From: Eric Biggers <ebiggers@kernel.org> To: linux-ext4@vger.kernel.org Cc: linux-fscrypt@vger.kernel.org Subject: [PATCH] ext4: fix deadlock allocating bio_post_read_ctx from mempool Date: Tue, 31 Dec 2019 12:12:22 -0600 Message-ID: <20191231181222.47684-1-ebiggers@kernel.org> (raw) From: Eric Biggers <ebiggers@google.com> 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> --- 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 fef7755300c3..410c904cf59b 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) -- 2.24.1
next reply index Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top 2019-12-31 18:12 Eric Biggers [this message] 2020-01-13 19:47 ` Theodore Y. Ts'o
Reply instructions: You may reply publicly to this message via plain-text email using any one of the following methods: * Save the following mbox file, import it into your mail client, and reply-to-all from there: mbox Avoid top-posting and favor interleaved quoting: https://en.wikipedia.org/wiki/Posting_style#Interleaved_style * Reply using the --to, --cc, and --in-reply-to switches of git-send-email(1): git send-email \ --in-reply-to=20191231181222.47684-1-ebiggers@kernel.org \ --to=ebiggers@kernel.org \ --cc=linux-ext4@vger.kernel.org \ --cc=linux-fscrypt@vger.kernel.org \ /path/to/YOUR_REPLY https://kernel.org/pub/software/scm/git/docs/git-send-email.html * If your mail client supports setting the In-Reply-To header via mailto: links, try the mailto: link
Linux-ext4 Archive on lore.kernel.org Archives are clonable: git clone --mirror https://lore.kernel.org/linux-ext4/0 linux-ext4/git/0.git # If you have public-inbox 1.1+ installed, you may # initialize and index your mirror using the following commands: public-inbox-init -V2 linux-ext4 linux-ext4/ https://lore.kernel.org/linux-ext4 \ linux-ext4@vger.kernel.org public-inbox-index linux-ext4 Example config snippet for mirrors Newsgroup available over NNTP: nntp://nntp.lore.kernel.org/org.kernel.vger.linux-ext4 AGPL code for this site: git clone https://public-inbox.org/public-inbox.git