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Subject[PATCH 5.5 072/399] ext4: fix deadlock allocating bio_post_read_ctx from mempool
Date
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)
--
2.20.1


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