Re: [PATCH RFC v3 30/35] arm64: mte: ptrace: Handle pages with missing tag storage

[Alexandru Elisei <alexandru.elisei@arm.com>]

Hi,

On Thu, Feb 01, 2024 at 02:51:39PM +0530, Anshuman Khandual wrote:
> 
> 
> On 1/25/24 22:12, Alexandru Elisei wrote:
> > A page can end up mapped in a MTE enabled VMA without the corresponding tag
> > storage block reserved. Tag accesses made by ptrace in this case can lead
> > to the wrong tags being read or memory corruption for the process that is
> > using the tag storage memory as data.
> > 
> > Reserve tag storage by treating ptrace accesses like a fault.
> > 
> > Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
> > ---
> > 
> > Changes since rfc v2:
> > 
> > * New patch, issue reported by Peter Collingbourne.
> > 
> >  arch/arm64/kernel/mte.c | 26 ++++++++++++++++++++++++--
> >  1 file changed, 24 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
> > index faf09da3400a..b1fa02dad4fd 100644
> > --- a/arch/arm64/kernel/mte.c
> > +++ b/arch/arm64/kernel/mte.c
> > @@ -412,10 +412,13 @@ static int __access_remote_tags(struct mm_struct *mm, unsigned long addr,
> >  	while (len) {
> >  		struct vm_area_struct *vma;
> >  		unsigned long tags, offset;
> > +		unsigned int fault_flags;
> > +		struct page *page;
> > +		vm_fault_t ret;
> >  		void *maddr;
> > -		struct page *page = get_user_page_vma_remote(mm, addr,
> > -							     gup_flags, &vma);
> >  
> > +get_page:
> > +		page = get_user_page_vma_remote(mm, addr, gup_flags, &vma);
> 
> But if there is valid page returned here in the first GUP attempt, will there
> still be a subsequent handle_mm_fault() on the same vma and addr ?

Only if it's missing tag storage. If it's missing tag storage, the page has
been mapped as arch_fault_on_access_pte(), and
handle_mm_fault()->..->arch_handle_folio_fault_on_access() will either
reserve tag storage, or migrate it.

> 
> >  		if (IS_ERR(page)) {
> >  			err = PTR_ERR(page);
> >  			break;
> > @@ -433,6 +436,25 @@ static int __access_remote_tags(struct mm_struct *mm, unsigned long addr,
> >  			put_page(page);
> >  			break;
> >  		}
> > +
> > +		if (tag_storage_enabled() && !page_tag_storage_reserved(page)) {
> 
> Should not '!page' be checked here as well ?

I was under the impression that get_user_page_vma_remote() returns an error
pointer if gup couldn't pin the page.

Thanks,
Alex

> 
> > +			fault_flags = FAULT_FLAG_DEFAULT | \
> > +				      FAULT_FLAG_USER | \
> > +				      FAULT_FLAG_REMOTE | \
> > +				      FAULT_FLAG_ALLOW_RETRY | \
> > +				      FAULT_FLAG_RETRY_NOWAIT;
> > +			if (write)
> > +				fault_flags |= FAULT_FLAG_WRITE;
> > +
> > +			put_page(page);
> > +			ret = handle_mm_fault(vma, addr, fault_flags, NULL);
> > +			if (ret & VM_FAULT_ERROR) {
> > +				err = -EFAULT;
> > +				break;
> > +			}
> > +			goto get_page;
> > +		}
> > +
> >  		WARN_ON_ONCE(!page_mte_tagged(page));
> >  
> >  		/* limit access to the end of the page */