[PATCH 1/3] io_uring/register: use stable SQ/CQ ring data during resize

[Jens Axboe <axboe@kernel.dk>]

Normally the kernel would not expect an application to modify any of
the data shared with the kernel during a resize operation, but of
course the kernel cannot always assume good intent on behalf of the
application.

As part of resizing the rings, existing SQEs and CQEs are copied over
to the new storage. Resizing uses the masks in the newly allocated
shared storage to index the arrays, however it's possible that malicious
userspace could modify these after they have been sanity checked.

Use the validated and locally stored CQ and SQ ring sizing for masking
to ensure the values are both stable and valid.

Fixes: 79cfe9e59c2a ("io_uring/register: add IORING_REGISTER_RESIZE_RINGS")
Reported-by: Jann Horn <jannh@google.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 io_uring/register.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/io_uring/register.c b/io_uring/register.c
index fdd44914c39c..5880eb75ae44 100644
--- a/io_uring/register.c
+++ b/io_uring/register.c
@@ -514,7 +514,7 @@ static int io_register_resize_rings(struct io_ring_ctx *ctx, void __user *arg)
 		goto overflow;
 	for (i = o.rings->sq.head; i < tail; i++) {
 		unsigned src_head = i & (ctx->sq_entries - 1);
-		unsigned dst_head = i & n.rings->sq_ring_mask;
+		unsigned dst_head = i & (p.sq_entries - 1);
 
 		n.sq_sqes[dst_head] = o.sq_sqes[src_head];
 	}
@@ -533,7 +533,7 @@ static int io_register_resize_rings(struct io_ring_ctx *ctx, void __user *arg)
 	}
 	for (i = o.rings->cq.head; i < tail; i++) {
 		unsigned src_head = i & (ctx->cq_entries - 1);
-		unsigned dst_head = i & n.rings->cq_ring_mask;
+		unsigned dst_head = i & (p.cq_entries - 1);
 
 		n.rings->cqes[dst_head] = o.rings->cqes[src_head];
 	}
-- 
2.47.1