ext4: fix deadlock allocating bio_post_read_ctx from mempool

[ 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>

diff --git a/fs/ext4/readpage.c b/fs/ext4/readpage.c
index a30b203fa461c7db9e87a7d98301e6f6087d7114..a5f55fece9b0472cd95d795eb45e7829eeac37f7 100644 (file)
--- 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)