From: Weiming Shi When bpf_msg_push_data() splits a scatterlist element into head and tail, the tail's page offset is advanced by `start` (absolute message byte offset) instead of `start - offset` (byte position within the element). This makes rsge.offset overshoot by `offset` bytes, pointing to the wrong location within the page or beyond its boundary. Consumers of the corrupted entry either silently read wrong data or trigger an out-of-bounds access. BUG: KASAN: slab-use-after-free in bpf_msg_pull_data (net/core/filter.c:2728) Read of size 32752 at addr ffff8881042f0010 by task poc/130 Call Trace: __asan_memcpy (mm/kasan/shadow.c:105) bpf_msg_pull_data (net/core/filter.c:2728) bpf_prog_run_pin_on_cpu (include/linux/bpf.h:1402) sk_psock_msg_verdict (net/core/skmsg.c:934) tcp_bpf_send_verdict (net/ipv4/tcp_bpf.c:421) sock_sendmsg_nosec (net/socket.c:727) Fixes: 6fff607e2f14 ("bpf: sk_msg program helper bpf_msg_push_data") Reported-by: Xiang Mei Reviewed-by: Jiayuan Chen Reviewed-by: Emil Tsalapatis Signed-off-by: Weiming Shi Signed-off-by: Jiayuan Chen --- To sashiko: Regarding bpf_msg_push_data() reading "copy = msg->sg.data[i].length" with i == msg->sg.end (appending at the very end of a full/near-full ring): This is pre-existing code, not touched by this series, and reproducing it needs a narrow combination -- a pure append at the end so the loop exits with i == msg->sg.end, a full/near-full ring, plus a prior push/pop history that leaves a stale length in the otherwise-unused end slot. A freshly built ring zeroes that slot, so copy stays 0. We don't consider it practically reproducible. Even then it's already covered: the overflow check in patch 1 ("copy + len < copy") rejects the dangerous case, and __GFP_ZERO in patch 3 prevents any data exposure. Not worth fixing here. --- net/core/filter.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/net/core/filter.c b/net/core/filter.c index 3c8f1cedb217f..3e555f276ba80 100644 --- a/net/core/filter.c +++ b/net/core/filter.c @@ -2872,7 +2872,7 @@ BPF_CALL_4(bpf_msg_push_data, struct sk_msg *, msg, u32, start, psge->length = start - offset; rsge.length -= psge->length; - rsge.offset += start; + rsge.offset += start - offset; sk_msg_iter_var_next(i); sg_unmark_end(psge); -- 2.43.0