linux-kernel - Re: [PATCH v7 2/4] mseal: add mseal syscall

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Date: Wed, 24 Jan 2024 18:04:24 -0800
From: Jeff Xu <jeffxu@...omium.org>
To: "Liam R. Howlett" <Liam.Howlett@...cle.com>, Jeff Xu <jeffxu@...omium.org>, 
	akpm@...ux-foundation.org, keescook@...omium.org, jannh@...gle.com, 
	sroettger@...gle.com, willy@...radead.org, gregkh@...uxfoundation.org, 
	torvalds@...ux-foundation.org, usama.anjum@...labora.com, 
	rdunlap@...radead.org, jeffxu@...gle.com, jorgelo@...omium.org, 
	groeck@...omium.org, linux-kernel@...r.kernel.org, 
	linux-kselftest@...r.kernel.org, linux-mm@...ck.org, pedro.falcato@...il.com, 
	dave.hansen@...el.com, linux-hardening@...r.kernel.org, deraadt@...nbsd.org
Subject: Re: [PATCH v7 2/4] mseal: add mseal syscall

On Wed, Jan 24, 2024 at 2:49 PM Jeff Xu <jeffxu@...omium.org> wrote:
>
> On Wed, Jan 24, 2024 at 12:06 PM Liam R. Howlett
> <Liam.Howlett@...cle.com> wrote:
> >
> > > Considering this is the MAP_FIXED case, and maybe that is not used
> > > that often in practice, I think this is acceptable performance-wise,
> > > unless you know another solution to help this.
> >
> > Okay, sure, I haven't been yelled at on the ML for a few weeks.  Here
> > goes:
> >
> > do_mmap() will call get_unmapped_area(), which will return an empty area
> > (no need to check mseal, I hope - or we have larger issues here) or a
> > MAP_FIXED address.
> >
> > do_mmap() will pass the address along to mmap_region()
> >
> > mmap_region() will then call do_vmi_munmap() - which will either remove
> > the VMA(s) in the way, or do nothing... or error.
> >
> > mmap_region() will return -ENOMEM in the case of an error returned from
> > do_vmi_munmap() today.  Change that to return the error code, and let
> > do_vmi_munmap() do the mseal check.  If mseal check fails then the error
> > is propagated the same way -ENOMEM is propagated today.
> >
> > This relies on the fact that we only really need to check the mseal
> > status of existing VMAs and we can only really map over existing VMAs by
> > first munmapping them.
> >
> > It does move your error return to much later in the call stack, but it
> > removes duplicate work and less code.  Considering this should be a rare
> > event, I don't think that's of concern.
> >
> I think that is a great idea, I will try to implement it and get back
> to you on this.
>
I confirm this works. I will add that in the next version. Thanks for
the suggestion.

-Jeff