Re: [PATCH] bcache: avoid oversized read request in cache missing code path

From: Coly Li
Date: Tue May 18 2021 - 00:03:22 EST

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Hi folks,

Please hold on, this patch fails on a large size read test case this
morning. I will post a new version after the issue fixed.

Thanks.

Coly Li

On 5/18/21 12:22 AM, colyli@xxxxxxx wrote:
> From: Coly Li <colyli@xxxxxxx>
>
> 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 from the oversized value
> s->insert_bio_sectors resulted by the oversized 'sectors'. 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, it is obvious that in order to avoid the kernel
> panics in code block 2 and code block 3, the calculated 'sectors' in
> code block 1 should not generate overflowed value in code block 5 and
> code block 6.
>
> To avoid overflow in code block 5, the maximum 'sectors' value should be
> equal or less than (1 << KEY_SIZE_BITS) - 1. And to avoid overflow in
> code block 6, the maximum 'sectors' value should be euqal or less than
> BIO_MAX_VECS * PAGE_SECTORS. Considering the kernel page size can be
> variable, a reasonable maximum limitation of 'sectors' in code block 1
> should be smaller on from "(1 << KEY_SIZE_BITS) - 1" and "BIO_MAX_VECS *
> PAGE_SECTORS".
>
> In this patch, a local variable inside cache_lookup_fn() is added as,
> max_cache_miss_size = min_t(uint32_t,
> (1 << KEY_SIZE_BITS) - 1, BIO_MAX_VECS * PAGE_SECTORS);
> Then code block 1 uses max_cache_miss_size to limit the maximum value of
> 'sectors' calculation as,
> 533 unsigned int sectors = KEY_INODE(k) == s->iop.inode
> 534 ? min_t(uint64_t, max_cache_miss_size,
> 535 KEY_START(k) - bio->bi_iter.bi_sector)
> 536 : max_cache_miss_size;
>
> Now the calculated 'sectors' value sent into cached_dev_cache_miss()
> won't trigger neither of the above kernel panics.
>
> Current problmatic code can be partially found since Linux v5.13-rc1,
> therefore all maintained stable kernels should try to apply this fix.
>
> Reported-by: Diego Ercolani <diego.ercolani@xxxxxxxxx>
> Reported-by: Jan Szubiak <jan.szubiak@xxxxxxxxxxxxxx>
> Reported-by: Marco Rebhan <me@xxxxxxxxxxxx>
> Reported-by: Matthias Ferdinand <bcache@xxxxxxxxx>
> Reported-by: Thorsten Knabe <linux@xxxxxxxxxxxxxxxxx>
> Reported-by: Victor Westerhuis <victor@xxxxxxxxxxx>
> Reported-by: Vojtech Pavlik <vojtech@xxxxxxx>
> Signed-off-by: Coly Li <colyli@xxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Cc: Takashi Iwai <tiwai@xxxxxxxx>
> Cc: Kent Overstreet <kent.overstreet@xxxxxxxxx>
> ---
> drivers/md/bcache/request.c | 15 +++++++++++----
> 1 file changed, 11 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/md/bcache/request.c b/drivers/md/bcache/request.c
> index 29c231758293..90fa9ac47661 100644
> --- a/drivers/md/bcache/request.c
> +++ b/drivers/md/bcache/request.c
> @@ -515,18 +515,25 @@ static int cache_lookup_fn(struct btree_op *op, struct btree *b, struct bkey *k)
> struct search *s = container_of(op, struct search, op);
> struct bio *n, *bio = &s->bio.bio;
> struct bkey *bio_key;
> - unsigned int ptr;
> + unsigned int ptr, max_cache_miss_size;
>
> if (bkey_cmp(k, &KEY(s->iop.inode, bio->bi_iter.bi_sector, 0)) <= 0)
> return MAP_CONTINUE;
>
> + /*
> + * Make sure the cache missing size won't exceed the restrictions of
> + * max bkey size and max bio's bi_max_vecs.
> + */
> + max_cache_miss_size = min_t(uint32_t,
> + (1 << KEY_SIZE_BITS) - 1, BIO_MAX_VECS * PAGE_SECTORS);
> +
> if (KEY_INODE(k) != s->iop.inode ||
> KEY_START(k) > bio->bi_iter.bi_sector) {
> unsigned int bio_sectors = bio_sectors(bio);
> unsigned int sectors = KEY_INODE(k) == s->iop.inode
> - ? min_t(uint64_t, INT_MAX,
> + ? min_t(uint64_t, max_cache_miss_size,
> KEY_START(k) - bio->bi_iter.bi_sector)
> - : INT_MAX;
> + : max_cache_miss_size;
> int ret = s->d->cache_miss(b, s, bio, sectors);
>
> if (ret != MAP_CONTINUE)
> @@ -547,7 +554,7 @@ static int cache_lookup_fn(struct btree_op *op, struct btree *b, struct bkey *k)
> if (KEY_DIRTY(k))
> s->read_dirty_data = true;
>
> - n = bio_next_split(bio, min_t(uint64_t, INT_MAX,
> + n = bio_next_split(bio, min_t(uint64_t, max_cache_miss_size,
> KEY_OFFSET(k) - bio->bi_iter.bi_sector),
> GFP_NOIO, &s->d->bio_split);
>
>

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Previous message: Anshuman Khandual: "Re: linux-next: Tree for May 13 (mm/page_alloc.c, <linux/mm.h>: sizeof(struct page))"
In reply to: colyli: "[PATCH] bcache: avoid oversized read request in cache missing code path"
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