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    SubjectRe: Bad ext3/nfs DoS bug
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    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


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