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Date: Thu, 21 Jun 2012 04:55:12 +0400 From: Solar Designer <solar@...nwall.com> To: kernel-hardening@...ts.openwall.com Cc: Pavel Kankovsky <peak@...o.troja.mff.cuni.cz> Subject: Re: RLIMIT_NPROC DoS fix Hi, Let's keep this on kernel-hardening only (dropping owl-dev, adding Pavel). FWIW, in pre-2.6 kernels there was no RLIMIT_NPROC enforcement on setuid() at all, so adding this check on execve() (like I did in -ow patches for 2.0-2.4 kernels) made things strictly more restricted. Now when we applied the same approach to mainline kernels, after RLIMIT_NPROC enforcement had been added to setuid(), we made things more relaxed in the way described by Vasily as compared to what they have been in 2.6 kernels for a long while. Yet I continue to think that the RLIMIT_NPROC enforcement on setuid() that existed in 2.6 kernels was wrong, and overall we're in a better state now. On Wed, Jun 13, 2012 at 02:22:41AM +0200, Pavel Kankovsky wrote: > On Tue, 12 Jun 2012, Vasily Kulikov wrote: > > >There is a problem with RLIMIT_NPROC patch: > >http://www.openwall.com/lists/kernel-hardening/2011/06/12/9 > > I'd like to point out there are at least two other ways setuid() et alii > can fail in recent kernels: LSM and memory allocation. LSM can break lots of things. ;-) As to memory allocation, we previously determined (in last year's discussion on kernel-hardening) that it currently can't fail - yet I proposed (and I continue to propose) that we make the code fail-close in this respect (have the process killed if this currently impossible condition does occur, such as in a future code revision or if we made an error in our analysis). (We already have it that way - with process killing - in the patch applied on top of RHEL5/OpenVZ kernels in Owl.) > >1) a root process sometimes does setrlimit()+setuid(U)+execve()/fork(). > >In this case U can do kill(pid, SIGSTOP) each time just after setuid(U). > >U may store unlimited number of processes this way. > >(Spender has learned this way.) > > You could probably refuse to handle SIGSTOP sent by a nonprivileged user > when PF_NPROC_EXCEEDED is set. I do not thing it would break anything. > But it would make things even dirtier. Yes, and we'd need to disallow setpriority() as well. > >2) there are nonroot users A and B. > >A makes a binary ./loop which does setuid(A)+for(;;) {} > >A does chmod u+s ./loop > >B does 'while :; do ./loop &; done' > > I think processes should count towards RLIMIT_NPROC of the user who forked > them (B in this example) for the rest of their CPU cycles. Even after they > exec a setuid program. I think this is already the case, but the crucial detail about Vasily's example above is that the process itself fully switches to the target user with a setuid() call - and indeed we perform set_user() on setuid(). Should we start to treat setuid() from non-root real UID specially for the purpose of RLIMIT_NPROC? I think not. Maybe we could postpone set_user() until execve() (which would never happen in the example above, thereby leaving the process B's for the purpose of RLIMIT_NPROC). This would deal with the SIGSTOP attack too. This needs further consideration. Alexander
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