In the cache missing code path of cached device, if a proper location from the internal B+ tree is matched for a cache miss range, function cached_dev_cache_miss() will be called in cache_lookup_fn() in the following code block, [code block 1] 526 unsigned int sectors = KEY_INODE(k) == s->iop.inode 527 ? min_t(uint64_t, INT_MAX, 528 KEY_START(k) - bio->bi_iter.bi_sector) 529 : INT_MAX; 530 int ret = s->d->cache_miss(b, s, bio, sectors);

Here s->d->cache_miss() is the call backfunction pointer initialized as cached_dev_cache_miss(), the last parameter 'sectors' is an important hint to calculate the size of read request to backing device of the missing cache data.

Current calculation in above code block may generate oversized value of 'sectors', which consequently may trigger 2 different potential kernel panics by BUG() or BUG_ON() as listed below,

1) BUG_ON() inside bch_btree_insert_key(), [code block 2] 886 BUG_ON(b->ops->is_extents && !KEY_SIZE(k)); 2) BUG() inside biovec_slab(), [code block 3] 51 default: 52 BUG(); 53 return NULL;

All the above panics are original from cached_dev_cache_miss() by the oversized parameter 'sectors'.

Inside cached_dev_cache_miss(), parameter 'sectors' is used to calculate the size of data read from backing device for the cache missing. This size is stored in s->insert_bio_sectors by the following lines of code, [code block 4] 909 s->insert_bio_sectors = min(sectors, bio_sectors(bio) + reada);

Then the actual key inserting to the internal B+ tree is generated and stored in s->iop.replace_key by the following lines of code, [code block 5] 911 s->iop.replace_key = KEY(s->iop.inode, 912 bio->bi_iter.bi_sector + s->insert_bio_sectors, 913 s->insert_bio_sectors); The oversized parameter 'sectors' may trigger panic 1) by BUG_ON() from the above code block.

And the bio sending to backing device for the missing data is allocated with hint from s->insert_bio_sectors by the following lines of code, [code block 6] 926 cache_bio = bio_alloc_bioset(GFP_NOWAIT, 927 DIV_ROUND_UP(s->insert_bio_sectors, PAGE_SECTORS), 928 &dc->disk.bio_split); The oversized parameter 'sectors' may trigger panic 2) by BUG() from the agove code block.

Now let me explain how the panics happen with the oversized 'sectors'. In code block 5, replace_key is generated by macro KEY(). From the definition of macro KEY(), [code block 7] 71 #define KEY(inode, offset, size) \ 72 ((struct bkey) { \ 73 .high = (1ULL << 63) | ((__u64) (size) << 20) | (inode), \ 74 .low = (offset) \ 75 })

Here 'size' is 16bits width embedded in 64bits member 'high' of struct bkey. But in code block 1, if "KEY_START(k) - bio->bi_iter.bi_sector" is very probably to be larger than (1<<16) - 1, which makes the bkey size calculation in code block 5 is overflowed. In one bug report the value of parameter 'sectors' is 131072 (= 1 << 17), the overflowed 'sectors' results the overflowed s->insert_bio_sectors in code block 4, then makes size field of s->iop.replace_key to be 0 in code block 5. Then the 0- sized s->iop.replace_key is inserted into the internal B+ tree as cache missing check key (a special key to detect and avoid a racing between normal write request and cache missing read request) as, [code block 8] 915 ret = bch_btree_insert_check_key(b, &s->op, &s->iop.replace_key);

Then the 0-sized s->iop.replace_key as 3rd parameter triggers the bkey size check BUG_ON() in code block 2, and causes the kernel panic 1).

Another kernel panic is from code block 6, is by the bvecs number oversized value s->insert_bio_sectors from code block 4, min(sectors, bio_sectors(bio) + reada) There are two possibility for oversized reresult, - bio_sectors(bio) is valid, but bio_sectors(bio) + reada is oversized. - sectors < bio_sectors(bio) + reada, but sectors is oversized.

