Re: [PATCH net v2] appletalk: fix TOCTOU race in atalk_sendmsg

[Simon Horman <horms@kernel.org>]

On Mon, Jun 15, 2026 at 05:06:33PM +0800, Yizhou Zhao wrote:
> atalk_sendmsg() looks up an AppleTalk route, stores the returned
> atalk_route and net_device pointers, and then drops the socket lock
> around sock_alloc_send_skb().  The route pointer returned by
> atrtr_find() is only protected while atalk_routes_lock is held; after
> that lock is dropped, a concurrent SIOCDELRT or device-down path can
> unlink the route, drop the device reference, and free the route.
> 
> When sendmsg resumes, it can still dereference the stale route and
> device pointers while building or transmitting the packet.  A KASAN
> reproducer using AF_APPLETALK sockets and SIOCADDRT/SIOCDELRT reports
> slab-use-after-free reads in atalk_sendmsg(), with the object allocated
> by atrtr_create() and freed by atrtr_delete().
> 
> Fix this by splitting the route lookup into a helper that is called with
> atalk_routes_lock already held.  atalk_sendmsg() now performs route
> lookup, copies the route fields it needs, and takes references to the
> selected devices with netdev_hold() while still holding
> atalk_routes_lock.  After the lock is dropped and skb allocation sleeps,
> the send path uses only the copied route data and the held net_device
> references, which are released with netdev_put() before returning.
> 
> This preserves the existing route selection behaviour, including the
> separate loopback route used for broadcast loopback, while removing the
> dangling route/device window.
> 
> Fixes: 60d9f461a20b ("appletalk: remove the BKL")
> Cc: stable@vger.kernel.org
> Reported-by: Yizhou Zhao <zhaoyz24@mails.tsinghua.edu.cn>
> Reported-by: Yuxiang Yang <yangyx22@mails.tsinghua.edu.cn>
> Reported-by: Ao Wang <wangao@seu.edu.cn>
> Reported-by: Xuewei Feng <fengxw06@126.com>
> Reported-by: Qi Li <qli01@tsinghua.edu.cn>
> Reported-by: Ke Xu <xuke@tsinghua.edu.cn>
> Assisted-by: GLM:GLM-5.1
> Signed-off-by: Yizhou Zhao <zhaoyz24@mails.tsinghua.edu.cn>
> ---
> Changes in v2:
> - Use netdev_hold()/netdev_put() instead of dev_hold()/dev_put().
> - Drop explicit NULL checks before releasing temporary device refs.
> - Link to v1: https://lore.kernel.org/netdev/20260610052315.64504-1-zhaoyz24@mails.tsinghua.edu.cn/

Reviewed-by: Simon Horman <horms@kernel.org>