Re: [PATCH v23 14/28] x86/mm: Shadow Stack page fault error checking

[Yu, Yu-cheng]

On 3/22/2021 3:38 AM, Kirill A. Shutemov wrote:
> On Tue, Mar 16, 2021 at 08:10:40AM -0700, Yu-cheng Yu wrote:
[...]
> >   
> > diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> > index a73347e2cdfc..4316732a18c6 100644
> > --- a/arch/x86/mm/fault.c
> > +++ b/arch/x86/mm/fault.c
> > @@ -1100,6 +1100,17 @@ access_error(unsigned long error_code, struct vm_area_struct *vma)
> >   				       (error_code & X86_PF_INSTR), foreign))
> >   		return 1;
> >   
> > +	/*
> > +	 * Verify a shadow stack access is within a shadow stack VMA.
> > +	 * It is always an error otherwise.  Normal data access to a
> > +	 * shadow stack area is checked in the case followed.
> > +	 */
> > +	if (error_code & X86_PF_SHSTK) {
> > +		if (!(vma->vm_flags & VM_SHSTK))
> > +			return 1;
> > +		return 0;
>
> Any reason to return 0 here? I would rather keep the single return 0 in
> the function, after all checks are done.

For shadow stack fault, X86_PF_SHSTK and X86_PF_WRITE both can be set. 
So for shadow stack fault, return from here and don't go into the normal 
write fault case.

Thanks,
Yu-cheng

> > +	}
> > +
> >   	if (error_code & X86_PF_WRITE) {
> >   		/* write, present and write, not present: */
> >   		if (unlikely(!(vma->vm_flags & VM_WRITE)))
> > @@ -1293,6 +1304,14 @@ void do_user_addr_fault(struct pt_regs *regs,
> >   
> >   	perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, address);
> >   
> > +	/*
> > +	 * Clearing _PAGE_DIRTY is used to detect shadow stack access.
> > +	 * This method cannot distinguish shadow stack read vs. write.
> > +	 * For valid shadow stack accesses, set FAULT_FLAG_WRITE to effect
> > +	 * copy-on-write.
> > +	 */
> > +	if (error_code & X86_PF_SHSTK)
> > +		flags |= FAULT_FLAG_WRITE;
> >   	if (error_code & X86_PF_WRITE)
> >   		flags |= FAULT_FLAG_WRITE;
> >   	if (error_code & X86_PF_INSTR)
> > --
> > 2.21.0