[PATCH net] net/rds: fix tcp stream corruption with large pages

rds_message_map_pages() assigns PAGE_SIZE bytes to every
scatterlist entry, even when total_len ends in a partial page. The RDS
congestion map is defined as 8192 bytes, so on systems with PAGE_SIZE
greater than 8192 the scatterlist maps bytes beyond the end of the
congestion map.  RDS-TCP transmits the SG contents according to those
lengths, so the extra bytes become part of the TCP RDS stream and are
interpreted as subsequent RDS message headers, corrupting the stream.

Limit the final scatterlist mapping to the number of bytes remaining.
This has no effect on systems with a 4K page size and allows RDS-TCP to
be used on systems with 16K and larger page sizes.  

The RDS selftest, which previously hung on 16K pages, now passes.

Fixes: 7875e18e0996 ("RDS: Message parsing")
Signed-off-by: Greg Marsden <redacted>
---
 net/rds/message.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/rds/message.c b/net/rds/message.c
index f25f2592586f..47d5e9ab9b10 100644
--- a/net/rds/message.c
+++ b/net/rds/message.c

@@ -431,7 +431,9 @@ struct rds_message *rds_message_map_pages(unsigned long *page_addrs, unsigned in
 	for (i = 0; i < rm->data.op_nents; ++i) {
 		sg_set_page(&rm->data.op_sg[i],
 				virt_to_page((void *)page_addrs[i]),
-				PAGE_SIZE, 0);
+				i == rm->data.op_nents - 1
+					? total_len - (i * PAGE_SIZE)
+					: PAGE_SIZE, 0);
 	}
 
 	return rm;

-- 
2.51.2

On Sat, 2026-09-05 at 10:00 -0700, Greg Marsden wrote:

rds_message_map_pages() assigns PAGE_SIZE bytes to every
scatterlist entry, even when total_len ends in a partial page. The RDS
congestion map is defined as 8192 bytes, so on systems with PAGE_SIZE
greater than 8192 the scatterlist maps bytes beyond the end of the
congestion map.  RDS-TCP transmits the SG contents according to those
lengths, so the extra bytes become part of the TCP RDS stream and are
interpreted as subsequent RDS message headers, corrupting the stream.

Limit the final scatterlist mapping to the number of bytes remaining.
This has no effect on systems with a 4K page size and allows RDS-TCP to
be used on systems with 16K and larger page sizes.  

The RDS selftest, which previously hung on 16K pages, now passes.

Fixes: 7875e18e0996 ("RDS: Message parsing")
Signed-off-by: Greg Marsden <redacted>

Looks good, thanks for the catch!
Reviewed-by: Allison Henderson <achender@kernel.org>

Hello:

This patch was applied to netdev/net.git (main)
by Jakub Kicinski [off-list ref]:

On Sat, 5 Sep 2026 10:00:41 -0700 you wrote:

rds_message_map_pages() assigns PAGE_SIZE bytes to every
scatterlist entry, even when total_len ends in a partial page. The RDS
congestion map is defined as 8192 bytes, so on systems with PAGE_SIZE
greater than 8192 the scatterlist maps bytes beyond the end of the
congestion map.  RDS-TCP transmits the SG contents according to those
lengths, so the extra bytes become part of the TCP RDS stream and are
interpreted as subsequent RDS message headers, corrupting the stream.

[...]

Here is the summary with links:
  - [net] net/rds: fix tcp stream corruption with large pages
    https://git.kernel.org/netdev/net/c/2ac09b5353fe

You are awesome, thank you!
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