[PATCH 6.19 136/311] io_uring/rsrc: reject zero-length fixed buffer import

Greg Kroah-Hartman
2026-04-08T18:57:29Z

From: Qi Tang <tpluszz77@gmail.com>

[ Upstream commit 111a12b422a8cfa93deabaef26fec48237163214 ]

validate_fixed_range() admits buf_addr at the exact end of the
registered region when len is zero, because the check uses strict
greater-than (buf_end > imu->ubuf + imu->len). io_import_fixed()
then computes offset == imu->len, which causes the bvec skip logic
to advance past the last bio_vec entry and read bv_offset from
out-of-bounds slab memory.

Return early from io_import_fixed() when len is zero. A zero-length
import has no data to transfer and should not walk the bvec array
at all.

BUG: KASAN: slab-out-of-bounds in io_import_reg_buf+0x697/0x7f0
Read of size 4 at addr ffff888002bcc254 by task poc/103
Call Trace:
io_import_reg_buf+0x697/0x7f0
io_write_fixed+0xd9/0x250
__io_issue_sqe+0xad/0x710
io_issue_sqe+0x7d/0x1100
io_submit_sqes+0x86a/0x23c0
__do_sys_io_uring_enter+0xa98/0x1590
Allocated by task 103:
The buggy address is located 12 bytes to the right of
allocated 584-byte region [ffff888002bcc000, ffff888002bcc248)

Fixes: 8622b20f23ed ("io_uring: add validate_fixed_range() for validate fixed buffer")
Signed-off-by: Qi Tang <tpluszz77@gmail.com>
Link: https://patch.msgid.link/20260329164936.240871-1-tpluszz77@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
io_uring/rsrc.c | 4 ++++
1 file changed, 4 insertions(+)

diff --git a/io_uring/rsrc.c b/io_uring/rsrc.c
index 41c89f5c616da..81446f9649ae9 100644
--- a/io_uring/rsrc.c
+++ b/io_uring/rsrc.c
@@ -1084,6 +1084,10 @@ static int io_import_fixed(int ddir, struct iov_iter *iter,
return ret;
if (!(imu->dir & (1 << ddir)))
return -EFAULT;
+ if (unlikely(!len)) {
+ iov_iter_bvec(iter, ddir, NULL, 0, 0);
+ return 0;
+ }

offset = buf_addr - imu->ubuf;

--
2.53.0