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net: skbuff: do not leave stale header offsets after pskb_carve()

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pskb_carve_inside_header() and pskb_carve_inside_nonlinear() remove
the first bytes of a packet and reallocate skb->head.

All the headers that were present before the operation are gone,
but both functions call skb_headers_offset_update(skb, 0), which
is a no-op : skb->mac_header, skb->network_header,
skb->transport_header and skb->csum_start keep their old values and
now describe bytes which are no longer there.

Both helpers size the new head from the old skb_end_offset(), so the
stale offsets still land inside the new allocation. They point past
skb_tail_pointer() though, to bytes that were never initialized.

pskb_carve_inside_nonlinear() is the worst case, because it leaves a
zombie skb with an empty linear part (skb->data ==
skb_tail_pointer(skb), skb_headlen(skb) == 0), while
skb_mac_header_was_set() is still true and skb->mac_header is way
ahead of skb->data.

The only user of pskb_extract() is rds_tcp_data_recv(), and the
carved skb is queued on tinc->ti_skb_list. When the RDS incoming
message is released, rds_tcp_inc_free() calls skb_queue_purge(),
which frees the skbs with SKB_DROP_REASON_QUEUE_PURGE. This is
visible from drop_monitor, which then tries to pull back to the
(bogus) mac header :

skbuff: __skb_pull(len=234)
skb len=6968 data_len=6968 headroom=0 headlen=0 tailroom=0
end-tail=384 mac=(234,14) mac_len=14 net=(248,40) trans=288
shinfo(txflags=0 nr_frags=1 gso(size=1428 type=16 segs=5))
csum(0x100120 start=288 offset=16 ip_summed=3 complete_sw=0 valid=1 level=0)
hash(0x7b446c6c sw=0 l4=1) proto=0x86dd pkttype=0 iif=60
kernel BUG at ./include/linux/skbuff.h:2847!

Add skb_carve_reset_headers() to mark the mac and transport headers
as not set, reset the network header, clear skb->mac_len, and drop
a now meaningless CHECKSUM_PARTIAL (csum_start no longer describes
anything).

Invalidate the inner offsets as well. Unlike mac_header and
transport_header they have no "unset" sentinel, so a leftover
non-zero value still looks like a real header. Zero
skb->inner_mac_header, skb->inner_network_header,
skb->inner_transport_header, skb->inner_protocol and
skb->encapsulation, so that all the header state is invalidated in
one place.

v2: fixed an inaccurate changelog. The stale offsets stay inside the
    new skb->head, which is never smaller than the old one, they
    simply point past skb_tail_pointer() to bytes that are gone.
    Thanks to Xuanqiang Luo for insisting on this.
    Also invalidate the inner header state, as suggested by the
    netdev AI review :
    https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20260911114922.621937-1-edumazet%40google.com

Fixes: 6fa01ccd88 ("skbuff: Add pskb_extract() helper function")
Reported-by: syzbot+586af68eb819833c2d91@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/netdev/6aa3e9d3.f2639fcc.29487d.0028.GAE@google.com/
Cc: Xuanqiang Luo <xuanqiang.luo@linux.dev>
Cc: Allison Henderson <achender@kernel.org>
Cc: rds-devel@oss.oracle.com
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Xuanqiang Luo <luoxuanqiang@kylinos.cn>
Link: https://patch.msgid.link/20260915130423.3956471-1-edumazet@google.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
This commit is contained in:
Eric Dumazet 2026-09-15 13:04:23 +00:00 committed by Paolo Abeni
parent ad9c65b8f9
commit a5117e1ecc
1 changed files with 30 additions and 2 deletions
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32
net/core/skbuff.c
View File

@ -6832,6 +6832,34 @@ struct sk_buff *alloc_skb_with_frags(unsigned long header_len,
}
EXPORT_SYMBOL(alloc_skb_with_frags);
/* pskb_carve_inside_header() and pskb_carve_inside_nonlinear()
* remove the first bytes of a packet and reallocate skb->head.
*
* Whatever headers were present before the operation are gone,
* we must not leave stale offsets, otherwise users of this skb
* (skb_dump(), drop_monitor, taps, ...) would read or pull garbage.
*/
static void skb_carve_reset_headers(struct sk_buff *skb)
{
skb_unset_mac_header(skb);
skb_unset_transport_header(skb);
skb_reset_network_header(skb);
skb->mac_len = 0;
/* Inner offsets have no "unset" marker, zero them so that
* skb_inner_network_header_was_set() becomes false and no
* consumer mistakes them for a real (and long gone) header.
*/
skb->inner_mac_header = 0;
skb->inner_network_header = 0;
skb->inner_transport_header = 0;
skb->inner_protocol = 0;
skb->encapsulation = 0;
if (skb->ip_summed == CHECKSUM_PARTIAL)
skb->ip_summed = CHECKSUM_NONE;
}
/* carve out the first off bytes from skb when off < headlen */
static int pskb_carve_inside_header(struct sk_buff *skb, const u32 off,
const int headlen, gfp_t gfp_mask)
@ -6887,7 +6915,7 @@ static int pskb_carve_inside_header(struct sk_buff *skb, const u32 off,
skb->head_frag = 0;
skb_set_end_offset(skb, size);
skb_set_tail_pointer(skb, skb_headlen(skb));
skb_headers_offset_update(skb, 0);
skb_carve_reset_headers(skb);
skb->cloned = 0;
skb->hdr_len = 0;
skb->nohdr = 0;
@ -7027,7 +7055,7 @@ static int pskb_carve_inside_nonlinear(struct sk_buff *skb, const u32 off,
skb->data = data;
skb_set_end_offset(skb, size);
skb_reset_tail_pointer(skb);
skb_headers_offset_update(skb, 0);
skb_carve_reset_headers(skb);
skb->cloned = 0;
skb->hdr_len = 0;
skb->nohdr = 0;