From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 3E23453B61F; Wed, 23 Sep 2026 14:44:16 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790174658; cv=none; b=lmiTWhZXygdxFrIUNUBcX/sDduXADbO/3OyPqR2NwBrUeqC7Z9OEZimhISVSq+XQ2tzhTTZmkHjksfomSkDU5Z//Iokn0EsTVZnxm0m0swIFNz77zuqeZw/nSEjdXBhfLdL0HVr0WMhucfYZTXsqSQV9MZX0sYNQF5NhcNE5OrU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790174658; c=relaxed/simple; bh=rjEFo491a/6uNWF7qdOVaZTH2JrI6ECUXBF5bJXJDKQ=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=iR9ned3QbbtGAfzCK0GOrpNBJ8URCM4NRs1cjgV5/7JJW7llHBnchL0AMDwBU6LbCoYQ7NkMX+I4BEaf9dOjIw9KeeDW07zx4s8EMDDfoAxPZ0HzNhkl0NC8ymDW6Tq1xPqOhw89d6EP3s8y/53sxg/8hyALvJnZUz5F42/lmYg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=TBtw85bh; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="TBtw85bh" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 5DCB21F000FF; Wed, 23 Sep 2026 14:44:16 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linuxfoundation.org; s=korg; t=1790174656; bh=apo86avu5GTeARfgPJOirHxDEmG3BQXhgUMSPXgu+bU=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=TBtw85bhq93xP+5fRgTOUF7qIa5nvl+aItpAil7uuE7/v0NypX8ujjQy3J7RB3SD9 xNeC+9uh1nXhsOV5EImCEMkUqr2PjpXzkM73RPKGLED8GH0B9QilS3QX33NVJIiY2P WdfCVR5Ljl3EBubhE6spyVaElsCDsXF+FNKb81AA= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, syzbot+586af68eb819833c2d91@syzkaller.appspotmail.com, Xuanqiang Luo , Allison Henderson , rds-devel@oss.oracle.com, Eric Dumazet , Xuanqiang Luo , Paolo Abeni , Sasha Levin Subject: [PATCH 6.18 185/398] net: skbuff: do not leave stale header offsets after pskb_carve() Date: Wed, 23 Sep 2026 16:04:19 +0200 Message-ID: <20260923140648.226834175@linuxfoundation.org> X-Mailer: git-send-email 2.55.0 In-Reply-To: <20260923140643.441954610@linuxfoundation.org> References: <20260923140643.441954610@linuxfoundation.org> User-Agent: quilt/0.69 X-stable: review X-Patchwork-Hint: ignore Precedence: bulk X-Mailing-List: patches@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 6.18-stable review patch. If anyone has any objections, please let me know. ------------------ From: Eric Dumazet [ Upstream commit a5117e1eccac6ee3bd4aed7cacf8ebcb6b3eb309 ] 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: 6fa01ccd8830 ("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 Cc: Allison Henderson Cc: rds-devel@oss.oracle.com Signed-off-by: Eric Dumazet Reviewed-by: Xuanqiang Luo Link: https://patch.msgid.link/20260915130423.3956471-1-edumazet@google.com Signed-off-by: Paolo Abeni Signed-off-by: Sasha Levin --- net/core/skbuff.c | 32 ++++++++++++++++++++++++++++++-- 1 file changed, 30 insertions(+), 2 deletions(-) diff --git a/net/core/skbuff.c b/net/core/skbuff.c index 0324292309921..594764eab40dc 100644 --- a/net/core/skbuff.c +++ b/net/core/skbuff.c @@ -6766,6 +6766,34 @@ 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) @@ -6821,7 +6849,7 @@ 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; @@ -6961,7 +6989,7 @@ 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; -- 2.53.0