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Tue, 30 Jun 2026 12:00:08 -0700 (PDT) Message-ID: <8c8b9ace-dc84-46bf-8495-44bf2f2b0680@kernel.dk> Date: Tue, 30 Jun 2026 13:00:06 -0600 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [BUG REPORT] btrfs/io_uring: GPF in tctx_task_work_run after encoded read error completion To: Yue Sun , Chris Mason , David Sterba Cc: linux-btrfs@vger.kernel.org, io-uring@vger.kernel.org, linux-kernel@vger.kernel.org References: <20260630091609.3414-1-samsun1006219@gmail.com> Content-Language: en-US From: Jens Axboe In-Reply-To: <20260630091609.3414-1-samsun1006219@gmail.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 6/30/26 3:16 AM, Yue Sun wrote: > Hello, > > I can reproduce a general protection fault on current upstream master by using > IORING_OP_URING_CMD with BTRFS_IOC_ENCODED_READ on a loop-backed btrfs image > while fail_make_request injects read errors. > > Summary > ------- > > The crash happens while io_uring is running task_work for a btrfs encoded read > completion: > > tctx_task_work_run() > mutex_lock(&ctx->uring_lock) > > The faulting mutex address is poisoned: > > RDI: dead000000001129 > KASAN: maybe wild-memory-access in range [0xdead000000001128-0xdead00000000112f] > > The root cause might be a double-completion/use-after-free race in the > btrfs io_uring encoded read error path. > > The timing appears to be: > > # CPU0: userspace task issues IORING_OP_URING_CMD. > io_uring_enter() > btrfs_uring_cmd() > btrfs_uring_encoded_read() > ret = btrfs_encoded_read(...) > if (ret == -EIOCBQUEUED) > btrfs_uring_read_extent(..., cmd) > > btrfs_uring_read_extent() > priv->cmd = cmd > ret = btrfs_encoded_read_regular_fill_pages(..., priv) > > # In this helper, priv is struct btrfs_encoded_read_private. > # uring_ctx points to the caller's struct btrfs_uring_priv. > btrfs_encoded_read_regular_fill_pages(..., uring_ctx=priv) > refcount_set(&priv->pending_refs, 1) > priv->uring_ctx = uring_ctx > refcount_inc(&priv->pending_refs) > btrfs_submit_bbio(bbio, 0) > > # CPU1: the submitted bio fails quickly, before CPU0 drops its owner ref. > btrfs_encoded_read_endio() > WRITE_ONCE(priv->status, bbio->bio.bi_status) > refcount_dec_and_test(&priv->pending_refs) > # pending_refs goes 2 -> 1, so this context does not queue completion. > > # CPU0: btrfs_submit_bbio() has returned and the uring branch continues. > btrfs_encoded_read_regular_fill_pages(..., uring_ctx=priv) > if (refcount_dec_and_test(&priv->pending_refs)) { > ret = blk_status_to_errno(READ_ONCE(priv->status)) > btrfs_uring_read_extent_endio(uring_ctx, ret) > kfree(priv) > return ret > } > > # Here priv is the caller's struct btrfs_uring_priv. > btrfs_uring_read_extent_endio(priv, err) > bc->priv = priv > io_uring_cmd_complete_in_task(priv->cmd, btrfs_uring_read_finished) > > # CPU0: task_work is queued, but the helper returns a normal error instead > # of -EIOCBQUEUED, so the caller takes the synchronous failure path. > btrfs_uring_read_extent() > if (ret && ret != -EIOCBQUEUED) > goto out_fail > out_fail: > btrfs_unlock_extent(...) > btrfs_inode_unlock(...) > kfree(priv) > __free_page(...) > kfree(pages) > return ret > > # Later, the same task waits for io_uring completions and runs task_work. > io_uring_enter() > io_cqring_wait() > io_run_task_work() > task_work_run() > tctx_task_work() > tctx_task_work_run() > req = container_of(node, struct io_kiocb, io_task_work.node) > ctx = req->ctx > mutex_lock(&ctx->uring_lock) > # Crash: req->ctx appears poisoned/stale before > # btrfs_uring_read_finished() is reached. If the work is passed to task_work, then btrfs must return -EIOCBQUEUED. Looks like a basic bug in btrfs, see below. Caveat - entirely untested/compiled/whatever. On vacation, btrfs guys can figure this out. diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c index 272598f6ae77..51c06618c733 100644 --- a/fs/btrfs/inode.c +++ b/fs/btrfs/inode.c @@ -9460,7 +9460,6 @@ int btrfs_encoded_read_regular_fill_pages(struct btrfs_inode *inode, ret = blk_status_to_errno(READ_ONCE(priv->status)); btrfs_uring_read_extent_endio(uring_ctx, ret); kfree(priv); - return ret; } return -EIOCBQUEUED; -- Jens Axboe