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 8440b56e385d5..b9dfebaa9eae5 100644 --- a/drivers/md/bcache/super.c +++ b/drivers/md/bcache/super.c @@ -1739,7 +1739,12 @@ 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) {