On Tue, Aug 01, 2023 at 05:07:00PM +0000, Edgecombe, Rick P wrote: > On Tue, 2023-08-01 at 15:01 +0100, Mark Brown wrote: > > > It could be a different flag, like SHADOW_STACK_SET_TOKEN_MARKER, > > > or it > > > could be SHADOW_STACK_SET_MARKER, and callers could pass > > > (SHADOW_STACK_SET_TOKEN | SHADOW_STACK_SET_MARKER) to get what you > > > have > > > implemented here. What do you think? > > For arm64 code this would mean that it would be possible (and fairly > > easy) to create stacks which don't have a termination record which > > would > > make life harder for unwinders to rely on. I don't think this is > > insurmountable, creating manually shouldn't be the standard and it'll > > already be an issue on x86 anyway. > If you are going to support optionally writing to shadow stacks (which > x86 needed for CRIU, and also seems like a nice thing for several other > reasons), you are already at that point. Can't you also do a bunch of > gcspopm's to the top of the GCS stack, and have no marker to hit before > the end of the stack? (maybe not in GCS, I don't know...) It's definitely possible to use writes or pops to achive the same effect, it's just that it's less likely to be something that happens through simple oversight than missing a flag off the initial map call would be. > > The other minor issue is that the current arm64 marker is all bits 0 > > so by itself for arm64 _MARKER would have no perceptible impact, it > > would only serve to push the token down a slot in the stack (I'm > > guessing that's the intended meaning?). > Pushing the token down a frame is what flags==0 does in this patch, > right? Yes, exactly - if we make the top of stack record optional then if that flag is omitted we'd not do that. > You don't have to support all the flags actually, you could just > support the one mode you already have and reject all other > combinations... Then it matches between arch's, and you still have the > guaranteed-ish end marker. Sure, though if we're going to the trouble of checking for the flag we probably may as well implement it. I guess x86 is locked in at this point by existing userspace. I guess I'll implement it assuming nobody from userspace complains, it's trivial for a kernel. > So the question is not what mode should arm support, but should we have > the flags match between x86 and ARM? The flags should definitely match, no disagreement there.
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