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    SubjectRe: file offset corruption on 32-bit machines?
    On Fri 11-04-08 21:26:56, Pavel Machek wrote:
    > On Thu 2008-04-10 16:27:00, Jan Kara wrote:
    > > > On Thu, 10 Apr 2008, Michal Hocko wrote:
    > > >
    > > > > > Jeff Robertson analyzes the behaviour of different operating systems'
    > > > > > 64-bit file offset implementation and concludes that on 32-bit
    > > > > > machines, Linux and Solaris lack any locking to keep the two 32-bit
    > > > > > halves in sync and this could cause rare file offset corruption.
    > > > > > http://jeffr-tech.livejournal.com/21014.html
    > > > > AFAICS, this race is theoretically possible, but it is very hard (almost
    > > > > impossible) to trigger with a sane file usage pattern. Note that you
    > > > > have to access shared struct file (same file descriptor) in different
    > > > > threads which should be synchronized by caller anyway (*).
    > > >
    > > > ... but not in cases the caller is an intentionally evil code, right? :)
    > > Yes.
    > >
    > > > > I also don't see any security implications from this race, but maybe
    > > > > someone with more knowlage about fs can see (f_pos is used at many
    > > > > places in the kernel code).
    > > >
    > > > The f_pos races are in fact exploitable, we've already been there. See
    > > > for example http://www.isec.pl/vulnerabilities/isec-0016-procleaks.txt
    > > Well, this race is more subtle - the window is just one instruction
    > > wide (stores to f_pos from CPU2 must come between the store of lower and
    > > upper 32-bits of f_pos on CPU1). And the only result is that f_pos has
    > > 32-bits from one file pointer and 32-bits from the other one. So I can
    > > hardly imagine this would be exploitable...
    >
    > Don't we have rlimit on max file size? I'd guess this could work
    > around it?
    There is this limit but AFAIK it limits max size of file you're able to
    create. And write/truncate checks already their local variable so the real
    value used later.

    Honza
    --
    Jan Kara <jack@suse.cz>
    SUSE Labs, CR


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