[PATCH 7.3 17/38] io_uring/waitid: prevent cancel reference wrap

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

7.3-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Junyeong Lee <jylee5498@gmail.com>

commit 98787d3fa4e99564d2b2bc21255b21f670c450d0 upstream.

io_waitid packs the cancellation state into bit 31 of ->refs and uses
bits 30:0 as a 31-bit reference counter:

  #define IO_WAITID_CANCEL_FLAG BIT(31)
  #define IO_WAITID_REF_MASK    GENMASK(30, 0)

__io_waitid_cancel() marks a request canceled, then increments the low
bits of ->refs to claim completion ownership. When another path owns the
request, the increment remains even though cancellation returns false.
The request remains discoverable by cancellation, so repeated failed
attempts can wrap the 31-bit reference field to zero. A later
cancellation can claim ownership and complete the request a second time.

Use atomic_fetch_or() when setting the cancel flag and inspect the old
flag value. The first cancel attempt continues to the existing ownership
increment, preserving its wake and issuer handoff semantics. Later
attempts see the cancel flag and return without incrementing the
reference field.

This makes repeated failed cancellation attempts idempotent while
leaving the first-cancel state transitions unchanged.

Fixes: f31ecf671ddc ("io_uring: add IORING_OP_WAITID support")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Junyeong Lee <jylee5498@gmail.com>
Link: https://patch.msgid.link/20261019152100.63565-1-jylee5498@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 io_uring/waitid.c |    7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

--- a/io_uring/waitid.c
+++ b/io_uring/waitid.c
@@ -154,9 +154,12 @@ static bool __io_waitid_cancel(struct io
 
 	/*
 	 * Mark us canceled regardless of ownership. This will prevent a
-	 * potential retry from a spurious wakeup.
+	 * potential retry from a spurious wakeup. Only the first cancel
+	 * attempt should take a reference for the ownership handoff.
 	 */
-	atomic_or(IO_WAITID_CANCEL_FLAG, &iw->refs);
+	if (atomic_fetch_or(IO_WAITID_CANCEL_FLAG, &iw->refs) &
+	    IO_WAITID_CANCEL_FLAG)
+		return false;
 
 	/* claim ownership */
 	if (atomic_fetch_inc(&iw->refs) & IO_WAITID_REF_MASK)