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SubjectRe: [PATCH] net/9p: avoid -ERESTARTSYS leak to userspace
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On 2018/3/10 4:41, Greg Kurz 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, clears TIF_SIGPENDING, handles
> the request and calls 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 fixes 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.
>

Looks good.

Reviewed-by: Yiwen Jiang <jiangyiwen@huawei.com>

> Signed-off-by: Greg Kurz <groug@kaod.org>
> ---
> 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 b433aff5ff13..e6cae8332e2e 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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