Dave Martin <Dave.Martin@xxxxxxx> writes: > On Mon, Oct 21, 2024 at 12:06:25PM +0200, Kevin Brodsky wrote: >> On 17/10/2024 17:48, Dave Martin wrote: >> > On Thu, Oct 17, 2024 at 02:39:04PM +0100, Kevin Brodsky wrote: >> >> This series is a follow-up to Joey's Permission Overlay Extension (POE) >> >> series [1] that recently landed on mainline. The goal is to improve the >> >> way we handle the register that governs which pkeys/POIndex are >> >> accessible (POR_EL0) during signal delivery. As things stand, we may >> >> unexpectedly fail to write the signal frame on the stack because POR_EL0 >> >> is not reset before the uaccess operations. See patch 3 for more details >> >> and the main changes this series brings. >> >> >> >> A similar series landed recently for x86/MPK [2]; the present series >> >> aims at aligning arm64 with x86. Worth noting: once the signal frame is >> >> written, POR_EL0 is still set to POR_EL0_INIT, granting access to pkey 0 >> >> only. This means that a program that sets up an alternate signal stack >> >> with a non-zero pkey will need some assembly trampoline to set POR_EL0 >> >> before invoking the real signal handler, as discussed here [3]. >> > This feels a bit bogus (though it's anyway orthogonal to this series). >> >> I'm not very fond of this either. However I believe this is the correct >> first step: bring arm64 in line with x86. Removing all restrictions >> before uaccess and then setting POR_EL0 to POR_EL0_INIT enables >> userspace to use any pkey for the alternate signal stack without an ABI >> change, albeit not in a very comfortable way (if the pkey is not 0). > > I see: we try not to prevent userspace from using whatever pkey it > likes for the alternate signal stack, but we are only permissive for > the bare minimum operations that userspace can't possibly control for > itself (i.e., writing the signal frame). > > This whole thing feels a bit of a botch, though. > > Do we know of anyone actually using a sigaltstack with a pkey other > than 0? Why the urgency? Code relying on an asm shim on x86 is > already nonportable, unless I've misunderstood something, so simply > turning on arm64 pkeys support in the kernel and libc shouldn't break > anything today? (At least, nothing that wasn't asking to be broken.) As far as I know, Chrome plans on using a sigaltstack with a non-zero pkey as part of the V8 CFI and W^X work [0][1][2]. IIUC that was is part of the motivation for the x86 change. I don't know if it's urgent though. I added Stephen on CC who might be able to comment on the current state of things in Chrome. I don't think the code that uses a pkey on a sigaltstack is upstream yet. [0]: https://v8.dev/blog/control-flow-integrity#signal-frame-corruption [1]: https://lore.kernel.org/lkml/CAEAAPHa3g0QwU=DZ2zVCqTCSh-+n2TtVKrQ07LvpwDjQ-F09gA@xxxxxxxxxxxxxx/ [2]: https://docs.google.com/document/d/1O2jwK4dxI3nRcOJuPYkonhTkNQfbmwdvxQMyXgeaRHo > >> >> > Really, we want some way for userspace to tell the kernel what >> > permissions to use for the alternate signal stack and signal handler >> > using it, and then honour that request consistently (just as we try to >> > do for the main stack today). >> > >> > ss_flags is mostly unused... I wonder whether we could add something in >> > there? Or add a sigaltstack2()? >> >> Yes, this would be sensible as a second step (backwards-compatible >> extension). Exactly how that API would look like is not trivial though: >> is the pkey implicitly derived from the pointer provided to >> sigaltstack()? Is there a need to specify another pkey for code, or do >> we just assume that the signal handler is only using code with pkey 0? >> (Not a concern on x86 as MPK doesn't restrict execution.) Would be very >> interested to hear opinions on this. I hadn't considered setting a non-zero pkey for code, but it sounds appealing. The general goal, IIUC, is for signal handlers to run in an isolated "context" using pkeys, in order to mitigate against an attacker trying to corrupt the CPU state on the stack from another thread. Then use this as a way to bypass any CFI mitigation, by setting an arbitrary PC and registers. So sigaltstack+pkey helps with isolating the stack. Then it's up to the programmer to carefully write the signal handler code so it only uses pkey-tagged data that other threads cannot corrupt in order to trick the signal handler into writing to its own stack. In this context, using a non-default pkey for code might be useful, in order to differentiate between "valid" signal handlers and other functions. It could help fend against an attacker being able to use sigaction as a whole-function gadget to install any function as a signal hander. Basically mitigating going from a limited CFI bypass to an arbitrary CFI bypass. That being said, regarding the kernel API, it might be possible to do the above with this patch. We'd be using the proposed "assembly prologues" that sets POR_EL0 as the first thing then continues to the real signal handler. But if we can avoid those and directly ask the kernel what POR_EL0 should be set to, it'd be simpler (and maybe safer). >> >> Kevin > > I would vote for specifying the pkey (or, if feasible, PKRU or > modifications to it) in some bits of ss_flags, or in an additional > flags argument to sigaltstack2(). > > Memory with a non-zero pkey cannot be used 100% portably, period, and > having non-RW(X) permissions on pkey 0 at any time is also not > portable, period. So I'm not sure that having libc magically guess > what userspace's pkeys policy is supposed to be based on racily digging > metadata out of /proc/self/maps or a cache of it etc. would be such a > good idea. > > There are other ways to approach (or not approach) this though -- > I would be interested to hear what other people think too... Thinking about this, I'm not sure about tying this API to sigaltstack, as this is about configuring the POR_EL0 register, which may control more than the stack. I actually have a concrete example of this in V8. There's a SetDefaultPermissionsForSignalHandler [3] function that needs to be called first thing on signal handlers to configure access to an allocated non-zero key. This is independent from having a pkey-tagged sigaltstack or not, but I suppose later on it will need to be replaced with assembly when the stack is no-longer accessible. [3]: https://source.chromium.org/chromium/chromium/src/+/main:v8/include/v8-platform.h;l=665;drc=0abf23ec2a1bb475b1555790fdc72ef630a43c2a;bpv=1;bpt=1 Doing this via sigaction as Catalin suggested makes sense to me, but I'm unsure how we express how POR_EL0 needs to be set solely using SA_* flags. Are we able to add a new architecture-specific payload to sigaction, or would that resort in a new syscall like sigaction2? As an alternative, I was wondering if this would warrant a new syscall like sigaltstack, but for CPU state. Thanks, Pierre
