6.18-stable review patch. If anyone has any objections, please let me know.
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From: huang-jl huang-jl@deepseek.com
[ Upstream commit 114ea9bbaf7681c4d363e13b7916e6fef6a4963a ]
io_import_kbuf() calculates nr_segs incorrectly when iov_offset is non-zero after iov_iter_advance(). It doesn't account for the partial consumption of the first bvec.
The problem comes when meet the following conditions: 1. Use UBLK_F_AUTO_BUF_REG feature of ublk. 2. The kernel will help to register the buffer, into the io uring. 3. Later, the ublk server try to send IO request using the registered buffer in the io uring, to read/write to fuse-based filesystem, with O_DIRECT.
From a userspace perspective, the ublk server thread is blocked in the
kernel, and will see "soft lockup" in the kernel dmesg.
When ublk registers a buffer with mixed-size bvecs like [4K]*6 + [12K] and a request partially consumes a bvec, the next request's nr_segs calculation uses bvec->bv_len instead of (bv_len - iov_offset).
This causes fuse_get_user_pages() to loop forever because nr_segs indicates fewer pages than actually needed.
Specifically, the infinite loop happens at: fuse_get_user_pages() -> iov_iter_extract_pages() -> iov_iter_extract_bvec_pages() Since the nr_segs is miscalculated, the iov_iter_extract_bvec_pages returns when finding that i->nr_segs is zero. Then iov_iter_extract_pages returns zero. However, fuse_get_user_pages does still not get enough data/pages, causing infinite loop.
Example: - Bvecs: [4K, 4K, 4K, 4K, 4K, 4K, 12K, ...] - Request 1: 32K at offset 0, uses 6*4K + 8K of the 12K bvec - Request 2: 32K at offset 32K - iov_offset = 8K (8K already consumed from 12K bvec) - Bug: calculates using 12K, not (12K - 8K) = 4K - Result: nr_segs too small, infinite loop in fuse_get_user_pages.
Fix by accounting for iov_offset when calculating the first segment's available length.
Fixes: b419bed4f0a6 ("io_uring/rsrc: ensure segments counts are correct on kbuf buffers") Signed-off-by: huang-jl huang-jl@deepseek.com Reviewed-by: Ming Lei ming.lei@redhat.com Signed-off-by: Jens Axboe axboe@kernel.dk Signed-off-by: Sasha Levin sashal@kernel.org --- io_uring/rsrc.c | 1 + 1 file changed, 1 insertion(+)
diff --git a/io_uring/rsrc.c b/io_uring/rsrc.c index 0010c4992490..5b6b73c6a62b 100644 --- a/io_uring/rsrc.c +++ b/io_uring/rsrc.c @@ -1057,6 +1057,7 @@ static int io_import_kbuf(int ddir, struct iov_iter *iter, if (count < imu->len) { const struct bio_vec *bvec = iter->bvec;
+ len += iter->iov_offset; while (len > bvec->bv_len) { len -= bvec->bv_len; bvec++;