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

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

[Shubham Antil]

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 <tjdqudcks0424@naver.com>
Closes: https://lore.kernel.org/netdev/20261006205204.1322102-1-tjdqudcks0424@naver.com/
Reported-by: Shubham Antil <shubham@octane.security>
Reported-by: Giovanni Vignone <gio@octane.security>
Reported-by: Robert van Eijk <robert@octane.security>
Reported-by: Paolo Gentry <paolo@octane.security>
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Shubham Antil <shubham@octane.security>
---
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

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

[netdev-bot+sinfo]

Hi!

This is an automated message. This series looks like a fix, but its
commit messages seem to be missing some information:

 - How the issue was discovered, e.g. hit in production, hit during
   development, syzbot report, manual code inspection, LLM or static
   analysis tool scan.

Please do not repost the series just to address the above. Instead,
reply to this email with the missing information, so that reviewers
can take it into account. If the series needs another revision for
other reasons, please include the information in the commit messages
then.

The evaluation is done by an LLM so it may be wrong, if you think
that is the case please reply and explain.