[PATCH] ublk: hold RCU read lock in ublk_try_buf_match()

[Paolo Bertinetti]

syzbot reports a lockdep splat when I/O is dispatched to a ublk device
with UBLK_F_SHMEM_ZC enabled:

  WARNING: suspicious RCU usage
  lib/maple_tree.c:720 suspicious rcu_dereference_check() usage!
  ...
  Call Trace:
   mas_root lib/maple_tree.c:720 [inline]
   mas_start+0x3cc/0x640 lib/maple_tree.c:1250
   mas_state_walk lib/maple_tree.c:3006 [inline]
   mas_walk+0x1db/0x490 lib/maple_tree.c:4458
   ublk_try_buf_match drivers/block/ublk_drv.c:5639 [inline]
   ublk_init_iod+0x5d2/0x1120 drivers/block/ublk_drv.c:521
   ublk_dispatch_req+0x97/0x9c0 drivers/block/ublk_drv.c:1776
   tctx_task_work_run+0x73d/0xa80 io_uring/tw.c:96

ublk_try_buf_match() calls mas_walk() on the buffer tree without
holding the RCU read lock or the tree lock. Writers are already
excluded because ublk_lock_buf_tree() freezes the queue, so there is
no real race, but maple tree readers must still hold one of the two.

Take the RCU read lock around the walk.

Fixes: 4d4a512a1f87 ("ublk: add PFN-based buffer matching in I/O path")
Reported-by: syzbot+b48868c46e62c1c4cb2c@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=b48868c46e62c1c4cb2c
Assisted-by: LLM
Signed-off-by: Paolo Bertinetti <paolobertinetti200f4@xxxxxxxxx>
---
syzbot tested this patch [1]: the RCU splat is gone, but the reproducer
then hits the already open "WARNING in io_wq_put_and_exit (2)" [2],
which syzbot has been reporting since May on kernels without this change.

[1] https://lore.kernel.org/all/6ab1eabe.71f81b7d.15fa6d.003b.GAE@xxxxxxxxxx/
[2] https://syzkaller.appspot.com/bug?extid=b0d54b9e81de55179e47

 drivers/block/ublk_drv.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c
index 66eb55e71..7a64745e1 100644
--- a/drivers/block/ublk_drv.c
+++ b/drivers/block/ublk_drv.c
@@ -5628,6 +5628,8 @@ static bool ublk_try_buf_match(struct ublk_device *ub,
 	unsigned long expected_offset = 0;
 	bool first = true;
 
+	guard(rcu)();
+
 	rq_for_each_bvec(bv, rq, iter) {
 		unsigned long pfn = page_to_pfn(bv.bv_page);
 		unsigned long end_pfn = pfn +

base-commit: 93f51579e7df248780214094418f205253383cc5
-- 
2.55.0