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X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: =?us-ascii?Q?zNZR01vbqMJX9M3oFQRoe8GitQ8B+GBXijjos8CJKuIwQ5kihn4ZXQw+NZPu?= =?us-ascii?Q?JZRCAa/0CDAOmPQHMIvve/MOgPDmwrm5KsDYArO+fZMN8HjGb/n95le9K7Jx?= =?us-ascii?Q?jwguFBH2hwrpho05dcLeHbl+Iu5F/EoX9rNHoLD0gn2LHRexm0tB04iMcv7w?= =?us-ascii?Q?fh35xs6w2SLuSZXIF15cuKY9lxjkMtE8wKXyKZVfRSunqd0GciJ8D2bYXJA+?= =?us-ascii?Q?BY8WL5+CDNvRtGXrWg3D2WecP5YAdXPuDw0qephv+qKoKM6y8hFleP8KP888?= =?us-ascii?Q?xCr3czxK4M8kCxNOGDb4UdFu16OgaeixQ5O7KLwFlfbEQKAFyUaT7Iq1jXc7?= =?us-ascii?Q?Rw8ZZSpf//2/fF0Zqd0wTImxtjyyABZtVlWpcY0YYyxB0XzPFrjt87NZLaE1?= =?us-ascii?Q?DfA9Sms9XkX4GknXYG9+3NlSriHmsupKfGdlLkkDaFUaWz2Kv8o2aPSSfw4c?= =?us-ascii?Q?5Fbvkhz9BXkE47joYF9K/cBYFV22pmOf1qM3RX1Fd77yFDoHcDrMV+fq0Y5y?= =?us-ascii?Q?f24iu3Qs6u4d0gZt6VTA9/5VZcdx88UfP37iWNnWIjmTmjIXyXLxeXl6BNOj?= =?us-ascii?Q?zNURXCwAhpBI6O6iyj8lojIVEAq6BUS3g6rWjnxrQUMUWRav+w7zddQvtUcp?= =?us-ascii?Q?0eM0YKTifut5TchtZflMAjE6a57MdqbI/zpt2Kc2NeDl0S5HfvBdwRvCn9n9?= =?us-ascii?Q?Spd3WL/HZ7tG377nIJkxSg3Rt7IK4BaJYkcmoTFbSHVbChnWrQg1ScvQ3gQJ?= =?us-ascii?Q?IcNPBjz2QESNh0iOL3EvAuhyk2fBcSJOvCFCIEskxc4pxc7jGMJG8+mylkdE?= =?us-ascii?Q?lA7m2Zj5iaoF6xg050ZryaQH9V1nrkHCYvyrK8h9d2pXBtYf336QkK0ouu6n?= =?us-ascii?Q?to1MOFb0oIthd1rtmwNqt1UDg2iC5Lu3om4dzkAZQnr/CqDvpQ6CfmjyIheT?= =?us-ascii?Q?eT9URFpm81aQLdl1a9wz1/cZsO7I7dUHv+VEgpDxAY2z0oy9YJCEgF/7B5gB?= =?us-ascii?Q?t0PAYRUUNVne7oWDgqY+Ia2YZL1UbqTj7kjbO+cD07TD2FrJUUUb8sG5hQD+?= =?us-ascii?Q?VrqncQzSkVeG3pu176uYk0OATXv0ec5rgWozGFhnIP55zPPOsqnd5mg8WQRB?= =?us-ascii?Q?8+jFx4QGs5VEM+b/dO7a/DbhzFTD7rU4xzPCat5Nw3mHn568xERJATAYoetM?= =?us-ascii?Q?pY6YFLPehdWpUtCeZ0/qt3sOHp2JaKSpvYfdfS2gXll0CTNxnGpRyOS0BLpK?= =?us-ascii?Q?PKfGdy56Y9D8dMqfzjmd4Luhr3RmMOClCiyfnZKNn67+LYJXRiZzzwo1FBzi?= =?us-ascii?Q?wrRAe2MBDLhab6QPzEwECQCuxHMSfhBynm0M4CNCx0kr4bBT2HVlysapQo4Q?= =?us-ascii?Q?gIGU4hr3Eo8bDing++KW2OeObmCV50lWYjznbtKtlN6SNjNzf95vX17HteXu?= =?us-ascii?Q?fFYCIzIqHlTZ6TBhKeK8qQ0qxXuvEbOoHywTk6tdMb27y1wr13BVJj2RfzIL?= =?us-ascii?Q?dIATBAZIawx6J+9CHJfnWBwOI7SC3kOm7ICVJ6/puWFaEhjOo2yEBf9ibPrf?= =?us-ascii?Q?289wTmly1/Ib+VTEnLTTVgNAIx177yti4zzQ41DuxH15W2fZwp1A2TtXKSAl?= =?us-ascii?Q?c01BqhHauGfHgKXJyxQp+rGz0TTYSu6dQOENfEraGZZmiPUTHrTfKNuVepbJ?= =?us-ascii?Q?5yV4ULCkc8oFkpqQLyflTrwsRh5ED5naElVQQr1FBUD9SivT?= X-OriginatorOrg: Nvidia.com X-MS-Exchange-CrossTenant-Network-Message-Id: 30c7fdbf-b5ce-45c1-61a0-08df1e47cd8f X-MS-Exchange-CrossTenant-AuthSource: PH0PR12MB7957.namprd12.