Re: [PATCH v4 1/2] vmalloc: Fix calculation of direct map addr range

["Edgecombe, Rick P" <rick.p.edgecombe@intel.com>]

On Mon, 2019-05-27 at 14:20 +0200, Peter Zijlstra wrote:
> On Tue, May 21, 2019 at 01:51:36PM -0700, Rick Edgecombe wrote:
> > The calculation of the direct map address range to flush was wrong.
> > This could cause problems on x86 if a RO direct map alias ever got
> > loaded
> > into the TLB. This shouldn't normally happen, but it could cause
> > the
> > permissions to remain RO on the direct map alias, and then the page
> > would return from the page allocator to some other component as RO
> > and
> > cause a crash.
> > 
> > So fix fix the address range calculation so the flush will include
> > the
> > direct map range.
> > 
> > Fixes: 868b104d7379 ("mm/vmalloc: Add flag for freeing of special
> > permsissions")
> > Cc: Meelis Roos <mroos@linux.ee>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: "David S. Miller" <davem@davemloft.net>
> > Cc: Dave Hansen <dave.hansen@intel.com>
> > Cc: Borislav Petkov <bp@alien8.de>
> > Cc: Andy Lutomirski <luto@kernel.org>
> > Cc: Ingo Molnar <mingo@redhat.com>
> > Cc: Nadav Amit <namit@vmware.com>
> > Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
> > ---
> >  mm/vmalloc.c | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> > 
> > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > index c42872ed82ac..836888ae01f6 100644
> > --- a/mm/vmalloc.c
> > +++ b/mm/vmalloc.c
> > @@ -2159,9 +2159,10 @@ static void vm_remove_mappings(struct
> > vm_struct *area, int deallocate_pages)
> >  	 * the vm_unmap_aliases() flush includes the direct map.
> >  	 */
> >  	for (i = 0; i < area->nr_pages; i++) {
> > -		if (page_address(area->pages[i])) {
> > +		addr = (unsigned long)page_address(area->pages[i]);
> > +		if (addr) {
> >  			start = min(addr, start);
> > -			end = max(addr, end);
> > +			end = max(addr + PAGE_SIZE, end);
> >  		}
> >  	}
> >  
> 
> Indeed; howevr I'm thinking this bug was caused to exist by the dual
> use
> of @addr in this function, so should we not, perhaps, do something
> like
> the below instead?
> 
> Also; having looked at this, it makes me question the use of
> flush_tlb_kernel_range() in _vm_unmap_aliases() and
> __purge_vmap_area_lazy(), it's potentially combining multiple ranges,
> which never really works well.
> 
> Arguably, we should just do flush_tlb_all() here, but that's for
> another
> patch I'm thinking.

Thanks. It mostly got broken implementing a style suggestion late in
the series. I'll change the addr variable around like you suggest to
make it more resistant.

The flush_tlb_all() suggestion makes sense to me, but I'll leave it for
now.

> ---
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -2123,7 +2123,6 @@ static inline void set_area_direct_map(c
>  /* Handle removing and resetting vm mappings related to the
> vm_struct. */
>  static void vm_remove_mappings(struct vm_struct *area, int
> deallocate_pages)
>  {
> -	unsigned long addr = (unsigned long)area->addr;
>  	unsigned long start = ULONG_MAX, end = 0;
>  	int flush_reset = area->flags & VM_FLUSH_RESET_PERMS;
>  	int i;
> @@ -2135,8 +2134,8 @@ static void vm_remove_mappings(struct vm
>  	 * execute permissions, without leaving a RW+X window.
>  	 */
>  	if (flush_reset && !IS_ENABLED(CONFIG_ARCH_HAS_SET_DIRECT_MAP))
> {
> -		set_memory_nx(addr, area->nr_pages);
> -		set_memory_rw(addr, area->nr_pages);
> +		set_memory_nx((unsigned long)area->addr, area-
> >nr_pages);
> +		set_memory_rw((unsigned long)area->addr, area-
> >nr_pages);
>  	}
>  
>  	remove_vm_area(area->addr);
> @@ -2160,9 +2159,10 @@ static void vm_remove_mappings(struct vm
>  	 * the vm_unmap_aliases() flush includes the direct map.
>  	 */
>  	for (i = 0; i < area->nr_pages; i++) {
> -		if (page_address(area->pages[i])) {
> +		unsigned long addr = (unsigned long)page_address(area-
> >pages[i]);
> +		if (addr) {
>  			start = min(addr, start);
> -			end = max(addr, end);
> +			end = max(addr + PAGE_SIZE, end);
>  		}
>  	}
>