From: Kees Cook <keescook@xxxxxxxxxxxx> Subject: slub: relocate freelist pointer to middle of object In a recent discussion[1] with Vitaly Nikolenko and Silvio Cesare, it became clear that moving the freelist pointer away from the edge of allocations would likely improve the overall defensive posture of the inline freelist pointer. My benchmarks show no meaningful change to performance (they seem to show it being faster), so this looks like a reasonable change to make. Instead of having the freelist pointer at the very beginning of an allocation (offset 0) or at the very end of an allocation (effectively offset -sizeof(void *) from the next allocation), move it away from the edges of the allocation and into the middle. This provides some protection against small-sized neighboring overflows (or underflows), for which the freelist pointer is commonly the target. (Large or well controlled overwrites are much more likely to attack live object contents, instead of attempting freelist corruption.) The vaunted kernel build benchmark, across 5 runs. Before: Mean: 250.05 Std Dev: 1.85 and after, which appears mysteriously faster: Mean: 247.13 Std Dev: 0.76 Attempts at running "sysbench --test=memory" show the change to be well in the noise (sysbench seems to be pretty unstable here -- it's not really measuring allocation). Hackbench is more allocation-heavy, and while the std dev is above the difference, it looks like may manifest as an improvement as well: 20 runs of "hackbench -g 20 -l 1000", before: Mean: 36.322 Std Dev: 0.577 and after: Mean: 36.056 Std Dev: 0.598 [1] https://twitter.com/vnik5287/status/1235113523098685440 Link: http://lkml.kernel.org/r/202003051624.AAAC9AECC@keescook Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx> Acked-by: Christoph Lameter <cl@xxxxxxxxx> Cc: Vitaly Nikolenko <vnik@xxxxxxxxxxx> Cc: Silvio Cesare <silvio.cesare@xxxxxxxxx> Cc: Christoph Lameter <cl@xxxxxxxxx>Cc: Pekka Enberg <penberg@xxxxxxxxxx> Cc: David Rientjes <rientjes@xxxxxxxxxx> Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- mm/slub.c | 7 +++++++ 1 file changed, 7 insertions(+) --- a/mm/slub.c~slub-relocate-freelist-pointer-to-middle-of-object +++ a/mm/slub.c @@ -3581,6 +3581,13 @@ static int calculate_sizes(struct kmem_c */ s->offset = size; size += sizeof(void *); + } else if (size > sizeof(void *)) { + /* + * Store freelist pointer near middle of object to keep + * it away from the edges of the object to avoid small + * sized over/underflows from neighboring allocations. + */ + s->offset = ALIGN(size / 2, sizeof(void *)); } #ifdef CONFIG_SLUB_DEBUG _
