Re: [PATCH hotfix 6.12 v2 4/8] mm: resolve faulty mmap_region() error path behaviour

[Catalin Marinas <catalin.marinas@arm.com>]

On Tue, Oct 29, 2024 at 03:16:00PM +0000, Lorenzo Stoakes wrote:
> On Tue, Oct 29, 2024 at 03:04:41PM +0000, Catalin Marinas wrote:
> > On Mon, Oct 28, 2024 at 10:14:50PM +0000, Lorenzo Stoakes wrote:
> > > So continue to check VM_MTE_ALLOWED which arch_calc_vm_flag_bits() sets if
> > > MAP_ANON.
> > [...]
> > > diff --git a/mm/shmem.c b/mm/shmem.c
> > > index 4ba1d00fabda..e87f5d6799a7 100644
> > > --- a/mm/shmem.c
> > > +++ b/mm/shmem.c
> > > @@ -2733,9 +2733,6 @@ static int shmem_mmap(struct file *file, struct vm_area_struct *vma)
> > >  	if (ret)
> > >  		return ret;
> > >
> > > -	/* arm64 - allow memory tagging on RAM-based files */
> > > -	vm_flags_set(vma, VM_MTE_ALLOWED);
> >
> > This breaks arm64 KVM if the VMM uses shared mappings for the memory
> > slots (which is possible). We have kvm_vma_mte_allowed() that checks for
> > the VM_MTE_ALLOWED flag as the VMM may not use PROT_MTE/VM_MTE directly.
> 
> Ugh yup missed that thanks.
> 
> > I need to read this thread properly but why not pass the file argument
> > to arch_calc_vm_flag_bits() and set VM_MTE_ALLOWED in there?
> 
> Can't really do that as it is entangled in a bunch of other stuff,
> e.g. calc_vm_prot_bits() would have to pass file and that's used in a bunch
> of places including arch code and... etc. etc.

Not calc_vm_prot_bits() but calc_vm_flag_bits().
arch_calc_vm_flag_bits() is only implemented by two architectures -
arm64 and parisc and calc_vm_flag_bits() is only called from do_mmap().

Basically we want to set VM_MTE_ALLOWED early during the mmap() call
and, at the time, my thinking was to do it in calc_vm_flag_bits(). The
calc_vm_prot_bits() OTOH is also called on the mprotect() path and is
responsible for translating PROT_MTE into a VM_MTE flag without any
checks. arch_validate_flags() would check if VM_MTE comes together with
VM_MTE_ALLOWED. But, as in the KVM case, that's not the only function
checking VM_MTE_ALLOWED.

Since calc_vm_flag_bits() did not take a file argument, the lazy
approach was to add the flag explicitly for shmem (and hugetlbfs in
-next). But I think it would be easier to just add the file argument to
calc_vm_flag_bits() and do the check in the arch code to return
VM_MTE_ALLOWED. AFAICT, this is called before mmap_region() and
arch_validate_flags() (unless I missed something in the recent
reworking).

> I suggest instead we instead don't drop the yucky shmem thing, which will
> set VM_MTE_ALLOWED for shmem, with arch_calc_vm_flag_bits() still setting
> it for MAP_ANON, but the other changes will mean the arch_validate_flags()
> will be fixed too.
> 
> So this just means not dropping the mm/shmem.c bit basically and everything
> should 'just work'?

If we can't get the calc_vm_flag_bits() approach to work, I'm fine with
this as a fix and we'll look to do it properly from 6.13.

-- 
Catalin