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ext4: fix deadlock allocating bio_post_read_ctx from mempool 

Without any form of coordination, any case where multiple allocations
from the same mempool are needed at a time to make forward progress can
deadlock under memory pressure.

This is the case for struct bio_post_read_ctx, as one can be allocated
to decrypt a Merkle tree page during fsverity_verify_bio(), which itself
is running from a post-read callback for a data bio which has its own
struct bio_post_read_ctx.

Fix this by freeing the first bio_post_read_ctx before calling
fsverity_verify_bio().  This works because verity (if enabled) is always
the last post-read step.

This deadlock can be reproduced by trying to read from an encrypted
verity file after reducing NUM_PREALLOC_POST_READ_CTXS to 1 and patching
mempool_alloc() to pretend that pool->alloc() always fails.

Note that since NUM_PREALLOC_POST_READ_CTXS is actually 128, to actually
hit this bug in practice would require reading from lots of encrypted
verity files at the same time.  But it's theoretically possible, as N
available objects isn't enough to guarantee forward progress when > N/2
threads each need 2 objects at a time.

Fixes: 22cfe4b48c ("ext4: add fs-verity read support")
Signed-off-by: Eric Biggers <ebiggers@google.com>
Link: https://lore.kernel.org/r/20191231181222.47684-1-ebiggers@kernel.org
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
This commit is contained in:
Eric Biggers 2019-12-31 12:12:22 -06:00 committed by Theodore Ts'o
parent 547c556f4d
commit 68e45330e3
1 changed files with 15 additions and 2 deletions
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17
fs/ext4/readpage.c
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@ -57,6 +57,7 @@ enum bio_post_read_step {
STEP_INITIAL = 0,
STEP_DECRYPT,
STEP_VERITY,
STEP_MAX,
};
struct bio_post_read_ctx {
@ -106,10 +107,22 @@ static void verity_work(struct work_struct *work)
{
struct bio_post_read_ctx *ctx =
container_of(work, struct bio_post_read_ctx, work);
struct bio *bio = ctx->bio;
fsverity_verify_bio(ctx->bio);
/*
* fsverity_verify_bio() may call readpages() again, and although verity
* will be disabled for that, decryption may still be needed, causing
* another bio_post_read_ctx to be allocated. So to guarantee that
* mempool_alloc() never deadlocks we must free the current ctx first.
* This is safe because verity is the last post-read step.
*/
BUILD_BUG_ON(STEP_VERITY + 1 != STEP_MAX);
mempool_free(ctx, bio_post_read_ctx_pool);
bio->bi_private = NULL;
bio_post_read_processing(ctx);
fsverity_verify_bio(bio);
__read_end_io(bio);
}
static void bio_post_read_processing(struct bio_post_read_ctx *ctx)