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Date: Mon, 13 Jan 2020 14:47:24 -0500 From: "Theodore Y. Ts'o" <tytso@....edu> To: Eric Biggers <ebiggers@...nel.org> Cc: linux-ext4@...r.kernel.org, linux-fscrypt@...r.kernel.org Subject: Re: [PATCH] ext4: fix deadlock allocating bio_post_read_ctx from mempool On Tue, Dec 31, 2019 at 12:12:22PM -0600, Eric Biggers wrote: > From: Eric Biggers <ebiggers@...gle.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@...gle.com> Thanks, applied. - Ted
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