Hy,
First of all, thanks a lot for your time.
On 4/25/24 10:36, Chengming Zhou wrote:
On 2024/4/24 20:47, Nicolas Bouchinet wrote:From: Nicolas Bouchinet <nicolas.bouchinet@xxxxxxxxxxx> Commit 284f17ac13fe ("mm/slub: handle bulk and single object freeing separately") splits single and bulk object freeing in two functions slab_free() and slab_free_bulk() which leads slab_free() to call slab_free_hook() directly instead of slab_free_freelist_hook().Right.If `init_on_free` is set, slab_free_hook() zeroes the object. Afterward, if `slub_debug=F` and `CONFIG_SLAB_FREELIST_HARDENED` are set, the do_slab_free() slowpath executes freelist consistency checks and try to decode a zeroed freepointer which leads to a "Freepointer corrupt" detection in check_object().IIUC, the "freepointer" can be checked on the free path only when it's outside the object memory. Here slab_free_hook() zeroed the freepointer and caused the problem. But why we should zero the memory outside the object_size? It seems more reasonable to only zero the object_size when init_on_free is set?
The original purpose was to avoid leaking information through the object and its metadata / tracking information as described in init_on_free initial Commit 6471384af2a6 ("mm: security: introduce init_on_alloc=1 and init_on_free=1 boot options").
I have to admit I didn't read the entire lore about the original
patchset yet, though it could be interesting to know a bit more
the threat models, specifically regarding the object metadata
init.
The patch could also be optimized a bit by restricting set_freepointer() call to the `CONFIG_SLAB_FREELIST_HARDENED` option value.
Thanks again, Nicolas
Thanks.Object's freepointer thus needs to be properly set using set_freepointer() after init_on_free. To reproduce, set `slub_debug=FU init_on_free=1 log_level=7` on the command line of a kernel build with `CONFIG_SLAB_FREELIST_HARDENED=y`. dmesg sample log: [ 10.708715] ============================================================================= [ 10.710323] BUG kmalloc-rnd-05-32 (Tainted: G B T ): Freepointer corrupt [ 10.712695] ----------------------------------------------------------------------------- [ 10.712695] [ 10.712695] Slab 0xffffd8bdc400d580 objects=32 used=4 fp=0xffff9d9a80356f80 flags=0x200000000000a00(workingset|slab|node=0|zone=2) [ 10.716698] Object 0xffff9d9a80356600 @offset=1536 fp=0x7ee4f480ce0ecd7c [ 10.716698] [ 10.716698] Bytes b4 ffff9d9a803565f0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ [ 10.720703] Object ffff9d9a80356600: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ [ 10.720703] Object ffff9d9a80356610: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ [ 10.724696] Padding ffff9d9a8035666c: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ [ 10.724696] Padding ffff9d9a8035667c: 00 00 00 00 .... [ 10.724696] FIX kmalloc-rnd-05-32: Object at 0xffff9d9a80356600 not freed Signed-off-by: Nicolas Bouchinet <nicolas.bouchinet@xxxxxxxxxxx> --- mm/slub.c | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-) diff --git a/mm/slub.c b/mm/slub.c index 3aa12b9b323d9..71dbff9ad8f17 100644 --- a/mm/slub.c +++ b/mm/slub.c @@ -4342,10 +4342,16 @@ static __fastpath_inline void slab_free(struct kmem_cache *s, struct slab *slab, void *object, unsigned long addr) { + bool init = false; + memcg_slab_free_hook(s, slab, &object, 1); + init = slab_want_init_on_free(s); - if (likely(slab_free_hook(s, object, slab_want_init_on_free(s)))) + if (likely(slab_free_hook(s, object, init))) { + if (init) + set_freepointer(s, object, NULL); do_slab_free(s, slab, object, object, 1, addr); + } } static __fastpath_inline
