Re: [PATCH] vhost/vsock: improve RCU read sections around vhost_vsock_get()

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Wed, Nov 26, 2025 at 02:38:26PM +0100, Stefano Garzarella wrote:
> From: Stefano Garzarella <sgarzare@redhat.com>
> 
> vhost_vsock_get() uses hash_for_each_possible_rcu() to find the
> `vhost_vsock` associated with the `guest_cid`. hash_for_each_possible_rcu()
> should only be called within an RCU read section, as mentioned in the
> following comment in include/linux/rculist.h:
> 
> /**
>  * hlist_for_each_entry_rcu - iterate over rcu list of given type
>  * @pos:	the type * to use as a loop cursor.
>  * @head:	the head for your list.
>  * @member:	the name of the hlist_node within the struct.
>  * @cond:	optional lockdep expression if called from non-RCU protection.
>  *
>  * This list-traversal primitive may safely run concurrently with
>  * the _rcu list-mutation primitives such as hlist_add_head_rcu()
>  * as long as the traversal is guarded by rcu_read_lock().
>  */
> 
> Currently, all calls to vhost_vsock_get() are between rcu_read_lock()
> and rcu_read_unlock() except for calls in vhost_vsock_set_cid() and
> vhost_vsock_reset_orphans(). In both cases, the current code is safe,
> but we can make improvements to make it more robust.
> 
> About vhost_vsock_set_cid(), when building the kernel with
> CONFIG_PROVE_RCU_LIST enabled, we get the following RCU warning when the
> user space issues `ioctl(dev, VHOST_VSOCK_SET_GUEST_CID, ...)` :
> 
>   WARNING: suspicious RCU usage
>   6.18.0-rc7 #62 Not tainted
>   -----------------------------
>   drivers/vhost/vsock.c:74 RCU-list traversed in non-reader section!!
> 
>   other info that might help us debug this:
> 
>   rcu_scheduler_active = 2, debug_locks = 1
>   1 lock held by rpc-libvirtd/3443:
>    #0: ffffffffc05032a8 (vhost_vsock_mutex){+.+.}-{4:4}, at: vhost_vsock_dev_ioctl+0x2ff/0x530 [vhost_vsock]
> 
>   stack backtrace:
>   CPU: 2 UID: 0 PID: 3443 Comm: rpc-libvirtd Not tainted 6.18.0-rc7 #62 PREEMPT(none)
>   Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.17.0-7.fc42 06/10/2025
>   Call Trace:
>    <TASK>
>    dump_stack_lvl+0x75/0xb0
>    dump_stack+0x14/0x1a
>    lockdep_rcu_suspicious.cold+0x4e/0x97
>    vhost_vsock_get+0x8f/0xa0 [vhost_vsock]
>    vhost_vsock_dev_ioctl+0x307/0x530 [vhost_vsock]
>    __x64_sys_ioctl+0x4f2/0xa00
>    x64_sys_call+0xed0/0x1da0
>    do_syscall_64+0x73/0xfa0
>    entry_SYSCALL_64_after_hwframe+0x76/0x7e
>    ...
>    </TASK>
> 
> This is not a real problem, because the vhost_vsock_get() caller, i.e.
> vhost_vsock_set_cid(), holds the `vhost_vsock_mutex` used by the hash
> table writers. Anyway, to prevent that warning, add lockdep_is_held()
> condition to hash_for_each_possible_rcu() to verify that either the
> caller is in an RCU read section or `vhost_vsock_mutex` is held when
> CONFIG_PROVE_RCU_LIST is enabled; and also clarify the comment for
> vhost_vsock_get() to better describe the locking requirements and the
> scope of the returned pointer validity.
> 
> About vhost_vsock_reset_orphans(), currently this function is only
> called via vsock_for_each_connected_socket(), which holds the
> `vsock_table_lock` spinlock (which is also an RCU read-side critical
> section). However, add an explicit RCU read lock there to make the code
> more robust and explicit about the RCU requirements, and to prevent
> issues if the calling context changes in the future or if
> vhost_vsock_reset_orphans() is called from other contexts.
> 
> Fixes: 834e772c8db0 ("vhost/vsock: fix use-after-free in network stack callers")
> Cc: stefanha@redhat.com
> Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
> ---
>  drivers/vhost/vsock.c | 15 +++++++++++----
>  1 file changed, 11 insertions(+), 4 deletions(-)

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

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