Re: [PATCH v2 2/5] mm: add PTE_MARKER_GUARD PTE marker

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Mon, Oct 21, 2024 at 03:45:31PM +0200, Vlastimil Babka wrote:
> On 10/20/24 18:20, Lorenzo Stoakes wrote:
> > Add a new PTE marker that results in any access causing the accessing
> > process to segfault.
>
> Should we distinguish it from other segfaults? Is there a way? I can see
> memory protection keys use SEGV_PKUERR, but no idea if we have any free values.

Wasn't even aware that existed!!

I'm not sure a process can do anything particularly useful with this
information though?  Hitting a guard page would indicate a programming
error rather than something that might allow meaningful feedback to a user
like memory protection keys.

Do you think there's enough value int his to warrant digging in? And indeed
I imagine bits are in short supply for this and would need a strong
argument to get... so yeah I don't think too worthwhile most likely!

Thanks for the suggestion though!

>
> > This is preferable to PTE_MARKER_POISONED, which results in the same
> > handling as hardware poisoned memory, and is thus undesirable for cases
> > where we simply wish to 'soft' poison a range.
> >
> > This is in preparation for implementing the ability to specify guard pages
> > at the page table level, i.e. ranges that, when accessed, should cause
> > process termination.
> >
> > Additionally, rename zap_drop_file_uffd_wp() to zap_drop_markers() - the
> > function checks the ZAP_FLAG_DROP_MARKER flag so naming it for this single
> > purpose was simply incorrect.
> >
> > We then reuse the same logic to determine whether a zap should clear a
> > guard entry - this should only be performed on teardown and never on
> > MADV_DONTNEED or the like.
> >
> > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
>
> Acked-by: Vlastimil Babka <vbabka@suse.cz>
>
> A nit below:
>
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index 906294ac85dc..50e3f6ed73ac 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -6353,6 +6353,9 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
> >  				ret = VM_FAULT_HWPOISON_LARGE |
> >  				      VM_FAULT_SET_HINDEX(hstate_index(h));
> >  				goto out_mutex;
> > +			} else if (marker & PTE_MARKER_GUARD) {
> > +				ret = VM_FAULT_SIGSEGV;
> > +				goto out_mutex;
>
> Given we don't support hugetlb, should we WARN_ON_ONCE() if such unexpected
> marker appears there?
>
> >  			}
> >  		}
> >
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 0f614523b9f4..551455cd453f 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -1455,7 +1455,7 @@ static inline bool should_zap_folio(struct zap_details *details,
> >  	return !folio_test_anon(folio);
> >  }
> >
> > -static inline bool zap_drop_file_uffd_wp(struct zap_details *details)
> > +static inline bool zap_drop_markers(struct zap_details *details)
> >  {
> >  	if (!details)
> >  		return false;
> > @@ -1476,7 +1476,7 @@ zap_install_uffd_wp_if_needed(struct vm_area_struct *vma,
> >  	if (vma_is_anonymous(vma))
> >  		return;
> >
> > -	if (zap_drop_file_uffd_wp(details))
> > +	if (zap_drop_markers(details))
> >  		return;
> >
> >  	for (;;) {
> > @@ -1671,7 +1671,15 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
> >  			 * drop the marker if explicitly requested.
> >  			 */
> >  			if (!vma_is_anonymous(vma) &&
> > -			    !zap_drop_file_uffd_wp(details))
> > +			    !zap_drop_markers(details))
> > +				continue;
> > +		} else if (is_guard_swp_entry(entry)) {
> > +			/*
> > +			 * Ordinary zapping should not remove guard PTE
> > +			 * markers. Only do so if we should remove PTE markers
> > +			 * in general.
> > +			 */
> > +			if (!zap_drop_markers(details))
> >  				continue;
> >  		} else if (is_hwpoison_entry(entry) ||
> >  			   is_poisoned_swp_entry(entry)) {
> > @@ -4003,6 +4011,10 @@ static vm_fault_t handle_pte_marker(struct vm_fault *vmf)
> >  	if (marker & PTE_MARKER_POISONED)
> >  		return VM_FAULT_HWPOISON;
> >
> > +	/* Hitting a guard page is always a fatal condition. */
> > +	if (marker & PTE_MARKER_GUARD)
> > +		return VM_FAULT_SIGSEGV;
> > +
> >  	if (pte_marker_entry_uffd_wp(entry))
> >  		return pte_marker_handle_uffd_wp(vmf);
> >
>