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Date: Fri, 5 Aug 2016 00:11:12 +0200
From: Daniel Borkmann <daniel@...earbox.net>
To: davem@...emloft.net
Cc: alexei.starovoitov@...il.com, tgraf@...g.ch,
netdev@...r.kernel.org, Daniel Borkmann <daniel@...earbox.net>
Subject: [PATCH net v2 2/3] bpf: fix checksum fixups on bpf_skb_store_bytes
bpf_skb_store_bytes() invocations above L2 header need BPF_F_RECOMPUTE_CSUM
flag for updates, so that CHECKSUM_COMPLETE will be fixed up along the way.
Where we ran into an issue with bpf_skb_store_bytes() is when we did a
single-byte update on the IPv6 hoplimit despite using BPF_F_RECOMPUTE_CSUM
flag; simple ping via ICMPv6 triggered a hw csum failure as a result. The
underlying issue has been tracked down to a buffer alignment issue.
Meaning, that csum_partial() computations via skb_postpull_rcsum() and
skb_postpush_rcsum() pair invoked had a wrong result since they operated on
an odd address for the hoplimit, while other computations were done on an
even address. This mix doesn't work as-is with skb_postpull_rcsum(),
skb_postpush_rcsum() pair as it always expects at least half-word alignment
of input buffers, which is normally the case. Thus, instead of these helpers
using csum_sub() and (implicitly) csum_add(), we need to use csum_block_sub(),
csum_block_add(), respectively. For unaligned offsets, they rotate the sum
to align it to a half-word boundary again, otherwise they work the same as
csum_sub() and csum_add().
Adding __skb_postpull_rcsum(), __skb_postpush_rcsum() variants that take the
offset as an input and adapting bpf_skb_store_bytes() to them fixes the hw
csum failures again. The skb_postpull_rcsum(), skb_postpush_rcsum() helpers
use a 0 constant for offset so that the compiler optimizes the offset & 1
test away and generates the same code as with csum_sub()/_add().
Fixes: 608cd71a9c7c ("tc: bpf: generalize pedit action")
Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
Acked-by: Alexei Starovoitov <ast@...nel.org>
---
include/linux/skbuff.h | 52 ++++++++++++++++++++++++++++++++------------------
net/core/filter.c | 4 ++--
2 files changed, 35 insertions(+), 21 deletions(-)
diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 6f0b3e0..0f665cb 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -2847,6 +2847,18 @@ static inline int skb_linearize_cow(struct sk_buff *skb)
__skb_linearize(skb) : 0;
}
+static __always_inline void
+__skb_postpull_rcsum(struct sk_buff *skb, const void *start, unsigned int len,
+ unsigned int off)
+{
+ if (skb->ip_summed == CHECKSUM_COMPLETE)
+ skb->csum = csum_block_sub(skb->csum,
+ csum_partial(start, len, 0), off);
+ else if (skb->ip_summed == CHECKSUM_PARTIAL &&
+ skb_checksum_start_offset(skb) < 0)
+ skb->ip_summed = CHECKSUM_NONE;
+}
+
/**
* skb_postpull_rcsum - update checksum for received skb after pull
* @skb: buffer to update
@@ -2857,36 +2869,38 @@ static inline int skb_linearize_cow(struct sk_buff *skb)
* update the CHECKSUM_COMPLETE checksum, or set ip_summed to
* CHECKSUM_NONE so that it can be recomputed from scratch.
*/
-
static inline void skb_postpull_rcsum(struct sk_buff *skb,
const void *start, unsigned int len)
{
- if (skb->ip_summed == CHECKSUM_COMPLETE)
- skb->csum = csum_sub(skb->csum, csum_partial(start, len, 0));
- else if (skb->ip_summed == CHECKSUM_PARTIAL &&
- skb_checksum_start_offset(skb) < 0)
- skb->ip_summed = CHECKSUM_NONE;
+ __skb_postpull_rcsum(skb, start, len, 0);
}
-unsigned char *skb_pull_rcsum(struct sk_buff *skb, unsigned int len);
+static __always_inline void
+__skb_postpush_rcsum(struct sk_buff *skb, const void *start, unsigned int len,
+ unsigned int off)
+{
+ if (skb->ip_summed == CHECKSUM_COMPLETE)
+ skb->csum = csum_block_add(skb->csum,
+ csum_partial(start, len, 0), off);
+}
+/**
+ * skb_postpush_rcsum - update checksum for received skb after push
+ * @skb: buffer to update
+ * @start: start of data after push
+ * @len: length of data pushed
+ *
+ * After doing a push on a received packet, you need to call this to
+ * update the CHECKSUM_COMPLETE checksum.
+ */
static inline void skb_postpush_rcsum(struct sk_buff *skb,
const void *start, unsigned int len)
{
- /* For performing the reverse operation to skb_postpull_rcsum(),
- * we can instead of ...
- *
- * skb->csum = csum_add(skb->csum, csum_partial(start, len, 0));
- *
- * ... just use this equivalent version here to save a few
- * instructions. Feeding csum of 0 in csum_partial() and later
- * on adding skb->csum is equivalent to feed skb->csum in the
- * first place.
- */
- if (skb->ip_summed == CHECKSUM_COMPLETE)
- skb->csum = csum_partial(start, len, skb->csum);
+ __skb_postpush_rcsum(skb, start, len, 0);
}
+unsigned char *skb_pull_rcsum(struct sk_buff *skb, unsigned int len);
+
/**
* skb_push_rcsum - push skb and update receive checksum
* @skb: buffer to update
diff --git a/net/core/filter.c b/net/core/filter.c
index c46244f..5ecd5c9 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -1401,7 +1401,7 @@ static u64 bpf_skb_store_bytes(u64 r1, u64 r2, u64 r3, u64 r4, u64 flags)
return -EFAULT;
if (flags & BPF_F_RECOMPUTE_CSUM)
- skb_postpull_rcsum(skb, ptr, len);
+ __skb_postpull_rcsum(skb, ptr, len, offset);
memcpy(ptr, from, len);
@@ -1410,7 +1410,7 @@ static u64 bpf_skb_store_bytes(u64 r1, u64 r2, u64 r3, u64 r4, u64 flags)
skb_store_bits(skb, offset, ptr, len);
if (flags & BPF_F_RECOMPUTE_CSUM)
- skb_postpush_rcsum(skb, ptr, len);
+ __skb_postpush_rcsum(skb, ptr, len, offset);
if (flags & BPF_F_INVALIDATE_HASH)
skb_clear_hash(skb);
--
1.9.3
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