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 29 Sep 2026 16:36:25.1930 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: 43083d15-7273-40c1-b7db-39efd9ccc17a X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: L3kqvOrRWbBdahPHDlRc6dGAJ/bTvcmRyrykT+Fy6VMrUDgiwgTvOQmIvZ7X9HJqkOR1MNpJjJjBmB5WA6SFIQ== X-MS-Exchange-Transport-CrossTenantHeadersStamped: PH7PR12MB8177 On Tue, Sep 29, 2026 at 12:25:34AM +0800, Chengfeng Ye wrote: > ip_mc_output() reads rt->dst.dev and stores it in skb->dev without RCU > protection. dst_dev_put() can concurrently replace the destination device It can also be rt_flush_dev(), if the dst entry is uncached. > and release the old device after an RCU grace period, leaving the multicast The reference release is immediate, only the free happens after a GP. > output path with a stale skb->dev. > > The stale device can be dereferenced by the post-routing path. In > particular, a socket using IP_PMTUDISC_PROBE reaches ip_skb_dst_mtu() from > ip_finish_output() and reads the freed device's MTU. ip_skb_dst_mtu() reads the device from the dst entry, not from skb->dev. > > KASAN reported: > > BUG: KASAN: slab-use-after-free in ip_skb_dst_mtu+0x634/0x740 > Read of size 4 at addr ffff88810921c038 by task poc/98 > Call Trace: > ip_skb_dst_mtu+0x634/0x740 > __ip_finish_output.part.0+0x22/0x2c0 > ip_mc_output+0x287/0x930 > ip_send_skb+0x11d/0x150 > udp_send_skb+0x63e/0xdf0 > udp_sendmsg+0x1235/0x1da0 > __sys_sendto+0x32c/0x3a0 > > Freed by task 99: > kfree+0x131/0x3c0 > device_release+0xc8/0x240 > kobject_put+0x14d/0x280 > netdev_run_todo+0x4cb/0xc70 > rtnl_dellink+0x362/0xa90 > > Protect the whole multicast output path with an RCU read-side critical > section, as ip_output() already does. Load the destination device through > skb_dst_dev_rcu() and keep it protected through the post-routing hooks and > ip_finish_output(). > > Fixes: 4a6ce2b6f2ec ("net: introduce a new function dst_dev_put()") > Cc: stable@vger.kernel.org > Signed-off-by: Chengfeng Ye > --- > Changes in v2: > - Protect the whole ip_mc_output() path instead of only the MTU read. > - Load the destination device with skb_dst_dev_rcu(), matching ip_output(). > - Use the commit that made dst device replacement RCU-protected as Fixes. > > Link: https://lore.kernel.org/r/20260927071051.3693368-1-nicoyip.dev@gmail.com/ [v1] > > net/ipv4/ip_output.c | 19 +++++++++++++------ > 1 file changed, 13 insertions(+), 6 deletions(-) The diff itself looks fine. FTR, I'm aware of the discussion that wants an RCU critical section in ip_local_out(), but: 1. Adding one in ip_mc_output() is consistent with other output functions: ip_output(), ip6_output(), ip_mr_output(), ip6_mr_output(), etc. 2. This patch doesn't affect the fast path, unlike an RCU critical section in ip_local_out() [1]. 3. Patching ip_local_out() doesn't cover all the process context callers of dst_output() either. And yes, I'm aware that more places suffer from this "problem". I opened an internal ticket after seeing [2]. I will get to it eventually and will target it at net-next since these UAFs are not very realistic as evident from [2]. [1] https://lore.kernel.org/netdev/85c6016d-fb15-4c61-aab9-61448911f934@redhat.com/ [2] https://lore.kernel.org/netdev/20260819164528.3761320-1-jjy600901@snu.ac.kr/