[PATCH] io_uring/rsrc: require cloned buffers to share accounting contexts

[Jann Horn]

When IORING_REGISTER_CLONE_BUFFERS is used to clone buffers from uring
instance A to uring instance B, where A and B use different MMs for
accounting, the accounting can go wrong:
If uring instance A is closed before uring instance B, the pinned memory
counters for uring instance B will be decremented, even though the pinned
memory was originally accounted through uring instance A; so the MM of
uring instance B can end up with negative locked memory.

Cc: stable@xxxxxxxxxxxxxxx
Closes: https://lore.kernel.org/r/CAG48ez1zez4bdhmeGLEFxtbFADY4Czn3CV0u9d_TMcbvRA01bg@xxxxxxxxxxxxxx
Fixes: 7cc2a6eadcd7 ("io_uring: add IORING_REGISTER_COPY_BUFFERS method")
Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
---
To be clear, I think this is a very minor issue, feel free to take your
time landing it.

I put a stable marker on this, but I'm ambivalent about whether this
issue even warrants landing a fix in stable - feel free to remove the
Cc stable marker if you think it's unnecessary.
---
 io_uring/rsrc.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/io_uring/rsrc.c b/io_uring/rsrc.c
index 077f84684c18a0b3f5e622adb4978b6a00353b2f..caecc18dd5be03054ae46179bc0918887bf609a4 100644
--- a/io_uring/rsrc.c
+++ b/io_uring/rsrc.c
@@ -931,6 +931,13 @@ static int io_clone_buffers(struct io_ring_ctx *ctx, struct io_ring_ctx *src_ctx
 	int i, ret, off, nr;
 	unsigned int nbufs;
 
+	/*
+	 * Accounting state is shared between the two rings; that only works if
+	 * both rings are accounted towards the same counters.
+	 */
+	if (ctx->user != src_ctx->user || ctx->mm_account != src_ctx->mm_account)
+		return -EINVAL;
+
 	/* if offsets are given, must have nr specified too */
 	if (!arg->nr && (arg->dst_off || arg->src_off))
 		return -EINVAL;

---
base-commit: c45323b7560ec87c37c729b703c86ee65f136d75
change-id: 20250114-uring-check-accounting-4356f8b91c37

-- 
Jann Horn <jannh@xxxxxxxxxx>