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Subject[PATCH 4.18 021/168] bpf: fix several offset tests 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 5b24109b0563d45094c470684c1f8cea1af269f8 ]

While recently going over bpf_msg_pull_data(), I noticed three
issues which are fixed in here:

1) When we attempt to find the first scatterlist element (sge)
for the start offset, we add len to the offset before we check
for start < offset + len, whereas it should come after when
we iterate to the next sge to accumulate the offsets. For
example, given a start offset of 12 with a sge length of 8
for the first sge in the list would lead us to determine this
sge as the first sge thinking it covers first 16 bytes where
start is located, whereas start sits in subsequent sges so
we would end up pulling in the wrong data.

2) After figuring out the starting sge, we have a short-cut test
in !msg->sg_copy[i] && bytes <= len. This checks whether it's
not needed to make the page at the sge private where we can
just exit by updating msg->data and msg->data_end. However,
the length test is not fully correct. bytes <= len checks
whether the requested bytes (end - start offsets) fit into the
sge's length. The part that is missing is that start must not
be sge length aligned. Meaning, the start offset into the sge
needs to be accounted as well on top of the requested bytes
as otherwise we can access the sge out of bounds. For example
the sge could have length of 8, our requested bytes could have
length of 8, but at a start offset of 4, so we also would need
to pull in 4 bytes of the next sge, when we jump to the out
label we do set msg->data to sg_virt(&sg[i]) + start - offset
and msg->data_end to msg->data + bytes which would be oob.

3) The subsequent bytes < copy test for finding the last sge has
the same issue as in point 2) but also it tests for less than
rather than less or equal to. Meaning if the sge length is of
8 and requested bytes of 8 while having the start aligned with
the sge, we would unnecessarily go and pull in the next sge as
well to make it private.

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 | 14 +++++++++-----
1 file changed, 9 insertions(+), 5 deletions(-)

--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -2276,10 +2276,10 @@ BPF_CALL_4(bpf_msg_pull_data,
struct sk_msg_buff *, msg, u32, start, u32, end, u64, flags)
{
unsigned int len = 0, offset = 0, copy = 0;
+ int bytes = end - start, bytes_sg_total;
struct scatterlist *sg = msg->sg_data;
int first_sg, last_sg, i, shift;
unsigned char *p, *to, *from;
- int bytes = end - start;
struct page *page;

if (unlikely(flags || end <= start))
@@ -2289,9 +2289,9 @@ BPF_CALL_4(bpf_msg_pull_data,
i = msg->sg_start;
do {
len = sg[i].length;
- offset += len;
if (start < offset + len)
break;
+ offset += len;
i++;
if (i == MAX_SKB_FRAGS)
i = 0;
@@ -2300,7 +2300,11 @@ BPF_CALL_4(bpf_msg_pull_data,
if (unlikely(start >= offset + len))
return -EINVAL;

- if (!msg->sg_copy[i] && bytes <= len)
+ /* The start may point into the sg element so we need to also
+ * account for the headroom.
+ */
+ bytes_sg_total = start - offset + bytes;
+ if (!msg->sg_copy[i] && bytes_sg_total <= len)
goto out;

first_sg = i;
@@ -2320,12 +2324,12 @@ BPF_CALL_4(bpf_msg_pull_data,
i++;
if (i == MAX_SKB_FRAGS)
i = 0;
- if (bytes < copy)
+ if (bytes_sg_total <= copy)
break;
} while (i != msg->sg_end);
last_sg = i;

- if (unlikely(copy < end - start))
+ if (unlikely(bytes_sg_total > copy))
return -EINVAL;

page = alloc_pages(__GFP_NOWARN | GFP_ATOMIC, get_order(copy));

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