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?