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Re: [PATCH] fexecve

On Fri, 16 Nov 2012 15:46:00 -0500 (EST)
Mouse <mouse%Rodents-Montreal.ORG@localhost> wrote:

> Nobody has quite come right out and said what the race supposedly
> remaining with fexecve is, or at least not that I've seen; the only
> one I've been able to think of is a TOCTOU race with someone
> overwriting the file between check and execute, and that one fexecve
> does not solve.

Yes, sorry, should have been explicit. That's the ridiculously obvious

> (It could be tweaked to solve it, by (for example) making O_EXEC (a)
> required for fexecve and (b) lock the file against writes in the same
> way executing it does, the way that's behind ETXTBSY.)

Which would be contrary to the spec anyway, no? If an implementation
needs to be non-compliant to be safe, is that better or worse than just
not having it at all? O_EXEC seems to exist on FreeBSD but not on OpenBSD
or Linux so I doubt there's much code out there that would work
unmodified with such an implementation.

> It's true fexecve doesn't solve the latter, but the bar _is_ higher;
> assuming checking involves checking ownership as well as contents,
> exploiting it requires the ability to overwrite a file owned by
> whatever user the check is checking for.

To make this statement, presumably you're also tacitly assuming checking
permissions? Doesn't matter who owns it if it's world writeable.

> (If, as is probably the case in many such uses, that user is root, I
> have trouble seeing _any_ issue here - anyone who can overwrite
> root-owned files pretty much pwnz0rz the system already.)

Depends whether they can overwrite all root-owned files, or just
specific ones (due to some other exploit).


3072D/F3A66B3A Julian Yon (2012 General Use) <>

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