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[bpf-next,v2,01/12] bpf: sockmap, msg_pop_data can incorrecty set an sge length

Message ID 158939715042.15176.13948401431530778597.stgit@john-Precision-5820-Tower
State Accepted
Delegated to: BPF Maintainers
Headers show
Series bpf: selftests, test_sockmap improvements | expand

Commit Message

John Fastabend May 13, 2020, 7:12 p.m. UTC 
When sk_msg_pop() is called where the pop operation is working on
the end of a sge element and there is no additional trailing data
and there _is_ data in front of pop, like the following case,


   |____________a_____________|__pop__|

We have out of order operations where we incorrectly set the pop
variable so that instead of zero'ing pop we incorrectly leave it
untouched, effectively. This can cause later logic to shift the
buffers around believing it should pop extra space. The result is
we have 'popped' more data then we expected potentially breaking
program logic.

It took us a while to hit this case because typically we pop headers
which seem to rarely be at the end of a scatterlist elements but
we can't rely on this.

Fixes: 7246d8ed4dcce ("bpf: helper to pop data from messages")
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
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 0 files changed
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Patch

diff --git a/net/core/filter.c b/net/core/filter.c
index da06349..dfb4f24 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -2579,8 +2579,8 @@  BPF_CALL_4(bpf_msg_pop_data, struct sk_msg *, msg, u32, start,
 			}
 			pop = 0;
 		} else if (pop >= sge->length - a) {
-			sge->length = a;
 			pop -= (sge->length - a);
+			sge->length = a;
 		}
 	}