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Hi! > > You contradict yourself. > > I don't see how that is. I understand that you could be unconvinced > by my reasoning and by my arguments, but I don't see how they are > contradictory. Well, you claim it is as safe as possible, and it is not quite. > The bottom line is that, whereas for root making syscalls fail (or, > worse, in the case of setuid(), behave subtly diffently) is a radical > change, for non-root it is something which should always be expected > (fork() can fail for lack of resources, write() can fail for quota > exhaution, etc.), and not something an attacker should be able to > exploit. I can bet someone will get the fork() case wrong: f = fork(); kill(f); fork will return -1, and kill will kill _all_ the processes. > > Yes, you are decreasing security of suid > > non-root programs, and yes, someone will take advantage of that. Plus, > > you can easily do away without this risk. > > I wish I could offer more assurance, but unfortunately the solutions > which do away with the risk come with a great cost: > > > Just add all "usual" capabilities when execing > > suid/sgid-anything. > > This makes it trivial to regain capabilities: just create a program > suid yourself and exec it. OK, we can say that "yourself" won't work, > but you still only need to find another uid to hijack... Not too If you can find another uid to hijack, that other uid has bad problems. And I do not think you'll commonly find another uid to hijack. And there are easier ways to get out of jail with your proposed capabilities: you do not restrict ptrace, so you can just ptrace any other process with same uid, and hijack it. (You probably want to introduce CAP_REG_PTRACE). Or just remove CAP_REG_XUID_EXEC when removing any other CAP_REG...? > > Alternatively disallow suid/sgid-anything exec > > when all "usual" capabilities are not present. > > This is probably too stringent: remove any trivial capability > whatsoever and you lose a rather important ability. It is not too bad; you'll usually not want restricted programs to exec anything setuid... (Do you have example where restricted-but-should-be-able-to-setuid-exec makes sense?) Pavel -- (english) http://www.livejournal.com/~pavelmachek (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ | ||||||||||||
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