From: Daniel Borkmann daniel@iogearbox.net
[ Upstream commit 2e43f95dd8ee62bc8bf57f2afac37fbd70c8d565 ]
If first_sg and last_sg wraps around in the scatterlist ring, then we need to account for that in the shift as well. E.g. crafting such msgs where this is the case leads to a hang as shift becomes negative. E.g. consider the following scenario:
first_sg := 14 |=> shift := -12 msg->sg_start := 10 last_sg := 3 | msg->sg_end := 5
round 1: i := 15, move_from := 3, sg[15] := sg[ 3] round 2: i := 0, move_from := -12, sg[ 0] := sg[-12] round 3: i := 1, move_from := -11, sg[ 1] := sg[-11] round 4: i := 2, move_from := -10, sg[ 2] := sg[-10] [...] round 13: i := 11, move_from := -1, sg[ 2] := sg[ -1] round 14: i := 12, move_from := 0, sg[ 2] := sg[ 0] round 15: i := 13, move_from := 1, sg[ 2] := sg[ 1] round 16: i := 14, move_from := 2, sg[ 2] := sg[ 2] round 17: i := 15, move_from := 3, sg[ 2] := sg[ 3] [...]
This means we will loop forever and never hit the msg->sg_end condition to break out of the loop. When we see that the ring wraps around, then the shift should be MAX_SKB_FRAGS - first_sg + last_sg - 1. Meaning, the remainder slots from the tail of the ring and the head until last_sg combined.
Fixes: 015632bb30da ("bpf: sk_msg program helper bpf_sk_msg_pull_data") Signed-off-by: Daniel Borkmann daniel@iogearbox.net Acked-by: John Fastabend john.fastabend@gmail.com Signed-off-by: Alexei Starovoitov ast@kernel.org Signed-off-by: Sasha Levin alexander.levin@microsoft.com --- net/core/filter.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-)
diff --git a/net/core/filter.c b/net/core/filter.c index e713a8791815..cc4d660aafe1 100644 --- a/net/core/filter.c +++ b/net/core/filter.c @@ -2360,7 +2360,10 @@ BPF_CALL_4(bpf_msg_pull_data, * had a single entry though we can just replace it and * be done. Otherwise walk the ring and shift the entries. */ - shift = last_sg - first_sg - 1; + WARN_ON_ONCE(last_sg == first_sg); + shift = last_sg > first_sg ? + last_sg - first_sg - 1 : + MAX_SKB_FRAGS - first_sg + last_sg - 1; if (!shift) goto out;