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

[Kirill A. Shutemov]

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.

> > 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:

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