* [PATCH AUTOSEL 6.1 03/16] bcache: fix NULL pointer in cache_set_flush()
[not found] <20250609134725.1344921-1-sashal@kernel.org>
@ 2025-06-09 13:47 ` Sasha Levin
0 siblings, 0 replies; only message in thread
From: Sasha Levin @ 2025-06-09 13:47 UTC (permalink / raw)
To: patches, stable
Cc: Linggang Zeng, Mingzhe Zou, Coly Li, Jens Axboe, Sasha Levin,
kent.overstreet, linux-bcache
From: Linggang Zeng <linggang.zeng@easystack.cn>
[ Upstream commit 1e46ed947ec658f89f1a910d880cd05e42d3763e ]
1. LINE#1794 - LINE#1887 is some codes about function of
bch_cache_set_alloc().
2. LINE#2078 - LINE#2142 is some codes about function of
register_cache_set().
3. register_cache_set() will call bch_cache_set_alloc() in LINE#2098.
1794 struct cache_set *bch_cache_set_alloc(struct cache_sb *sb)
1795 {
...
1860 if (!(c->devices = kcalloc(c->nr_uuids, sizeof(void *), GFP_KERNEL)) ||
1861 mempool_init_slab_pool(&c->search, 32, bch_search_cache) ||
1862 mempool_init_kmalloc_pool(&c->bio_meta, 2,
1863 sizeof(struct bbio) + sizeof(struct bio_vec) *
1864 bucket_pages(c)) ||
1865 mempool_init_kmalloc_pool(&c->fill_iter, 1, iter_size) ||
1866 bioset_init(&c->bio_split, 4, offsetof(struct bbio, bio),
1867 BIOSET_NEED_BVECS|BIOSET_NEED_RESCUER) ||
1868 !(c->uuids = alloc_bucket_pages(GFP_KERNEL, c)) ||
1869 !(c->moving_gc_wq = alloc_workqueue("bcache_gc",
1870 WQ_MEM_RECLAIM, 0)) ||
1871 bch_journal_alloc(c) ||
1872 bch_btree_cache_alloc(c) ||
1873 bch_open_buckets_alloc(c) ||
1874 bch_bset_sort_state_init(&c->sort, ilog2(c->btree_pages)))
1875 goto err;
^^^^^^^^
1876
...
1883 return c;
1884 err:
1885 bch_cache_set_unregister(c);
^^^^^^^^^^^^^^^^^^^^^^^^^^^
1886 return NULL;
1887 }
...
2078 static const char *register_cache_set(struct cache *ca)
2079 {
...
2098 c = bch_cache_set_alloc(&ca->sb);
2099 if (!c)
2100 return err;
^^^^^^^^^^
...
2128 ca->set = c;
2129 ca->set->cache[ca->sb.nr_this_dev] = ca;
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
...
2138 return NULL;
2139 err:
2140 bch_cache_set_unregister(c);
2141 return err;
2142 }
(1) If LINE#1860 - LINE#1874 is true, then do 'goto err'(LINE#1875) and
call bch_cache_set_unregister()(LINE#1885).
(2) As (1) return NULL(LINE#1886), LINE#2098 - LINE#2100 would return.
(3) As (2) has returned, LINE#2128 - LINE#2129 would do *not* give the
value to c->cache[], it means that c->cache[] is NULL.
LINE#1624 - LINE#1665 is some codes about function of cache_set_flush().
As (1), in LINE#1885 call
bch_cache_set_unregister()
---> bch_cache_set_stop()
---> closure_queue()
-.-> cache_set_flush() (as below LINE#1624)
1624 static void cache_set_flush(struct closure *cl)
1625 {
...
1654 for_each_cache(ca, c, i)
1655 if (ca->alloc_thread)
^^
1656 kthread_stop(ca->alloc_thread);
...
1665 }
(4) In LINE#1655 ca is NULL(see (3)) in cache_set_flush() then the
kernel crash occurred as below:
[ 846.712887] bcache: register_cache() error drbd6: cannot allocate memory
[ 846.713242] bcache: register_bcache() error : failed to register device
[ 846.713336] bcache: cache_set_free() Cache set 2f84bdc1-498a-4f2f-98a7-01946bf54287 unregistered
[ 846.713768] BUG: unable to handle kernel NULL pointer dereference at 00000000000009f8
[ 846.714790] PGD 0 P4D 0
[ 846.715129] Oops: 0000 [#1] SMP PTI
[ 846.715472] CPU: 19 PID: 5057 Comm: kworker/19:16 Kdump: loaded Tainted: G OE --------- - - 4.18.0-147.5.1.el8_1.5es.3.x86_64 #1
[ 846.716082] Hardware name: ESPAN GI-25212/X11DPL-i, BIOS 2.1 06/15/2018
[ 846.716451] Workqueue: events cache_set_flush [bcache]
[ 846.716808] RIP: 0010:cache_set_flush+0xc9/0x1b0 [bcache]
[ 846.717155] Code: 00 4c 89 a5 b0 03 00 00 48 8b 85 68 f6 ff ff a8 08 0f 84 88 00 00 00 31 db 66 83 bd 3c f7 ff ff 00 48 8b 85 48 ff ff ff 74 28 <48> 8b b8 f8 09 00 00 48 85 ff 74 05 e8 b6 58 a2 e1 0f b7 95 3c f7
[ 846.718026] RSP: 0018:ffffb56dcf85fe70 EFLAGS: 00010202
[ 846.718372] RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
[ 846.718725] RDX: 0000000000000001 RSI: 0000000040000001 RDI: 0000000000000000
[ 846.719076] RBP: ffffa0ccc0f20df8 R08: ffffa0ce1fedb118 R09: 000073746e657665
[ 846.719428] R10: 8080808080808080 R11: 0000000000000000 R12: ffffa0ce1fee8700
[ 846.719779] R13: ffffa0ccc0f211a8 R14: ffffa0cd1b902840 R15: ffffa0ccc0f20e00
[ 846.720132] FS: 0000000000000000(0000) GS:ffffa0ce1fec0000(0000) knlGS:0000000000000000
[ 846.720726] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 846.721073] CR2: 00000000000009f8 CR3: 00000008ba00a005 CR4: 00000000007606e0
[ 846.721426] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[ 846.721778] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[ 846.722131] PKRU: 55555554
[ 846.722467] Call Trace:
[ 846.722814] process_one_work+0x1a7/0x3b0
[ 846.723157] worker_thread+0x30/0x390
[ 846.723501] ? create_worker+0x1a0/0x1a0
[ 846.723844] kthread+0x112/0x130
[ 846.724184] ? kthread_flush_work_fn+0x10/0x10
[ 846.724535] ret_from_fork+0x35/0x40
Now, check whether that ca is NULL in LINE#1655 to fix the issue.
