[PATCH v2] io_uring/bpf_filter: Set src->bpf_filters_cow in io_bpf_filter_clone()

[Hui Peng]

When io_bpf_filter_clone() clones a struct io_bpf_filters table from a
source restriction set to a destination restriction set, it increments
src->bpf_filters->refs and sets dst->bpf_filters_cow = true, but forgets
to set src->bpf_filters_cow = true.

As a result, subsequent IORING_REGISTER_BPF_FILTER registrations on an
io_uring instance or task holding the source restriction set bypass
copy-on-write and mutate the shared io_bpf_filters table in place,
corrupting the BPF filter rules of already-cloned rings.

Fix this by setting src->bpf_filters_cow = true alongside
dst->bpf_filters_cow = true in io_bpf_filter_clone().

Fixes: ed82f35b926b ("io_uring: allow registration of per-task restrictions")
Assisted-by: LLM
Signed-off-by: Hui Peng <benquike@xxxxxxxxx>
---
v2: Resend as plain text. v1 went out as PGP/MIME, which I now
understand is not wanted on the lists - apologies for the noise. Also
add a Fixes: tag and the Assisted-by: LLM tag.

io_bpf_filter_clone(), the bpf_filters_cow flag and its only consumer in
io_register_bpf_filter() were all added together by ed82f35b926b
("io_uring: allow registration of per-task restrictions"), first
released in v7.0-rc1, so that is the tag. Note d42eb05e60fe
("io_uring: add support for BPF filtering for opcode restrictions")
created bpf_filter.c and has a later author date because of a rebase,
but it predates ed82f35b926b in the history and contains neither
io_bpf_filter_clone() nor bpf_filters_cow.

To be clear about severity: this is a restriction-bypass / filter-set
corruption issue, not a memory-safety one. The refcount is taken
correctly and there is no use-after-free; the problem is purely that the
source side of the clone is never marked COW, so a later
IORING_REGISTER_BPF_FILTER on the source mutates the table that the
cloned ring is still using.

Found by code inspection; build tested only, no reproducer.

 io_uring/bpf_filter.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/io_uring/bpf_filter.c b/io_uring/bpf_filter.c
index c0037632b7af..4a21511c4811 100644
--- a/io_uring/bpf_filter.c
+++ b/io_uring/bpf_filter.c
@@ -253,6 +253,7 @@ void io_bpf_filter_clone(struct io_restriction *dst, struct io_restriction *src)
 	 * If the src filter is going away, just ignore it.
 	 */
 	if (refcount_inc_not_zero(&src->bpf_filters->refs)) {
+		src->bpf_filters_cow = true;
 		dst->bpf_filters = src->bpf_filters;
 		dst->bpf_filters_cow = true;
 	}