On Wed, 13 Jul 2011 10:31:42 +0400 Solar Designer <solar@xxxxxxxxxxxx> wrote: > Linus, Neil, Motohiro - thank you for your comments! > > On Wed, Jul 13, 2011 at 09:14:08AM +1000, NeilBrown wrote: > > The contrast is really "failing when trying to use reduced privileges is > > safer than failing to reduce privileges - if the reduced privileges are not > > available". > > Right. > > > Note that there is room for a race that could have unintended consequences. > > > > Between the 'setuid(ordinary-user)' and a subsequent 'exit()' after execve() > > has failed, any other process owned by the same user (and we know where are > > quite a few) would fail an execve() where it really should not. > > It is not obvious to me that this is unintended, and that dealing with > it in some way makes much of a difference. (Also, it's not exactly "any > other process owned by the same user" - this only affects processes that > also run with similar or lower RLIMIT_NPROC. So, for example, if a web > server is set to use RLIMIT_NPROC of 30, but interactive logins use 40, > then the latter may succeed and allow for shell commands to succeed. > This is actually a common combination of settings that we've been using > on some systems for years.) I don't think it can be intended to cause 'execve' to fail when a user is at the NPROC limit - except in the specific case that the process has previously called setuid. So I feel justified in calling it an unintended consequence. It my not be a very common consequence but but we all know that uncommon things do happen. I agree that having different limits for different cases could make this much less of a problem, but it doesn't necessarily remove it. > > > I think it would be safer to add a test for PF_SUPERPRIV and PF_FORKNOEXEC > > in current->flags and only fail the execve if both are set. > > i.e. > > (current->flags & (PF_SUPERPRIV|PF_FORKNOEXEC)) == (PF_SUPERPRIV|PF_FORKNOEXEC) > > > > That should narrow it down to only failing in the particular case that we are > > interested in. > > That's a curious idea, and apparently this is what NetBSD does, but > unfortunately it does not match a common use case that we are interested > in - specifically, Apache with suEXEC (which is part of the Apache > distribution). Here's what happens: > > httpd runs as non-root. It forks, execs suexec (SUID root). suexec > calls setuid() to the target non-root user and execve() on the CGI > program (script, interpreter, whatever). > > Notice how the fork() and the setuid() are separated by execve() of > suexec itself. Thus, we need to apply the RLIMIT_NPROC check on > execve() unconditionally (well, we may allow processes with > CAP_SYS_RESOURCE to proceed despite of the failed check, like it's > done in -ow patches), or at least not on the condition proposed above. > > Alexander Yes, the PF_FORKNOEXEC test causes problems in that case. Using just the PF_SUPERPRIV test would still be a good idea I think, but would not be quite as thorough a check. Adding a new PF flag would be possible (there seem to be 3 unused) but is probably not justified. NeilBrown -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
