[PATCH 1/1] mm: slub: fix corrupted freechain in deactivate_slab()

[Dongli Zhang]

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;
-- 
2.17.1