RE: [PATCH net v2] vsock: fix potential null pointer dereference in vsock_poll()

[Jorgen Hansen]

> From: Stefano Garzarella <sgarzare@xxxxxxxxxx>
> Sent: Wednesday, August 12, 2020 2:56 PM
> To: davem@xxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx; Dexuan Cui <decui@xxxxxxxxxxxxx>;
> netdev@xxxxxxxxxxxxxxx; Stefan Hajnoczi <stefanha@xxxxxxxxxx>; Jakub
> Kicinski <kuba@xxxxxxxxxx>; Jorgen Hansen <jhansen@xxxxxxxxxx>;
> Stefano Garzarella <sgarzare@xxxxxxxxxx>
> Subject: [PATCH net v2] vsock: fix potential null pointer dereference in
> vsock_poll()
> 
> syzbot reported this issue where in the vsock_poll() we find the
> socket state at TCP_ESTABLISHED, but 'transport' is null:
>   general protection fault, probably for non-canonical address
> 0xdffffc0000000012: 0000 [#1] PREEMPT SMP KASAN
>   KASAN: null-ptr-deref in range [0x0000000000000090-0x0000000000000097]
>   CPU: 0 PID: 8227 Comm: syz-executor.2 Not tainted 5.8.0-rc7-syzkaller #0
>   Hardware name: Google Google Compute Engine/Google Compute Engine,
> BIOS Google 01/01/2011
>   RIP: 0010:vsock_poll+0x75a/0x8e0 net/vmw_vsock/af_vsock.c:1038
>   Call Trace:
>    sock_poll+0x159/0x460 net/socket.c:1266
>    vfs_poll include/linux/poll.h:90 [inline]
>    do_pollfd fs/select.c:869 [inline]
>    do_poll fs/select.c:917 [inline]
>    do_sys_poll+0x607/0xd40 fs/select.c:1011
>    __do_sys_poll fs/select.c:1069 [inline]
>    __se_sys_poll fs/select.c:1057 [inline]
>    __x64_sys_poll+0x18c/0x440 fs/select.c:1057
>    do_syscall_64+0x60/0xe0 arch/x86/entry/common.c:384
>    entry_SYSCALL_64_after_hwframe+0x44/0xa9
> 
> This issue can happen if the TCP_ESTABLISHED state is set after we read
> the vsk->transport in the vsock_poll().
> 
> We could put barriers to synchronize, but this can only happen during
> connection setup, so we can simply check that 'transport' is valid.
> 
> Fixes: c0cfa2d8a788 ("vsock: add multi-transports support")
> Reported-and-tested-by:
> syzbot+a61bac2fcc1a7c6623fe@xxxxxxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Stefano Garzarella <sgarzare@xxxxxxxxxx>
> ---
> v2:
>  - removed cleanups patch from the series [David]
> 
> v1:
> https://nam04.safelinks.protection.outlook.com/?url=https%3A%2F%2Fpatc
> hwork.ozlabs.org%2Fproject%2Fnetdev%2Fcover%2F20200811095504.25051-
> 1-
> sgarzare%40redhat.com%2F&amp;data=02%7C01%7Cjhansen%40vmware.co
> m%7C32b3919883a448f56a8708d83ebf1dce%7Cb39138ca3cee4b4aa4d6cd83d
> 9dd62f0%7C0%7C0%7C637328337851992525&amp;sdata=CSo8PEJJwyDE75Qz
> n3lmasJFSNaNChiRXjoy%2FfoJ8Vs%3D&amp;reserved=0
> ---
>  net/vmw_vsock/af_vsock.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/vmw_vsock/af_vsock.c b/net/vmw_vsock/af_vsock.c
> index 27bbcfad9c17..9e93bc201cc0 100644
> --- a/net/vmw_vsock/af_vsock.c
> +++ b/net/vmw_vsock/af_vsock.c
> @@ -1032,7 +1032,7 @@ static __poll_t vsock_poll(struct file *file, struct
> socket *sock,
>  		}
> 
>  		/* Connected sockets that can produce data can be written.
> */
> -		if (sk->sk_state == TCP_ESTABLISHED) {
> +		if (transport && sk->sk_state == TCP_ESTABLISHED) {
>  			if (!(sk->sk_shutdown & SEND_SHUTDOWN)) {
>  				bool space_avail_now = false;
>  				int ret = transport->notify_poll_out(
> --
> 2.26.2

Thanks for fixing this!

Reviewed-by: Jorgen Hansen <jhansen@xxxxxxxxxx>