[PATCH 5.10.y v2] SUNRPC: Copy the deferred RPC Call from the head buffer

[Bjoern Doebel <doebel@amazon.de>]

From: Chuck Lever <cel@kernel.org>

commit 2192a56b0686599358909c3e12e9a366f5b5d452 upstream.

svc_defer() snapshots the RPC Call for replay once a pending cache
upcall resolves. It copies the Call starting (rq_arg.len -
head[0].iov_len) bytes before head[0].iov_base, a back-up that once
covered a transport header. No transport has had one since commit
983084b2672c ("SUNRPC: Remove svc_rqst::rq_xprt_hlen"), so the two
lengths are meant to be equal.

An RPCSEC_GSS Call breaks that assumption. Unwrapping the integrity
service truncates rq_arg.len by the length of the trailing checksum,
and unwrapping the privacy service leaves rq_arg.len short by an
amount the client sets with trailing bytes after the wrap token.
Neither adjusts head[0].iov_len. Both lengths are size_t, so the
subtraction underflows and the copy starts past head[0].iov_base.
Every deferred integrity Call replays a shifted snapshot.

A deferred replay skips GSS verification, so a shifted snapshot runs
as an authenticated Call. With the privacy service the offset is
client-chosen. A forged Call placed in that window, naming another
user's GSS context handle, runs under that identity.

Copy the Call from head[0].iov_base. Refuse to defer a Call whose
length runs past the head buffer, so the copy cannot read out of
bounds.

Fixes: 260c1d1298f6 ("svc: Add transport hdr size for defer/revisit")
Cc: stable@vger.kernel.org
Reported-by: Kimi Security Team <bug-report@moonshot.ai>
Reported-by: Yilin Zhang <yilinzhang@moonshot.ai>
Link: https://patch.msgid.link/20260902133101.48914-1-cel@kernel.org
Signed-off-by: Chuck Lever <cel@kernel.org>
[doebel: 5.10 lacks 983084b2672c; keep xprt_hlen and its comments in
 svc_deferred_recv(). The removed skip is zero for all 5.10 transports.]
Signed-off-by: Bjoern Doebel <doebel@amazon.de>
---
v2:
- Add the "commit <sha1> upstream." line.
- Document why the backport differs from upstream.

 net/sunrpc/svc_xprt.c | 18 +++++++-----------
 1 file changed, 7 insertions(+), 11 deletions(-)

diff --git a/net/sunrpc/svc_xprt.c b/net/sunrpc/svc_xprt.c
index 60782504ad3e7..a8bd4f7f9299d 100644
--- a/net/sunrpc/svc_xprt.c
+++ b/net/sunrpc/svc_xprt.c
@@ -1181,13 +1181,8 @@ static void svc_revisit(struct cache_deferred_req *dreq, int too_many)
 }
 
 /*
- * Save the request off for later processing. The request buffer looks
- * like this:
- *
- * <xprt-header><rpc-header><rpc-pagelist><rpc-tail>
- *
- * This code can only handle requests that consist of an xprt-header
- * and rpc-header.
+ * Save the request off for later processing. Only a Call that fits
+ * entirely in rq_arg.head[0] can be deferred.
  */
 static struct cache_deferred_req *svc_defer(struct cache_req *req)
 {
@@ -1200,8 +1195,11 @@ static struct cache_deferred_req *svc_defer(struct cache_req *req)
 		dr = rqstp->rq_deferred;
 		rqstp->rq_deferred = NULL;
 	} else {
-		size_t skip;
 		size_t size;
+
+		if (rqstp->rq_arg.len > rqstp->rq_arg.head[0].iov_len)
+			return NULL;
+
 		/* FIXME maybe discard if size too large */
 		size = sizeof(struct svc_deferred_req) + rqstp->rq_arg.len;
 		dr = kmalloc(size, GFP_KERNEL);
@@ -1216,9 +1214,7 @@ static struct cache_deferred_req *svc_defer(struct cache_req *req)
 		dr->argslen = rqstp->rq_arg.len >> 2;
 		dr->xprt_hlen = rqstp->rq_xprt_hlen;
 
-		/* back up head to the start of the buffer and copy */
-		skip = rqstp->rq_arg.len - rqstp->rq_arg.head[0].iov_len;
-		memcpy(dr->args, rqstp->rq_arg.head[0].iov_base - skip,
+		memcpy(dr->args, rqstp->rq_arg.head[0].iov_base,
 		       dr->argslen << 2);
 	}
 	trace_svc_defer(rqstp);
-- 
2.50.1