Re: [PATCH hotfix 6.12 v4 4/5] mm: refactor arch_calc_vm_flag_bits() and arm64 MTE handling

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

On Wed, Oct 30, 2024 at 12:00:12PM +0000, Catalin Marinas wrote:
> On Tue, Oct 29, 2024 at 06:11:47PM +0000, Lorenzo Stoakes wrote:
> > Currently MTE is permitted in two circumstances (desiring to use MTE having
> > been specified by the VM_MTE flag) - where MAP_ANONYMOUS is specified, as
> > checked by arch_calc_vm_flag_bits() and actualised by setting the
> > VM_MTE_ALLOWED flag, or if the file backing the mapping is shmem, in which
> > case we set VM_MTE_ALLOWED in shmem_mmap() when the mmap hook is activated
> > in mmap_region().
> >
> > The function that checks that, if VM_MTE is set, VM_MTE_ALLOWED is also set
> > is the arm64 implementation of arch_validate_flags().
> >
> > Unfortunately, we intend to refactor mmap_region() to perform this check
> > earlier, meaning that in the case of a shmem backing we will not have
> > invoked shmem_mmap() yet, causing the mapping to fail spuriously.
> >
> > It is inappropriate to set this architecture-specific flag in general mm
> > code anyway, so a sensible resolution of this issue is to instead move the
> > check somewhere else.
> >
> > We resolve this by setting VM_MTE_ALLOWED much earlier in do_mmap(), via
> > the arch_calc_vm_flag_bits() call.
> >
> > This is an appropriate place to do this as we already check for the
> > MAP_ANONYMOUS case here, and the shmem file case is simply a variant of the
> > same idea - we permit RAM-backed memory.
> >
> > This requires a modification to the arch_calc_vm_flag_bits() signature to
> > pass in a pointer to the struct file associated with the mapping, however
> > this is not too egregious as this is only used by two architectures anyway
> > - arm64 and parisc.
> >
> > So this patch performs this adjustment and removes the unnecessary
> > assignment of VM_MTE_ALLOWED in shmem_mmap().
> >
> > Suggested-by: Catalin Marinas <catalin.marinas@arm.com>
> > Reported-by: Jann Horn <jannh@google.com>
> > Fixes: deb0f6562884 ("mm/mmap: undo ->mmap() when arch_validate_flags() fails")
> > Cc: stable <stable@kernel.org>
> > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
>
> Thanks for respinning this. FTR,
>
> Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>

Thanks!

>
> > @@ -151,13 +152,13 @@ calc_vm_prot_bits(unsigned long prot, unsigned long pkey)
> >   * Combine the mmap "flags" argument into "vm_flags" used internally.
> >   */
> >  static inline unsigned long
> > -calc_vm_flag_bits(unsigned long flags)
> > +calc_vm_flag_bits(struct file *file, unsigned long flags)
> >  {
> >  	return _calc_vm_trans(flags, MAP_GROWSDOWN,  VM_GROWSDOWN ) |
> >  	       _calc_vm_trans(flags, MAP_LOCKED,     VM_LOCKED    ) |
> >  	       _calc_vm_trans(flags, MAP_SYNC,	     VM_SYNC      ) |
> >  	       _calc_vm_trans(flags, MAP_STACK,	     VM_NOHUGEPAGE) |
> > -	       arch_calc_vm_flag_bits(flags);
> > +		arch_calc_vm_flag_bits(file, flags);
>
> Nitpick (but please ignore, Andrew picked them up already): one space
> alignment off.

Ack yeah, I saw that at the time, didn't quite know how best to resolve as
my editor wanted to put in tabs, but was already mix of tabs + spaces,
which renders different in diff than in the actual code... but in case good
that it's resolvd!

>
> --
> Catalin

Cheers, Lorenzo