Re: Bad ext3/nfs DoS bug

Marcel Holtmann
2006-07-18T07:54:57Z

Hi James,

> I've tried contacting the relevant maintainers directly,
> and it's even in the kernel bugzilla, but nothing's happened
> and it's been over a month now. No-one seems to be doing anyting
> about this. Is one meant to post this to bugtraq or what?
>
> Here's the bug: http://bugzilla.kernel.org/show_bug.cgi?id=6828
> (exploit code follows)
>
> > We found this rather surprising behaviour when debugging a
> > network card for one of our embedded systems. There was a
> > bus problem that occasionally caused the network card to
> > place random data in the outgoing packets. We were using
> > NFS root, as we hadn't written drivers for the block
> > devices yet, and discovered our Linux NFS servers getting
> > ext3 errors. It turned out that the 3com cards we have in
> > the servers lie about checking UDP checksums, and passed
> > the rubbish to knfsd where it was causing the problem.
> >
> > Here's an example one of our widgets (dcm503) is talking
> > to an NFS server (dufftown)
> >
> > 17:28:38.535011 dcm503.guralp.local.984095109 > dufftown.guralp.local.nfs: 116
> > lookup fh Unknown/1 "" (DF) (ttl 64, id 0, len 144)
> > 4500 0090 0000 4000 4011 3d45 0a52 01fa
> > c0a8 3024 03ff 0801 007c 8e9c 3aa8 1985
> > 0000 0000 0000 0002 0001 86a3 0000 0002
> > 0000 0004 0000 0001 0000 001c 028f 5b0c
> > 0000 0006 6463 6d35 3033 0000 0000 0000
> > 0000 0000 0000 0000 0000 0000 0000 0000
> > 0100 0001 0021 0003 3d26 3d00 4a2f ffff
> > 3d00 2c08 c923 0000 0000 0000 0000 0000
> > 0000 0000 000a 6d6f 756e 7470 6f69 6e74
> >
> > so what's happened here is 4a2f ffff should have been 4a2f
> > xxxx but the network card has missed the clock on the bus
> > and gotten ffff instead
> >
> > nfsd_dispatch: vers 2 proc 4
> > nfsd: LOOKUP 32: 01000001 03002100 003d263d ffff2f4a 082c003d 000023c9
> > nfsd: nfsd_lookup(fh 32: 01000001 03002100 003d263d ffff2f4a 082c003d
> > 000023c9, )
> > nfsd: fh_verify(32: 01000001 03002100 003d263d ffff2f4a 082c003d 000023c9)
> >
> > so here the client does a V2 lookup with a DH which has
> > gotten screwed up by my clients network card, this is
> > received by my server, gets past the UDP checksum code
> > (thank you 3com) and ends up at knfsd.
> >
> > knfsd passes this to fh_verify which decodes it to be hde3
> > and inode 4294913866 (0xffff2f4a)
> >
> > that then gets passed to ext3 which then panics.
> >
> > EXT3-fs error (device hde3): ext3_get_inode_block: bad inode number:
> > 4294913866
> >
> > marks the file system as containing an error, and remounts
> > the system read only.
> >
> > Obviously this is sub optimal, and a fairly horrid DoS
> > since anyone can craft a UDP packet, with a bogus FH in
> > it. Whilst this is for V2_LOOKUP it works for all of the
> > V2 procedures we tried.
> >
>
> exploit code is available at
>
> http://www.madingley.org/uploaded/crash-nfs.tar.gz

so I used your exploit and I could reproduce it on every 2.6 kernel, I
tried so far. However with a 2.4 kernel I see the error messages, but it
doesn't get remounted read-only. Did you run tests with 2.4 kernels?

Regards

Marcel