Hi,
May I get the feedback for this patch?
This reduces the chance of page fault when freepointer is corrupted while
"slub_debug=F" is set.
Thank you very much!
Dongli Zhang
On 3/30/20 8:14 PM, Dongli Zhang wrote:
> The slub_debug is able to fix the corrupted slab freelist/page. However,
> alloc_debug_processing() only checks the validity of current and next
> freepointer during allocation path. As a result, once some objects have
> their freepointers corrupted, deactivate_slab() may lead to page fault.
>
> Below is from a test kernel module when
> 'slub_debug=PUF,kmalloc-128 slub_nomerge'. The test kernel corrupts the
> freepointer of one free object on purpose. Unfortunately, deactivate_slab()
> does not detect it when iterating the freechain.
>
> [ 92.665260] BUG: unable to handle page fault for address: 00000000123456f8
> [ 92.671597] #PF: supervisor read access in kernel mode
> [ 92.676159] #PF: error_code(0x0000) - not-present page
> [ 92.681666] PGD 0 P4D 0
> [ 92.684923] Oops: 0000 [#1] SMP PTI
> ... ...
> [ 92.706684] RIP: 0010:deactivate_slab.isra.92+0xed/0x490
> ... ...
> [ 92.819781] Call Trace:
> [ 92.823129] ? ext4_htree_store_dirent+0x30/0xf0
> [ 92.829488] ? ext4_htree_store_dirent+0x30/0xf0
> [ 92.834852] ? stack_trace_save+0x46/0x70
> [ 92.839342] ? init_object+0x66/0x80
> [ 92.843729] ? ___slab_alloc+0x536/0x570
> [ 92.847664] ___slab_alloc+0x536/0x570
> [ 92.851696] ? __find_get_block+0x23d/0x2c0
> [ 92.856763] ? ext4_htree_store_dirent+0x30/0xf0
> [ 92.862258] ? _cond_resched+0x10/0x40
> [ 92.866925] ? __getblk_gfp+0x27/0x2a0
> [ 92.872136] ? ext4_htree_store_dirent+0x30/0xf0
> [ 92.878394] ? __slab_alloc+0x17/0x30
> [ 92.883222] __slab_alloc+0x17/0x30
> [ 92.887210] __kmalloc+0x1d9/0x200
> [ 92.891448] ext4_htree_store_dirent+0x30/0xf0
> [ 92.896748] htree_dirblock_to_tree+0xcb/0x1c0
> [ 92.902398] ext4_htree_fill_tree+0x1bc/0x2d0
> [ 92.907749] ext4_readdir+0x54f/0x920
> [ 92.912725] iterate_dir+0x88/0x190
> [ 92.917072] __x64_sys_getdents+0xa6/0x140
> [ 92.922760] ? fillonedir+0xb0/0xb0
> [ 92.927020] ? do_syscall_64+0x49/0x170
> [ 92.931603] ? __ia32_sys_getdents+0x130/0x130
> [ 92.937012] do_syscall_64+0x49/0x170
> [ 92.940754] entry_SYSCALL_64_after_hwframe+0x44/0xa9
>
> Therefore, this patch adds extra consistency check in deactivate_slab().
> Once an object's freepointer is corrupted, all following objects starting
> at this object are isolated.
>
> Signed-off-by: Dongli Zhang <dongli.zhang@xxxxxxxxxx>
> ---
> mm/slub.c | 14 ++++++++++++++
> 1 file changed, 14 insertions(+)
>
> diff --git a/mm/slub.c b/mm/slub.c
> index 6589b41d5a60..c27e2d993535 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2082,6 +2082,20 @@ static void deactivate_slab(struct kmem_cache *s, struct page *page,
> void *prior;
> unsigned long counters;
>
> + if ((s->flags & SLAB_CONSISTENCY_CHECKS) &&
> + !check_valid_pointer(s, page, nextfree)) {
> + /*
> + * If 'nextfree' is invalid, it is possible that
> + * the object at 'freelist' is already corrupted.
> + * Therefore, all objects starting at 'freelist'
> + * are isolated.
> + */
> + object_err(s, page, freelist, "Freechain corrupt");
> + freelist = NULL;
> + slab_fix(s, "Isolate corrupted freechain");
> + break;
> + }
> +
> do {
> prior = page->freelist;
> counters = page->counters;
>
