BIT))
+/*
+ Safe-Linking:
+ Use randomness from ASLR (mmap_base) to protect single-linked lists
+ of fastbins. Together with allocation alignment checks, this mechanism
+ reduces the risk of pointer hijacking, as was done with Safe-Unlinking
+ in the double-linked lists of smallbins.
+*/
+#define PROTECT_PTR(pos, ptr) ((mchunkptr)((((size_t)pos) >> PAGE_SHIFT) ^ ((size_t)ptr)))
+#define REVEAL_PTR(pos, ptr) PROTECT_PTR(pos, ptr)
+#define CHECK_PTR(P) \
+ if (!aligned_OK(P)) \
+ abort();
/*
morecore_properties is a status word holding dynamically discovered