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

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Date:   Fri, 09 Mar 2018 21:41:38 +0100
From:   Greg Kurz <groug@...d.org>
To:     linux-kernel@...r.kernel.org
Cc:     netdev@...r.kernel.org, v9fs-developer@...ts.sourceforge.net,
        Eric Van Hensbergen <ericvh@...il.com>,
        Ron Minnich <rminnich@...dia.gov>,
        Latchesar Ionkov <lucho@...kov.net>,
        "David S. Miller" <davem@...emloft.net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Yiwen Jiang <jiangyiwen@...wei.com>
Subject: [PATCH] net/9p: avoid -ERESTARTSYS leak to userspace

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.

Signed-off-by: Greg Kurz <groug@...d.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();