Re: [PATCHv5 06/13] x86/mm: Provide ARCH_GET_UNTAG_MASK and ARCH_ENABLE_TAGGED_ADDR

[Alexander Potapenko <glider@google.com>]

On Wed, Jul 20, 2022 at 2:47 PM Kirill A. Shutemov
<kirill.shutemov@linux.intel.com> wrote:
>
> On Wed, Jul 20, 2022 at 10:19:36AM +0200, Alexander Potapenko wrote:
> > > > >  long do_arch_prctl_64(struct task_struct *task, int option, unsigned long arg2)
> > > > >  {
> > > > >         int ret = 0;
> > > > > @@ -829,7 +883,11 @@ long do_arch_prctl_64(struct task_struct *task, int option, unsigned long arg2)
> > > > >         case ARCH_MAP_VDSO_64:
> > > > >                 return prctl_map_vdso(&vdso_image_64, arg2);
> > > > >  #endif
> > > > > -
> > > > > +       case ARCH_GET_UNTAG_MASK:
> > > > > +               return put_user(task->mm->context.untag_mask,
> > > > > +                               (unsigned long __user *)arg2);
> > > >
> > > > Can we have ARCH_GET_UNTAG_MASK return the same error value (ENODEV or
> > > > EINVAL) as ARCH_ENABLE_TAGGED_ADDR in the case the host doesn't
> > > > support LAM?
> > > > After all, the mask does not make much sense in this case.
> > >
> > > I'm not sure about this.
> > >
> > > As it is ARCH_GET_UNTAG_MASK returns -1UL mask if LAM is not present or
> > > not enabled. Applying this mask will give correct result for both.
> >
> > Is anyone going to use this mask if tagging is unsupported?
> > Tools like HWASan won't even try to proceed in that case.
>
> I can imagine the code that tries to be indifferent to whether a pointer
> has tags. It gets mask from ARCH_GET_UNTAG_MASK and applies it to the
> pointer without any conditions.

In that case there would still be just one call to ARCH_GET_UNTAG_MASK
to get the mask that will probably be applied many times.
So there's not a big difference with checking for -ENODEV and setting
that mask manually.
But your proposal with a special arch_prctl indeed looks cleaner.

> > > Why is -ENODEV better here? Looks like just more work for userspace.
> >
> > This boils down to the question of detecting LAM support I raised previously.
> > It's nice to have a syscall without side effects to check whether LAM
> > can be enabled at all (e.g. one can do the check in the parent process
> > and conditionally enable LAM in certain, but not all, child processes)
> > CPUID won't help here, because the presence of the LAM bit in CPUID
> > doesn't guarantee its support in the kernel, and every other solution
> > is more complicated than just issuing a system call.
> >
> > Note that TBI has PR_GET_TAGGED_ADDR_CTRL, which can be used to detect
> > the presence of memory tagging support.
>
> I would rather make enumeration explicit:

Ok, this would also work. Thanks!

> diff --git a/arch/x86/include/uapi/asm/prctl.h b/arch/x86/include/uapi/asm/prctl.h
> index 38164a05c23c..a31e27b95b19 100644
> --- a/arch/x86/include/uapi/asm/prctl.h
> +++ b/arch/x86/include/uapi/asm/prctl.h
> @@ -22,5 +22,6 @@
>
>  #define ARCH_GET_UNTAG_MASK            0x4001
>  #define ARCH_ENABLE_TAGGED_ADDR                0x4002
> +#define ARCH_GET_MAX_TAG_BITS          0x4003
>
>  #endif /* _ASM_X86_PRCTL_H */
> diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
> index cfa2e42a135a..2e4df63b775f 100644
> --- a/arch/x86/kernel/process_64.c
> +++ b/arch/x86/kernel/process_64.c
> @@ -911,6 +911,13 @@ long do_arch_prctl_64(struct task_struct *task, int option, unsigned long arg2)
>                                 (unsigned long __user *)arg2);
>         case ARCH_ENABLE_TAGGED_ADDR:
>                 return prctl_enable_tagged_addr(task->mm, arg2);
> +       case ARCH_GET_MAX_TAG_BITS:
> +               if (!cpu_feature_enabled(X86_FEATURE_LAM))
> +                       return put_user(0, (unsigned long __user *)arg2);
> +               else if (lam_u48_allowed())
> +                       return put_user(15, (unsigned long __user *)arg2);
> +               else
> +                       return put_user(6, (unsigned long __user *)arg2);
>         default:
>                 ret = -EINVAL;
>                 break;
> --
>  Kirill A. Shutemov



-- 
Alexander Potapenko
Software Engineer

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