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@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=b48868c46e62c1c4cb2c Assisted-by: LLM Signed-off-by: Paolo Bertinetti --- 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@google.com/ [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