[PATCH] net/rds: Do not unpin a page after a failed atomic pin

[Kyle Zeng <kylebot@openai.com>]

rds_cmsg_atomic() assumes that a non-NULL output from rds_pin_pages()
represents a pin that it owns. However, pin_user_pages_fast() can roll
back a fast-GUP pin after racing fork() and then fail in the slow path,
for example after a concurrent mprotect(PROT_NONE). The output slot can
still contain the already-unpinned page.

The common error path calls unpin_user_page() on that stale pointer.
This can consume another subsystem's pin and allow the page to be
freed while it is still in use.

Clear page when rds_pin_pages() does not return one. The error path
continues to release the page on errors after a successful pin, and
successful requests retain their existing ownership and behavior. All
four RDS atomic ancillary operations share this path.

Signed-off-by: Kyle Zeng <kylebot@openai.com>
---
I have a reproducer for this issue and can share via DM

 net/rds/rdma.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/net/rds/rdma.c b/net/rds/rdma.c
index 078090d292fa..71b57abb96c4 100644
--- a/net/rds/rdma.c
+++ b/net/rds/rdma.c
@@ -957,8 +957,11 @@ int rds_cmsg_atomic(struct rds_sock *rs, struct rds_message *rm,
 	}
 
 	ret = rds_pin_pages(args->local_addr, 1, &page, 1);
-	if (ret != 1)
+	if (ret != 1) {
+		/* A failed pin can leave a stale page pointer. */
+		page = NULL;
 		goto err;
+	}
 	ret = 0;
 
 	sg_set_page(rm->atomic.op_sg, page, 8, offset_in_page(args->local_addr));

base-commit: d89e1d186ec9271bf78e62771779e9fc03bf0fc3
-- 
2.56.0