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>

1 files changed, 12 insertions, 0 deletions

diff --git a/libc/stdlib/malloc-standard/malloc.h b/libc/stdlib/malloc-standard/malloc.h
index 44120d388..30a696e5a 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