CVE-2021-47275

In the Linux kernel, the following vulnerability has been resolved:

bcache: avoid oversized read request in cache missing code path

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 ke —truncated—

Affected Software

References

• https://git.kernel.org/stable/c/41fe8d088e96472f63164e213de44ec77be69478
• https://git.kernel.org/stable/c/555002a840ab88468e252b0eedf0b05e2ce7099c