On Thu, May 4, 2023 at 10:06 PM Peter Xu <peterx@xxxxxxxxxx> wrote: > > On Thu, May 04, 2023 at 07:09:38PM +0200, Florent Revest wrote: > > Joey recently introduced a Memory-Deny-Write-Executable (MDWE) prctl which tags > > current with a flag that prevents pages that were previously not executable from > > becoming executable. > > This tag always gets inherited by children tasks. (it's in MMF_INIT_MASK) > > > > At Google, we've been using a somewhat similar downstream patch for a few years > > now. To make the adoption of this feature easier, we've had it support a mode in > > which the W^X flag does not propagate to children. For example, this is handy if > > a C process which wants W^X protection suspects it could start children > > processes that would use a JIT. > > > > I'd like to align our features with the upstream prctl. This series proposes a > > new NO_INHERIT flag to the MDWE prctl to make this kind of adoption easier. It > > sets a different flag in current that is not in MMF_INIT_MASK and which does not > > propagate. > > I don't think I have enough context, so sorry if I'm going to ask a naive > question.. Not at all! :) You're absolutely right, it's important to address these points. > I can understand how current MDWE helps on not allowing any modifi-able > content from becoming executable. How could NO_INHERIT help if it won't > inherit and not in MMF_INIT_MASK? The way I see it, enabling MDWE is just a small step towards hardening a binary anyway. It can possibly make exploitation a bit harder in the case where the attacker has _just_: a write primitive they can use to write a shellcode somewhere and a primitive to make that page executable later. It's a fairly narrow protection already and I think it only really helps as part of a broader "defense in depth" strategy. > IIUC it means the restriction will only apply to the current process. Then > I assume the process can escape from this rule simply by a fork(). If so, > what's the point to protect at all? If we assume enough control from the attacker, then MDWE is already useless since it can be bypassed by writing to a file and then mmapping that file with PROT_EXEC. I think that's a good example of how "perfect can be the enemy of good" in security hardening. MDWE isn't a silver-bullet but it's a cheap trick and it makes a small dent in reducing the attack surface so it seems worth having anyway ? But indeed, to address your question, if you choose to use this NO_INHERIT flag: you're no longer protected if the attacker can fork() as part of their exploitation. I think it's been a useful trade-off for our internal users since, on the other hand, it also makes adoption a lot easier: our C++ services developers can trivially opt into a potpourri of hardening features without having to think too much about how they work under-the-hood. The default behavior has been to use a NO_INHERIT strategy so users don't get bad surprises the day when they try to spawn a JITted subcommand. In the meantime, their C++ service still gets a little bit of extra protection. > And, what's the difference of this comparing to disabling MDWE after being > enabled (which seems to be forbidden for now, but it seems fork() can play > a similar role of disabling it)? That would be functionally somewhat similar, yes. I think it mostly comes down to ease of adoption. I imagine that users who would opt into NO_INHERIT are those who are interested in MDWE for the binary they are writing but aren't 100% confident in what subprocesses they will run and so they don't have to think about disabling it after every fork.
