From: Eric Biggers ebiggers@google.com
[ Upstream commit 68e45330e341dad2d3a0a3f8ef2ec46a2a0a3bbc ]
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: 22cfe4b48ccb ("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 Signed-off-by: Sasha Levin sashal@kernel.org --- fs/ext4/readpage.c | 17 +++++++++++++++-- 1 file changed, 15 insertions(+), 2 deletions(-)
diff --git a/fs/ext4/readpage.c b/fs/ext4/readpage.c index fef7755300c35..410c904cf59b9 100644 --- a/fs/ext4/readpage.c +++ b/fs/ext4/readpage.c @@ -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)