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@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 6f0b3e0..0f665cb 100644 (file)
--- 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 (file)
--- 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);