linux-kernel - Wondering: why capabilities system is broken?

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Message-ID: <46252471.2030006@tls.msk.ru>
Date:	Tue, 17 Apr 2007 23:48:01 +0400
From:	Michael Tokarev <mjt@....msk.ru>
To:	linux-kernel@...r.kernel.org
Subject: Wondering: why capabilities system is broken?

Hopefully not a flamewar question...

Currently, capabilities of a process are reset during exec()
system call.  At least effective+permitted set.

1) In case new uid != 0, all the caps are cleared, so it is not
possible to execute a program as non-root but still give it some
capabilities (like, say, CAP_NET_BIND_SERVICE).

2) In case new uid == 0, effective and permitted sets are restored
to all-ones.

This is regardless of other settings, like prctl(KEEPCAPS), or
the current set of capabilities.

I partly understand why 2) is done - in case of setuid binary being
executed, all the capabilities are set for it.  But this breaks
executing non-setuid binaries too -- for example, it'd be very nice
to be able to chroot to some directory, and remove CAP_SYS_CHROOT
(and other evil caps like CAP_SYS_MODULE, CAP_SETPCAP) -- this way,
with minimal efforts, chroot will work almost (yes, I understand
it's not entirely the same) the same as BSD jail(2) concept.

So the question is: why capability sets are being reinitialized during
exec()?  At least in 2.4 era, they weren't...  and stuff like
execcap, sucaps etc was working.  Now they aren't anymore.

Thanks.

/mjt
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