Subject: Re: [V9fs-developer] [PATCH] net/9p: avoid -ERESTARTSYS leak to userspace

From: Greg Kurz <[email-masked]>
Date: Tue, 20 Feb 2018 17:48:16 +0100

Hi Al,

It's been two years without any sign of life from 9p maintainers... :-\

Would you apply (or nack) this patch ?

Thanks,

--
Greg

PS: in the case you apply it, probable Cc [email-masked] as well

On Thu, 08 Feb 2018 18:38:49 +0100

Greg Kurz <[email-masked]> wrote:

> If it was interrupted by a signal, the 9p client may need to send some
> more requests to the server for cleanup before returning to userspace.
> 
> To avoid such a last minute request to be interrupted right away, the
> client memorizes if a signal is pending, clear TIF_SIGPENDING, handle
> the request and call recalc_sigpending() before returning.
> 
> Unfortunately, if the transmission of this cleanup request fails for any
> reason, the transport returns an error and the client propagates it right
> away, without calling recalc_sigpending().
> 
> This ends up with -ERESTARTSYS from the initially interrupted request
> crawling up to syscall exit, with TIF_SIGPENDING cleared by the cleanup
> request. The specific signal handling code, which is responsible for
> converting -ERESTARTSYS to -EINTR is not called, and userspace receives
> the confusing errno value:
> 
> open: Unknown error 512 (512)
> 
> This is really hard to hit in real life. I discovered the issue while
> working on hot-unplug of a virtio-9p-pci device with an instrumented
> QEMU allowing to control request completion.
> 
> Both p9_client_zc_rpc() and p9_client_rpc() functions have this buggy
> error path actually. Their code flow is a bit obscure and the best
> thing to do would probably be a full rewrite: to really ensure this
> situation of clearing TIF_SIGPENDING and returning -ERESTARTSYS can
> never happen.
> 
> But given the general lack of interest for the 9p code, I won't risk
> breaking more things. So this patch simply fix the buggy paths in both
> functions with a trivial label+goto.
> 
> Thanks to Laurent Dufour for his help and suggestions on how to find
> the root cause and how to fix it.
> 
> Signed-off-by: Greg Kurz <[email-masked]>
> ---
>  net/9p/client.c |    6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/net/9p/client.c b/net/9p/client.c
> index 4c8cf9c1631a..5154eaf19fff 100644
> --- a/net/9p/client.c
> +++ b/net/9p/client.c
> @@ -769,7 +769,7 @@ p9_client_rpc(struct p9_client *c, int8_t type, const char *fmt, ...)
>  	if (err < 0) {
>  		if (err != -ERESTARTSYS && err != -EFAULT)
>  			c->status = Disconnected;
> -		goto reterr;
> +		goto recalc_sigpending;
>  	}
>  again:
>  	/* Wait for the response */
> @@ -804,6 +804,7 @@ p9_client_rpc(struct p9_client *c, int8_t type, const char *fmt, ...)
>  		if (req->status == REQ_STATUS_RCVD)
>  			err = 0;
>  	}
> +recalc_sigpending:
>  	if (sigpending) {
>  		spin_lock_irqsave(&current->sighand->siglock, flags);
>  		recalc_sigpending();
> @@ -867,7 +868,7 @@ static struct p9_req_t *p9_client_zc_rpc(struct p9_client *c, int8_t type,
>  		if (err == -EIO)
>  			c->status = Disconnected;
>  		if (err != -ERESTARTSYS)
> -			goto reterr;
> +			goto recalc_sigpending;
>  	}
>  	if (req->status == REQ_STATUS_ERROR) {
>  		p9_debug(P9_DEBUG_ERROR, "req_status error %d\n", req->t_err);
> @@ -885,6 +886,7 @@ static struct p9_req_t *p9_client_zc_rpc(struct p9_client *c, int8_t type,
>  		if (req->status == REQ_STATUS_RCVD)
>  			err = 0;
>  	}
> +recalc_sigpending:
>  	if (sigpending) {
>  		spin_lock_irqsave(&current->sighand->siglock, flags);
>  		recalc_sigpending();
> 
> 
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