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net: fix recent csum changes

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Vladimir reported csum issues after my recent change in skb_postpull_rcsum()

Issue here is the following:

initial skb->csum is the csum of

[part to be pulled][rest of packet]

Old code:
 skb->csum = csum_sub(skb->csum, csum_partial(pull, pull_length, 0));

New code:
 skb->csum = ~csum_partial(pull, pull_length, ~skb->csum);

This is broken if the csum of [pulled part]
happens to be equal to skb->csum, because end
result of skb->csum is 0 in new code, instead
of being 0xffffffff

David Laight suggested to use

skb->csum = -csum_partial(pull, pull_length, -skb->csum);

I based my patches on existing code present in include/net/seg6.h,
update_csum_diff4() and update_csum_diff16() which might need
a similar fix.

I guess that my tests, mostly pulling 40 bytes of IPv6 header
were not providing enough entropy to hit this bug.

v2: added wsum_negate() to make sparse happy.

Fixes: 29c3002644 ("net: optimize skb_postpull_rcsum()")
Fixes: 0bd28476f6 ("gro: optimize skb_gro_postpull_rcsum()")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Suggested-by: David Laight <David.Laight@ACULAB.COM>
Cc: David Lebrun <dlebrun@google.com>
Tested-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://lore.kernel.org/r/20211204045356.3659278-1-eric.dumazet@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
This commit is contained in:
Eric Dumazet 2021-12-03 20:53:56 -08:00 committed by Jakub Kicinski
parent 4c375272fb
commit 45cac67545
3 changed files with 8 additions and 3 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

3
include/linux/skbuff.h
View File

@ -3486,7 +3486,8 @@ 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_partial(start, len, ~skb->csum);
skb->csum = wsum_negate(csum_partial(start, len,
wsum_negate(skb->csum)));
else if (skb->ip_summed == CHECKSUM_PARTIAL &&
skb_checksum_start_offset(skb) < 0)
skb->ip_summed = CHECKSUM_NONE;

4
include/net/checksum.h
View File

@ -180,4 +180,8 @@ static inline void remcsum_unadjust(__sum16 *psum, __wsum delta)
*psum = csum_fold(csum_sub(delta, (__force __wsum)*psum));
}
static inline __wsum wsum_negate(__wsum val)
{
return (__force __wsum)-((__force u32)val);
}
#endif

4
include/net/gro.h
View File

@ -173,8 +173,8 @@ static inline void skb_gro_postpull_rcsum(struct sk_buff *skb,
const void *start, unsigned int len)
{
if (NAPI_GRO_CB(skb)->csum_valid)
NAPI_GRO_CB(skb)->csum = ~csum_partial(start, len,
~NAPI_GRO_CB(skb)->csum);
NAPI_GRO_CB(skb)->csum = wsum_negate(csum_partial(start, len,
wsum_negate(NAPI_GRO_CB(skb)->csum)));
}
/* GRO checksum functions. These are logical equivalents of the normal