From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-qk1-f198.google.com (mail-qk1-f198.google.com [209.85.222.198]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 8BABA3B995B for ; Tue, 15 Sep 2026 13:04:26 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.222.198 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789477468; cv=none; b=Y689l8/meaOGb67FxQ/+1YYiQkOsQgMKQIix+oh5mzqK+EKhJwxKYwm2S/zZzTs8G6LnBd+PZEgAmS6qvT6wm6j+/wToq9fz7TLVMYpJJvIALI98jeMVza5URDbasKZ1xWvbq2RcS1nom0Ex2xXai8hasX99FsqiXItFbdZXDiw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789477468; c=relaxed/simple; bh=+aXEc6Jqx+Mbu6F6Kqxo+P3lKBuQ+xVoQz+rwS7WUlU=; h=Date:Mime-Version:Message-ID:Subject:From:To:Cc:Content-Type; b=qiHCDISVpPnS3KhH7jvqaf6xObBdrbCRbyRCShWtY2jUl7YG6TxF7JjEotAilW5pVWfCj7RL78vyCIoAOSgmhNsCjCGHAAu6hLtiP7qSah7qGcUs3dbeKCFbvkcgn9SPOBLtvawZOzTXydlrQsHiAX8t4s629vKkLUmqCcxyRC8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com; spf=pass smtp.mailfrom=flex--edumazet.bounces.google.com; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b=tFw7iiob; arc=none smtp.client-ip=209.85.222.198 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=flex--edumazet.bounces.google.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="tFw7iiob" Received: by mail-qk1-f198.google.com with SMTP id af79cd13be357-93a0050a554so829197085a.1 for ; Tue, 15 Sep 2026 06:04:26 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20251104; t=1789477465; x=1790082265; darn=vger.kernel.org; h=content-type:cc:to:from:subject:message-id:mime-version:date:from :to:cc:subject:date:message-id:reply-to:content-type; bh=r1vBkUy6uw3dX2vrbVvLOBWOMA9pJO/uSvFCFG1VGuc=; b=tFw7iiobqvvHsiMV4GYthKRmJ1z6FGg2ec+0a2Sg0uvtOyJpInJSvMoTU/HApJ70vX WSHH7Kc/etzQiogCU93HAZ1+FQUioIBXqKUIwpOO4fiIj9h1zmU/tTp6df10zyEJJEgW M03CBUkI9rbtwPDyxypYS6icvCfA+JVICkTkXrbcmA0EDJhKH+VDWJiIW7SUews+3CTF eovqxCnA5YRe7Bacx5Je4Sg2osi2EqI5e7cb5W9EzYxXVnY2xyufLT6Hhxm6guS5IM02 F13ijX0NrvQQWIwhI+wol+WBYV2hwaE6SXeu24DkcXmy5V5a5Smh3g02ZkWGOjiDXiOt 2lzQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20260707; t=1789477465; x=1790082265; h=content-type:cc:to:from:subject:message-id:mime-version:date :x-gm-message-state:from:to:cc:subject:date:message-id:reply-to :content-type; bh=r1vBkUy6uw3dX2vrbVvLOBWOMA9pJO/uSvFCFG1VGuc=; b=EtFLZ2oV21TeWGGwAmV9k0vIdfTmTobyjCZTfRCj/8+FxMWYigGj6tREL+CQffJICE BhgDZrjY6odeP4sVqx+9UVtFJ1neskf4iaAJ2DbQZOnva/GZiR/qwKZbmfeOv5LlAwqf k1OadHsuoigjbvmrHrCY0vGCU6721anbZQwlonVOkBztKg4OfnjbvrBazt0IsHNl9VKk WX2Iiin3kDYG1kDBpHkWF+8chnWU8gWI1hXRBb1zgQKap9THaqB3uN3iMyGfHFa7r1qw sxhtE7VH579Uyw3M2pv4WPbatWVujB6uA/GTJGFa8oXTAZTBuoFC3YRWyIZCI3AXVqBJ Rf/Q== X-Forwarded-Encrypted: i=1; AKwUvByG0w48C/gR8mL8n7YO6vx5hOJD++gRJZt0HNDQgKcJiygjD6F9tPAmPx9Kd7K1LUQd8yQx+sg=@vger.kernel.org X-Gm-Message-State: AFuF++ng3wLd9UDUvYLb6F9BTKGuZF7P1SlnEWpHU+M32mHZeYjw5xF5 AZYewG1u6i3GkR3W4uqGT8owsxaU92fmXzRwBdriyZJslVEXrw8MGjGOlVJNOUWZrvRPmP7/oP8 jNVAxhIv9rFk1+g== X-Received: from qknpy1.prod.google.com ([2002:a05:620a:8781:b0:939:f90e:6b97]) (user=edumazet job=prod-delivery.src-stubby-dispatcher) by 2002:a05:620a:6cc8:b0:92e:cf8e:67b9 with SMTP id af79cd13be357-93a2987c980mr977181585a.21.1789477465104; Tue, 15 Sep 2026 06:04:25 -0700 (PDT) Date: Tue, 15 Sep 2026 13:04:23 +0000 Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 X-Mailer: git-send-email 2.55.0.1032.g73a4cd73de-goog Message-ID: <20260915130423.3956471-1-edumazet@google.com> Subject: [PATCH v2 net] net: skbuff: do not leave stale header offsets after pskb_carve() From: Eric Dumazet To: "David S . Miller" , Jakub Kicinski , Paolo Abeni Cc: Simon Horman , netdev@vger.kernel.org, eric.dumazet@gmail.com, Eric Dumazet , syzbot+586af68eb819833c2d91@syzkaller.appspotmail.com, Xuanqiang Luo , Allison Henderson , rds-devel@oss.oracle.com Content-Type: text/plain; charset="UTF-8" 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 ------------[ cut here ]------------ 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 --- 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 cc3b4b70288b4e3f17984cd4f4e4b7a330353e15..609f2c7f4a47ad7a81426af149727fa47be12bb5 100644 --- a/net/core/skbuff.c +++ b/net/core/skbuff.c @@ -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; -- 2.55.0.1032.g73a4cd73de-goog