[PATCH 1/2] 9p/xen: check logical size for buffer size

[Dominique Martinet <asmadeus@codewreck.org>]

trans_xen did not check the data fits into the buffer before copying
from the xen ring, but we probably should.
Add a check that just skips the request and return an error to
userspace if it did not fit

Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>
---

This comes more or less as a follow up of a fix for trans_fd:
https://lkml.kernel.org/r/20221117091159.31533-1-guozihua@huawei.com
Where msize should be replaced by capacity check, except trans_xen
did not actually use to check the size fits at all.

While we normally trust the hypervisor (they can probably do whatever
they want with our memory), a bug in the 9p server is always possible so
sanity checks never hurt, especially now buffers got drastically smaller
with a recent patch.

My setup for xen is unfortunately long dead so I cannot test this:
Stefano, you've tested v9fs xen patches in the past, would you mind
verifying this works as well?

 net/9p/trans_xen.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/net/9p/trans_xen.c b/net/9p/trans_xen.c
index b15c64128c3e..66ceb3b3ae30 100644
--- a/net/9p/trans_xen.c
+++ b/net/9p/trans_xen.c
@@ -208,6 +208,14 @@ static void p9_xen_response(struct work_struct *work)
 			continue;
 		}
 
+		if (h.size > req->rc.capacity) {
+			dev_warn(&priv->dev->dev,
+				 "requested packet size too big: %d for tag %d with capacity %zd\n",
+		                 h.size, h.tag, rreq->rc.capacity);
+			req->status = REQ_STATUS_ERROR;
+			goto recv_error;
+		}
+
 		memcpy(&req->rc, &h, sizeof(h));
 		req->rc.offset = 0;
 
@@ -217,6 +225,7 @@ static void p9_xen_response(struct work_struct *work)
 				     masked_prod, &masked_cons,
 				     XEN_9PFS_RING_SIZE(ring));
 
+recv_error:
 		virt_mb();
 		cons += h.size;
 		ring->intf->in_cons = cons;
-- 
2.38.1