[PATCH net v3] net/rds: ib: zero the unwritten tail of a short receive fragment

rds_ib_process_recv() accepts an RDS/IB fragment once the receive
completion reports at least an RDS header, and then trusts the
header-declared message length h_len: rds_ib_inc_copy_to_user() later
copies up to h_len bytes from the fragment pages to userspace on
recvmsg().

When a fragment carries fewer payload bytes than h_len accounts for, the
bytes between the end of the received payload and h_len are read from the
unwritten tail of the fragment page.  That page comes from the per-CPU
receive cache and is not zeroed, so recvmsg() returns stale page contents
to userspace.

Zero the unwritten tail of a short fragment page on receipt, so the copy
can only ever return received payload or zeros.  Fragments that fill the
page are unaffected.

Confirmed under KMSAN: without this change recvmsg() returns stale
frag-page bytes and KMSAN flags an uninitialised copy in
rds_ib_inc_copy_to_user(); with it the same over-read returns zeros and
KMSAN is clean.

Fixes: 1e23b3ee0e94 ("RDS/IB: Receive datagrams via IB")
Reported-by: sungbyeongchan <redacted>
Closes: https://lore.kernel.org/netdev/20261006205204.1322102-1-tjdqudcks0424@naver.com/ (local)
Reported-by: Shubham Antil <redacted>
Reported-by: Giovanni Vignone <redacted>
Reported-by: Robert van Eijk <redacted>
Reported-by: Paolo Gentry <redacted>
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Shubham Antil <redacted>
---
v3:
 - Zero the unwritten fragment tail instead of dropping the connection.
   A zero-copy send with a small iovec segment can legitimately produce a
   non-final fragment shorter than RDS_FRAG_SIZE, so the v2 length check
   could force a reconnect on well-formed traffic; zeroing the tail avoids
   that while still keeping uninitialised bytes out of recvmsg().
   (Raised by the Sashiko review.)
 - Drop Reviewed-by, since the approach changed.
v2:
 - Add Reported-by (sungbyeongchan, first to report, and the Octane
   Security reporters).
 net/rds/ib_recv.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/net/rds/ib_recv.c b/net/rds/ib_recv.c
index bd6cb3ffa..9695c7f24 100644
--- a/net/rds/ib_recv.c
+++ b/net/rds/ib_recv.c

@@ -35,6 +35,7 @@
 #include <linux/slab.h>
 #include <linux/pci.h>
 #include <linux/dma-mapping.h>
+#include <linux/highmem.h>
 #include <rdma/rdma_cm.h>
 
 #include "rds_single_path.h"

@@ -949,6 +950,19 @@ static void rds_ib_process_recv(struct rds_connection *conn,
 		}
 	}
 
+	/* The device wrote only data_len bytes into this recycled fragment
+	 * page.  Zero the unwritten tail so a later rds_ib_inc_copy_to_user(),
+	 * which copies up to the peer-declared h_len, cannot hand stale page
+	 * contents to userspace.
+	 */
+	if (data_len < RDS_FRAG_SIZE) {
+		void *frag = kmap_local_page(sg_page(&recv->r_frag->f_sg));
+
+		memset(frag + recv->r_frag->f_sg.offset + data_len, 0,
+		       RDS_FRAG_SIZE - data_len);
+		kunmap_local(frag);
+	}
+
 	list_add_tail(&recv->r_frag->f_item, &ibinc->ii_frags);
 	recv->r_frag = NULL;
 

-- 
2.43.0