lists.openwall.net | lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC | |
Open Source and information security mailing list archives
| ||
|
Message-ID: <87mwij4nih.fsf@natisbad.org> Date: Sun, 26 Jan 2014 01:04:06 +0100 From: arno@...isbad.org (Arnaud Ebalard) To: Willy Tarreau <w@....eu> Cc: Eric Dumazet <eric.dumazet@...il.com>, David Miller <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Daniel Borkmann <dborkman@...hat.com>, Herbert Xu <herbert@...dor.apana.org.au>, netdev@...r.kernel.org Subject: Re: [BUG] null pointer dereference in tcp_gso_segment() Hi, Willy Tarreau <w@....eu> writes: > On Wed, Jan 22, 2014 at 02:18:45PM -0800, Eric Dumazet wrote: >> On Wed, 2014-01-22 at 23:02 +0100, Arnaud Ebalard wrote: >> > Hi Eric, >> > >> > Eric Dumazet <eric.dumazet@...il.com> writes: >> > >> > >> Unless there is an assumption I missed somewhere in the function, the >> > >> problem may occur during the first round of the loop, because (unlike >> > >> the 'while' condition does at line 21) skb->next is not checked against >> > >> null at lines 17 above before it is passed to tcp_hdr() at line 18. >> > >> >> > >> To be honest, I am asking because I am not familiar w/ the code and it >> > >> is somewhat old so I wonder why noone got hit before. AFAICT, >> > >> f4c50d990dcf ([NET]: Add software TSOv4) added TSOv4 support in 2006 via >> > >> introduction of tcp_tso_segmen() (with the same kind of deref but >> > >> possibly different assumptions) which was more recently modified via >> > >> 28850dc7c7 (net: tcp: move GRO/GSO functions to tcp_offload) to become >> > >> tcp_gso_segment(). >> > >> >> > >> David, can you confirm the analysis and possibly comment on the >> > >> conditions needed for the bug to manifest? >> > > >> > > A gso packet contains at least 2 segments. >> > >> > By whom / where is it enforced? >> >> For example, tcp_gso_segment() does the following check : >> >> if (unlikely(skb->len <= mss)) >> goto out; >> >> If there was one segment, then skb->len should also be smaller than mss >> >> Since TCP stack seemed to be the provider of the packet in your stack >> trace, check tcp_set_skb_tso_segs() > > Thanks Eric for the explanation. From Arnaud's trace, I suspect that he's > received an ACK which has released some pending data, so it's very likely > indeed that at least two segments were released at once given that the > receiver is likely to ACK every two segments. > > Also we can expect that the received ACK was copy-breaked. I don't know > if some sort of skb recycling may happen at this stage and reveal some > bad corner cases (eg: improperly initialized skb during the rx path that > causes everything to break when it's recycled for the tx path), but Arnaud > you can easily disable the rx_copybreak feature by setting the rx_copybreak > module argument to zero. You can change it at run time in /sys/module. At > least it will tell us if it could be related or not. The problem is that I cannot simply use that trick to test your hypothesis as the bug is not easily reproducible. I was lucky to trigger it twice but never got it then (when I tested with an additonal BUG_ON(skb->next == NULL) before the main loop in tcp_gso_segment()). Cheers, a+ -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Powered by blists - more mailing lists