Re: [PATCH] arm64/mm: fold check for KFENCE into can_set_direct_map()

[Mike Rapoport]

Hi Anshuman,

On Wed, Sep 21, 2022 at 05:09:19PM +0530, Anshuman Khandual wrote:
> 
> 
> On 9/21/22 13:18, Mike Rapoport wrote:
> > From: Mike Rapoport <rppt@xxxxxxxxxxxxx>
> > 
> > KFENCE requires linear map to be mapped at page granularity, so that it
> > is possible to protect/unprotect single pages, just like with
> > rodata_full and DEBUG_PAGEALLOC.
> > 
> > Instead of repating
> > 
> > 	can_set_direct_map() || IS_ENABLED(CONFIG_KFENCE)
> > 
> > make can_set_direct_map() handle the KFENCE case.
> > 
> > This also prevents potential false positives in kernel_page_present()
> > that may return true for non-present page if CONFIG_KFENCE is enabled.
> > 
> > Signed-off-by: Mike Rapoport <rppt@xxxxxxxxxxxxx>
> > ---
> >  arch/arm64/mm/mmu.c      | 8 ++------
> >  arch/arm64/mm/pageattr.c | 8 +++++++-
> >  2 files changed, 9 insertions(+), 7 deletions(-)
> > 
> > diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> > index e7ad44585f40..c5065abec55a 100644
> > --- a/arch/arm64/mm/mmu.c
> > +++ b/arch/arm64/mm/mmu.c
> > @@ -535,7 +535,7 @@ static void __init map_mem(pgd_t *pgdp)
> >  	 */
> >  	BUILD_BUG_ON(pgd_index(direct_map_end - 1) == pgd_index(direct_map_end));
> >  
> > -	if (can_set_direct_map() || IS_ENABLED(CONFIG_KFENCE))
> > +	if (can_set_direct_map())
> >  		flags |= NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS;
> >  
> >  	/*
> > @@ -1547,11 +1547,7 @@ int arch_add_memory(int nid, u64 start, u64 size,
> >  
> >  	VM_BUG_ON(!mhp_range_allowed(start, size, true));
> >  
> > -	/*
> > -	 * KFENCE requires linear map to be mapped at page granularity, so that
> > -	 * it is possible to protect/unprotect single pages in the KFENCE pool.
> > -	 */
> > -	if (can_set_direct_map() || IS_ENABLED(CONFIG_KFENCE))
> > +	if (can_set_direct_map())
> >  		flags |= NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS;
> >  
> >  	__create_pgd_mapping(swapper_pg_dir, start, __phys_to_virt(start),
> > diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c
> > index 64e985eaa52d..d107c3d434e2 100644
> > --- a/arch/arm64/mm/pageattr.c
> > +++ b/arch/arm64/mm/pageattr.c
> > @@ -21,7 +21,13 @@ bool rodata_full __ro_after_init = IS_ENABLED(CONFIG_RODATA_FULL_DEFAULT_ENABLED
> >  
> >  bool can_set_direct_map(void)
> >  {
> > -	return rodata_full || debug_pagealloc_enabled();
> > +	/*
> > +	 * rodata_full, DEBUG_PAGEALLOC and KFENCE require linear map to be
> > +	 * mapped at page granularity, so that it is possible to
> > +	 * protect/unprotect single pages.
> > +	 */
> > +	return rodata_full || debug_pagealloc_enabled() ||
> > +		IS_ENABLED(CONFIG_KFENCE);
> >  }
> 
> Changing can_set_direct_map() also changes behaviour for other functions such as 
> 
> set_direct_map_default_noflush()
> set_direct_map_invalid_noflush()
> __kernel_map_pages()
> 
> Is that okay ?
 
Yes. Since KFENCE disables block mappings, these will actually change the
page tables.
Actually, before this change the test for can_set_direct_map() in these
functions was false negative when CONFIG_KFENCE=y
  
> >  static int change_page_range(pte_t *ptep, unsigned long addr, void *data)

-- 
Sincerely yours,
Mike.