From a81f8ce78353cd921c446e84adb8519d65e222d4 Mon Sep 17 00:00:00 2001
From: Pavel Begunkov <asml.silence@gmail.com>
Date: Fri, 2 Dec 2022 14:27:13 +0000
Subject: [PATCH 3/5] io_uring: cmpxchg for poll arm refs release
Patch-mainline: v6.1-rc7
Git-commit: 2f3893437a4ebf2e892ca172e9e122841319d675
References: bsc#1207511 CVE-2023-0468
[ upstream commit 2f3893437a4ebf2e892ca172e9e122841319d675 ]
Replace atomically substracting the ownership reference at the end of
arming a poll with a cmpxchg. We try to release ownership by setting 0
assuming that poll_refs didn't change while we were arming. If it did
change, we keep the ownership and use it to queue a tw, which is fully
capable to process all events and (even tolerates spurious wake ups).
It's a bit more elegant as we reduce races b/w setting the cancellation
flag and getting refs with this release, and with that we don't have to
worry about any kinds of underflows. It's not the fastest path for
polling. The performance difference b/w cmpxchg and atomic dec is
usually negligible and it's not the fastest path.
Cc: stable@vger.kernel.org
Fixes: aa43477b04025 ("io_uring: poll rework")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/0c95251624397ea6def568ff040cad2d7926fd51.1668963050.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: David Disseldorp <ddiss@suse.de>
---
fs/io_uring.c | 8 +++-----
1 file changed, 3 insertions(+), 5 deletions(-)
diff --git a/fs/io_uring.c b/fs/io_uring.c
index 5518a3132232b..78402694570d8 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -6763,7 +6763,6 @@ static int __io_arm_poll_handler(struct io_kiocb *req,
struct io_poll_table *ipt, __poll_t mask)
{
struct io_ring_ctx *ctx = req->ctx;
- int v;
INIT_HLIST_NODE(&req->hash_node);
req->work.cancel_seq = atomic_read(&ctx->cancel_seq);
@@ -6812,11 +6811,10 @@ static int __io_arm_poll_handler(struct io_kiocb *req,
}
/*
- * Release ownership. If someone tried to queue a tw while it was
- * locked, kick it off for them.
+ * Try to release ownership. If we see a change of state, e.g.
+ * poll was waken up, queue up a tw, it'll deal with it.
*/
- v = atomic_dec_return(&req->poll_refs);
- if (unlikely(v & IO_POLL_REF_MASK))
+ if (atomic_cmpxchg(&req->poll_refs, 1, 0) != 1)
__io_poll_execute(req, 0, poll->events);
return 0;
}
--
2.35.3