Re: [PATCH v10 13/40] arm64/mm: Map pages for guarded control stack

Mark Brown
2024-08-20T15:28:31Z

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On Tue, Aug 20, 2024 at 03:59:21PM +0100, Catalin Marinas wrote:
> On Mon, Aug 19, 2024 at 05:33:24PM +0100, Mark Brown wrote:
> > On Mon, Aug 19, 2024 at 10:10:36AM +0100, Catalin Marinas wrote:

> > > At a quick look, do_mmap() seems to always set VM_MAYEXEC but discard it
> > > for non-executable file mmap. Last time I looked (when doing MTE) there
> > > wasn't a way for the arch code to clear specific VM_* flags, only to
> > > validate them. But I think we should just clear VM_MAYEXEC and also
> > > return an error for VM_EXEC in the core do_mmap() if VM_SHADOW_STACK. It
> > > would cover the other architectures doing shadow stacks.

> > Yes, I think adding something generic would make sense here. That feels
> > like a cleanup which could be split out?

> It can be done separately. It doesn't look like x86 has such checks.
> Adding it generically would be a slight ABI tightening but I doubt it
> matters, no sane software would use an executable shadow stack.

OK.

> > > Is there any arch restriction with setting BTI and GCS? It doesn't make
> > > sense but curious if it matters. We block the exec permission anyway
> > > (unless the BTI pages moved to PIE as well, I don't remember).

> > As you say BTI should be meaningless for a non-executable page like GCS,
> > I'm not aware of any way in which it matters. BTI is separate to PIE.

> My thoughts were whether we can get rid of this hunk entirely by
> handling it in the core code. We'd allow BTI if one wants such useless
> combination but clear VM_MAYEXEC in the core code (and ignore VM_SHARED
> since you can't set it anyway).

I have to admit that the BTI because I was shoving _EXEC in there rather
than because it specifically needed to be blocked. So change the check
for VM_SHARED to a VM_WARN_ON(), and leave the _EXEC check for now
pending the above core change?

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