From a bug report the result of "DIV_ROUND_UP(s->insert_bio_sectors,

PAGE_SECTORS)" from code block 6 can be 344, 282, 946, 342 and many other values which larther than BIO_MAX_VECS (a.k.a 256). When calling bio_alloc_bioset() with such larger-than-256 value as the 2nd parameter, this value will eventually be sent to biovec_slab() as parameter 'nr_vecs' in following code path, bio_alloc_bioset() ==> bvec_alloc() ==> biovec_slab() Because parameter 'nr_vecs' is larger-than-256 value, the panic by BUG() in code block 3 is triggered inside biovec_slab().

From the above analysis, we know that the 4th parameter 'sector' sent

into cached_dev_cache_miss() may cause overflow in code block 5 and 6, and finally cause kernel panic in code block 2 and 3. And if result of bio_sectors(bio) + reada exceeds valid bvecs number, it may also trigger kernel panic in code block 3 from code block 6.

Now the almost-useless readahead size for cache missing request back to backing device is removed, this patch can fix the oversized issue with more simpler method. - add a local variable size_limit, set it by the minimum value from the max bkey size and max bio bvecs number. - set s->insert_bio_sectors by the minimum value from size_limit, sectors, and the sectors size of bio. - replace sectors by s->insert_bio_sectors to do bio_next_split.

By the above method with size_limit, s->insert_bio_sectors will never result oversized replace_key size or bio bvecs number. And split bio 'miss' from bio_next_split() will always match the size of 'cache_bio', that is the current maximum bio size we can sent to backing device for fetching the cache missing data.

Current problmatic code can be partially found since Linux v3.13-rc1, therefore all maintained stable kernels should try to apply this fix.

Reported-by: Alexander Ullrich ealex1979@gmail.com Reported-by: Diego Ercolani diego.ercolani@gmail.com Reported-by: Jan Szubiak jan.szubiak@linuxpolska.pl Reported-by: Marco Rebhan me@dblsaiko.net Reported-by: Matthias Ferdinand bcache@mfedv.net Reported-by: Thorsten Knabe linux@thorsten-knabe.de Reported-by: Victor Westerhuis victor@westerhu.is Reported-by: Vojtech Pavlik vojtech@suse.cz Signed-off-by: Coly Li colyli@suse.de Cc: stable@vger.kernel.org Cc: Christoph Hellwig hch@lst.de Cc: Kent Overstreet kent.overstreet@gmail.com Cc: Nix nix@esperi.org.uk Cc: Takashi Iwai tiwai@suse.com --- Changelog, v5, improvement and fix based on v4 comments from Christoph Hellwig and Nix. v4, not directly access BIO_MAX_VECS and reduce reada value to avoid oversized bvecs number, by hint from Christoph Hellwig. v3, fix typo in v2. v2, fix the bypass bio size calculation in v1. v1, the initial version.

drivers/md/bcache/request.c | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/md/bcache/request.c b/drivers/md/bcache/request.c index ab8ff18df32a..d855a8882cbc 100644 --- a/drivers/md/bcache/request.c +++ b/drivers/md/bcache/request.c @@ -882,6 +882,7 @@ static int cached_dev_cache_miss(struct btree *b, struct search *s, int ret = MAP_CONTINUE; struct cached_dev *dc = container_of(s->d, struct cached_dev, disk); struct bio *miss, *cache_bio; + unsigned int size_limit;

s->cache_missed = 1;

@@ -891,7 +892,10 @@ static int cached_dev_cache_miss(struct btree *b, struct search *s, goto out_submit; }

- s->insert_bio_sectors = min(sectors, bio_sectors(bio)); + /* Limitation for valid replace key size and cache_bio bvecs number */ + size_limit = min_t(unsigned int, bio_max_segs(UINT_MAX) * PAGE_SECTORS, + (1 << KEY_SIZE_BITS) - 1); + s->insert_bio_sectors = min3(size_limit, sectors, bio_sectors(bio));

s->iop.replace_key = KEY(s->iop.inode, bio->bi_iter.bi_sector + s->insert_bio_sectors, @@ -903,7 +907,7 @@ static int cached_dev_cache_miss(struct btree *b, struct search *s,

s->iop.replace = true;

- miss = bio_next_split(bio, sectors, GFP_NOIO, &s->d->bio_split); + miss = bio_next_split(bio, s->insert_bio_sectors, GFP_NOIO, &s->d->bio_split);

/* btree_search_recurse()'s btree iterator is no good anymore */ ret = miss == bio ? MAP_DONE : -EINTR;