Re: [PATCH 14/15] mm: Align THP mappings for non-DAX

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Tue, Oct 01, 2019 at 06:18:28AM -0600, William Kucharski wrote:
> 
> 
> On 10/1/19 5:32 AM, Kirill A. Shutemov wrote:
> > On Tue, Oct 01, 2019 at 05:21:26AM -0600, William Kucharski wrote:
> > > 
> > > 
> > > > On Oct 1, 2019, at 4:45 AM, Kirill A. Shutemov <kirill@shutemov.name> wrote:
> > > > 
> > > > On Tue, Sep 24, 2019 at 05:52:13PM -0700, Matthew Wilcox wrote:
> > > > > 
> > > > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > > > > index cbe7d0619439..670a1780bd2f 100644
> > > > > --- a/mm/huge_memory.c
> > > > > +++ b/mm/huge_memory.c
> > > > > @@ -563,8 +563,6 @@ unsigned long thp_get_unmapped_area(struct file *filp, unsigned long addr,
> > > > > 
> > > > > 	if (addr)
> > > > > 		goto out;
> > > > > -	if (!IS_DAX(filp->f_mapping->host) || !IS_ENABLED(CONFIG_FS_DAX_PMD))
> > > > > -		goto out;
> > > > > 
> > > > > 	addr = __thp_get_unmapped_area(filp, len, off, flags, PMD_SIZE);
> > > > > 	if (addr)
> > > > 
> > > > I think you reducing ASLR without any real indication that THP is relevant
> > > > for the VMA. We need to know if any huge page allocation will be
> > > > *attempted* for the VMA or the file.
> > > 
> > > Without a properly aligned address the code will never even attempt allocating
> > > a THP.
> > > 
> > > I don't think rounding an address to one that would be properly aligned to map
> > > to a THP if possible is all that detrimental to ASLR and without the ability to
> > > pick an aligned address it's rather unlikely anyone would ever map anything to
> > > a THP unless they explicitly designate an address with MAP_FIXED.
> > > 
> > > If you do object to the slight reduction of the ASLR address space, what
> > > alternative would you prefer to see?
> > 
> > We need to know by the time if THP is allowed for this
> > file/VMA/process/whatever. Meaning that we do not give up ASLR entropy for
> > nothing.
> > 
> > For instance, if THP is disabled globally, there is no reason to align the
> > VMA to the THP requirements.
> 
> I understand, but this code is in thp_get_unmapped_area(), which is only called
> if THP is configured and the VMA can support it.
> 
> I don't see it in Matthew's patchset, so I'm not sure if it was inadvertently
> missed in his merge or if he has other ideas for how it would eventually be
> called, but in my last patch revision the code calling it in do_mmap()
> looked like this:
> 
> #ifdef CONFIG_RO_EXEC_FILEMAP_HUGE_FAULT_THP
>         /*
>          * If THP is enabled, it's a read-only executable that is
>          * MAP_PRIVATE mapped, the length is larger than a PMD page
>          * and either it's not a MAP_FIXED mapping or the passed address is
>          * properly aligned for a PMD page, attempt to get an appropriate
>          * address at which to map a PMD-sized THP page, otherwise call the
>          * normal routine.
>          */
>         if ((prot & PROT_READ) && (prot & PROT_EXEC) &&
>                 (!(prot & PROT_WRITE)) && (flags & MAP_PRIVATE) &&
>                 (!(flags & MAP_FIXED)) && len >= HPAGE_PMD_SIZE) {

len and MAP_FIXED is already handled by thp_get_unmapped_area().

	if (prot & (PROT_READ|PROT_WRITE|PROT_READ) == (PROT_READ|PROT_EXEC) &&
		(flags & MAP_PRIVATE)) {


>                 addr = thp_get_unmapped_area(file, addr, len, pgoff, flags);
> 
>                 if (addr && (!(addr & ~HPAGE_PMD_MASK))) {

This check is broken.

For instance, if pgoff is one, (addr & ~HPAGE_PMD_MASK) has to be equal to
PAGE_SIZE to have chance to get a huge page in the mapping.

>                         /*
>                          * If we got a suitable THP mapping address, shut off
>                          * VM_MAYWRITE for the region, since it's never what
>                          * we would want.
>                          */
>                         vm_maywrite = 0;

Wouldn't it break uprobe, for instance?

>                 } else
>                         addr = get_unmapped_area(file, addr, len, pgoff, flags);
>         } else {
> #endif
> 
> So I think that meets your expectations regarding ASLR.
> 
>    -- Bill

-- 
 Kirill A. Shutemov