6.8-stable review patch. If anyone has any objections, please let me know.
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From: Florian Westphal fw@strlen.de
commit 025f8ad20f2e3264d11683aa9cbbf0083eefbdcd upstream.
mpls_gso_segment() assumes skb_inner_network_header() returns a valid result:
mpls_hlen = skb_inner_network_header(skb) - skb_network_header(skb); if (unlikely(!mpls_hlen || mpls_hlen % MPLS_HLEN)) goto out; if (unlikely(!pskb_may_pull(skb, mpls_hlen)))
With syzbot reproducer, skb_inner_network_header() yields 0, skb_network_header() returns 108, so this will "pskb_may_pull(skb, -108)))" which triggers a newly added DEBUG_NET_WARN_ON_ONCE() check:
------------[ cut here ]------------ WARNING: CPU: 0 PID: 5068 at include/linux/skbuff.h:2723 pskb_may_pull_reason include/linux/skbuff.h:2723 [inline] WARNING: CPU: 0 PID: 5068 at include/linux/skbuff.h:2723 pskb_may_pull include/linux/skbuff.h:2739 [inline] WARNING: CPU: 0 PID: 5068 at include/linux/skbuff.h:2723 mpls_gso_segment+0x773/0xaa0 net/mpls/mpls_gso.c:34 [..] skb_mac_gso_segment+0x383/0x740 net/core/gso.c:53 nsh_gso_segment+0x40a/0xad0 net/nsh/nsh.c:108 skb_mac_gso_segment+0x383/0x740 net/core/gso.c:53 __skb_gso_segment+0x324/0x4c0 net/core/gso.c:124 skb_gso_segment include/net/gso.h:83 [inline] [..] sch_direct_xmit+0x11a/0x5f0 net/sched/sch_generic.c:327 [..] packet_sendmsg+0x46a9/0x6130 net/packet/af_packet.c:3113 [..]
First iteration of this patch made mpls_hlen signed and changed test to error out to "mpls_hlen <= 0 || ..".
Eric Dumazet said:
I was thinking about adding a debug check in skb_inner_network_header() if inner_network_header is zero (that would mean it is not 'set' yet), but this would trigger even after your patch.
So add new skb_inner_network_header_was_set() helper and use that.
The syzbot reproducer injects data via packet socket. The skb that gets allocated and passed down the stack has ->protocol set to NSH (0x894f) and gso_type set to SKB_GSO_UDP | SKB_GSO_DODGY.
This gets passed to skb_mac_gso_segment(), which sees NSH as ptype to find a callback for. nsh_gso_segment() retrieves next type:
proto = tun_p_to_eth_p(nsh_hdr(skb)->np);
... which is MPLS (TUN_P_MPLS_UC). It updates skb->protocol and then calls mpls_gso_segment(). Inner offsets are all 0, so mpls_gso_segment() ends up with a negative header size.
In case more callers rely on silent handling of such large may_pull values we could also 'legalize' this behaviour, either replacing the debug check with (len > INT_MAX) test or removing it and instead adding a comment before existing
if (unlikely(len > skb->len)) return SKB_DROP_REASON_PKT_TOO_SMALL;
test in pskb_may_pull_reason(), saying that this check also implicitly takes care of callers that miscompute header sizes.
Cc: Simon Horman horms@kernel.org Fixes: 219eee9c0d16 ("net: skbuff: add overflow debug check to pull/push helpers") Reported-by: syzbot+99d15fcdb0132a1e1a82@syzkaller.appspotmail.com Closes: https://lore.kernel.org/netdev/00000000000043b1310611e388aa@google.com/raw Signed-off-by: Florian Westphal fw@strlen.de Link: https://lore.kernel.org/r/20240222140321.14080-1-fw@strlen.de Signed-off-by: Jakub Kicinski kuba@kernel.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- include/linux/skbuff.h | 5 +++++ net/mpls/mpls_gso.c | 3 +++ 2 files changed, 8 insertions(+)
--- a/include/linux/skbuff.h +++ b/include/linux/skbuff.h @@ -2847,6 +2847,11 @@ static inline void skb_set_inner_network skb->inner_network_header += offset; }
+static inline bool skb_inner_network_header_was_set(const struct sk_buff *skb) +{ + return skb->inner_network_header > 0; +} + static inline unsigned char *skb_inner_mac_header(const struct sk_buff *skb) { return skb->head + skb->inner_mac_header; --- a/net/mpls/mpls_gso.c +++ b/net/mpls/mpls_gso.c @@ -27,6 +27,9 @@ static struct sk_buff *mpls_gso_segment( __be16 mpls_protocol; unsigned int mpls_hlen;
+ if (!skb_inner_network_header_was_set(skb)) + goto out; + skb_reset_network_header(skb); mpls_hlen = skb_inner_network_header(skb) - skb_network_header(skb); if (unlikely(!mpls_hlen || mpls_hlen % MPLS_HLEN))