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Subject[PATCH 4.18 029/168] bpf: fix shift upon scatterlist ring wrap-around in bpf_msg_pull_data
Date
4.18-stable review patch.  If anyone has any objections, please let me know.

------------------

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
net/core/filter.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)

--- 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;


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