Re: [PATCH v7 6/7] mseal, system mappings: uprobe mapping

[Oleg Nesterov <oleg@redhat.com>]

On 02/26, Lorenzo Stoakes wrote:
>
> On Wed, Feb 26, 2025 at 05:26:04PM +0100, Oleg Nesterov wrote:
> > On 02/24, jeffxu@chromium.org wrote:
> > >
> > > Unlike other system mappings, the uprobe mapping is not
> > > established during program startup. However, its lifetime is the same
> > > as the process's lifetime. It could be sealed from creation.
> >
> > Agreed, VM_SEALED should be always for the "[uprobes]" vma, regardless
> > of config options.
>
> If you think this ought to be the case generally, then perhaps we should
> drop this patch from the commit and just do this separately as a
> permanent-on thing, if you are sure this is fine + want it?

See below...

> An aside - we _definitely_ cannot allow this -system mapping stuff- to be
> enabled without a config option,

This is clear.

But as for uprobes in particular I do think that VM_SEALED is always fine.

Do we really want it? I dunno. If a task unmaps its "[uprobes]" vma it
will crash when it hits the uprobes bp next time. Unless the probed insn
can be emulated and it is not ret-probe. Do we really care? Again, I don't
know.

Should this change come as a separate patch? I don't understand why it should
but I am fine either way.

In short. please do what you think is right,  VM_SEALED can't hurt uprobes ;)

> > 	#ifdef CONFIG_64BIT
> > 	/* VM is sealed, in vm_flags */
> > 	#define VM_SEALED	_BITUL(63)
> > +	#else
> > +	#define VM_SEALED	0
> > 	#endif
>
> This has been raised a few times. Jeff objects to this

OK,

> > and then simply
> >
> > 	vma = _install_special_mapping(mm, area->vaddr, PAGE_SIZE,
> > -				VM_EXEC|VM_MAYEXEC|VM_DONTCOPY|VM_IO,
> > +				VM_EXEC|VM_MAYEXEC|VM_DONTCOPY|VM_IO|VM_SEALED,
> >
> > ?
>
> Nah you'd have to do:
>
>
> > 	vma = _install_special_mapping(mm, area->vaddr, PAGE_SIZE,
> 				VM_EXEC|VM_MAYEXEC|VM_DONTCOPY|VM_IO
> #ifdef CONFIG_64BIT
> 				VM_SEALED
> #endif
> 				,

Why??? With the proposed change above VM_SEALED == 0 if !CONFIG_64BIT.

Oleg.