Signed-off-by: Linggang Zeng <linggang.zeng@easystack.cn>
Signed-off-by: Mingzhe Zou <mingzhe.zou@easystack.cn>
Signed-off-by: Coly Li <colyli@kernel.org>
Link: https://lore.kernel.org/r/20250527051601.74407-2-colyli@kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
**YES**
This commit should be backported to stable kernel trees. Here's my
extensive analysis:
## Bug Severity and Impact
This commit fixes a **NULL pointer dereference** in the bcache subsystem
that causes a kernel crash. The crash trace shows:
```
BUG: unable to handle kernel NULL pointer dereference at
00000000000009f8
```
The crash occurs during cache set registration failure in an out-of-
memory (OOM) condition, which is a real-world scenario that can happen
in production systems under memory pressure.
## Root Cause Analysis
The commit message provides excellent debugging information showing the
precise execution flow:
1. In `bch_cache_set_alloc()`, if memory allocation fails (lines
1860-1874), it goes to the error path which calls
`bch_cache_set_unregister()` and returns NULL
2. In `register_cache_set()`, when `bch_cache_set_alloc()` returns NULL,
the function returns early (line 2100)
3. Because of the early return, `ca->set` is never assigned (lines
2128-2129), leaving `c->cache[]` as NULL
4. When `cache_set_flush()` is called via the closure mechanism, it
tries to access `ca->alloc_thread` where `ca` is NULL, causing the
crash
## Code Change Analysis
The fix is minimal and surgical:
```diff
- if (ca->alloc_thread)
+ /*
+ * If the register_cache_set() call to bch_cache_set_alloc()
failed,
+ * ca has not been assigned a value and return error.
+ * So we need check ca is not NULL during
bch_cache_set_unregister().
+ */
+ if (ca && ca->alloc_thread)
kthread_stop(ca->alloc_thread);
```
This adds a NULL check before dereferencing `ca`, which is a defensive
programming practice that prevents the crash.
## Comparison with Similar Commits
Looking at the similar commits with "Backport Status: YES":
1. **Similar commits #1, #2, #3**: All fix NULL pointer dereferences in
bcache during OOM conditions
2. **Similar commits #4, #5**: Fix potential NULL pointer issues in
btree node allocation
3. All these commits share characteristics:
- Fix real crashes with clear reproducers
- Minimal code changes (often just adding NULL checks)
- No architectural changes
- No new features
- Clear bug fixes in error paths
## Stable Tree Criteria Assessment
This commit meets all the stable tree criteria:
1. **Fixes a real bug**: Yes - NULL pointer dereference causing kernel
crash
2. **Already in Linus' tree**: Yes - has been merged by Jens Axboe
3. **Small and contained**: Yes - single line change adding NULL check
4. **No new features**: Correct - purely a bug fix
5. **Minimal risk**: Very low risk - adds defensive NULL check
6. **Clear issue**: The crash trace and detailed analysis make the issue
crystal clear
## Additional Considerations
- The bug occurs in an error path during OOM conditions, which might be
considered rare but is definitely possible in production
- The fix prevents a kernel crash, which is always high priority for
stable backports
- The bcache subsystem is used in production systems for caching, making
stability critical
- The fix pattern (adding NULL checks in error paths) is well-
established and safe
## Conclusion
This commit is an excellent candidate for stable backporting. It fixes a
real kernel crash with a minimal, safe change that adds proper NULL
checking in an error path. The detailed commit message with line-by-line
analysis demonstrates thorough understanding of the issue, and the fix
follows established patterns seen in other successful bcache stable
backports.
drivers/md/bcache/super.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)
diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
index 1ddae5c972398..2c7b3c8673de2 100644
--- a/drivers/md/bcache/super.c
+++ b/drivers/md/bcache/super.c
@@ -1741,7 +1741,12 @@ static void cache_set_flush(struct closure *cl)
mutex_unlock(&b->write_lock);
}
- if (ca->alloc_thread)
+ /*
+ * If the register_cache_set() call to bch_cache_set_alloc() failed,
+ * ca has not been assigned a value and return error.
+ * So we need check ca is not NULL during bch_cache_set_unregister().
+ */
+ if (ca && ca->alloc_thread)
kthread_stop(ca->alloc_thread);
if (c->journal.cur) {
--
2.39.5
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