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From: John Fastabend <john.fastabend@gmail.com>
To: jakub@cloudflare.com, daniel@iogearbox.net
Cc: netdev@vger.kernel.org, bpf@vger.kernel.org,
	john.fastabend@gmail.com, ast@kernel.org
Subject: [PATCH 1/2] bpf: sockmap, msg_pop_data can incorrecty set an sge length
Date: Mon, 04 May 2020 10:21:23 -0700
Message-ID: <158861288359.14306.7654891716919968144.stgit@john-Precision-5820-Tower> (raw)
In-Reply-To: <158861271707.14306.15853815339036099229.stgit@john-Precision-5820-Tower>

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>
---
 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 7d6ceaa..5cc9276 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -2590,8 +2590,8 @@ 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;
 		}
 	}
 


  reply index

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-05-04 17:21 [PATCH 0/2] sockmap, fix for some error paths with helpers John Fastabend
2020-05-04 17:21 ` John Fastabend [this message]
2020-05-05 18:59   ` [PATCH 1/2] bpf: sockmap, msg_pop_data can incorrecty set an sge length Martin KaFai Lau
2020-05-04 17:21 ` [PATCH 2/2] bpf: sockmap, bpf_tcp_ingress needs to subtract bytes from sg.size John Fastabend
2020-05-05 19:05   ` Martin KaFai Lau
2020-05-05  9:30 ` [PATCH 0/2] sockmap, fix for some error paths with helpers Jakub Sitnicki
2020-05-05 22:28 ` Daniel Borkmann

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