[PATCH] net/9p: avoid -ERESTARTSYS leak to userspace

[Greg Kurz]

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 <gr...@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 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();