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From: Jeff Xu <jeffxu@chromium.org>
Date: Wed, 24 Jan 2024 18:04:24 -0800
Message-ID: <CABi2SkUxDJXEGVa7gTSPMroX6_Mx8f6HEhp_xBM3PDq6nnt7vw@mail.gmail.com>
Subject: Re: [PATCH v7 2/4] mseal: add mseal syscall
To: "Liam R. Howlett" <Liam.Howlett@oracle.com>, Jeff Xu <jeffxu@chromium.org>, 
	akpm@linux-foundation.org, keescook@chromium.org, jannh@google.com, 
	sroettger@google.com, willy@infradead.org, gregkh@linuxfoundation.org, 
	torvalds@linux-foundation.org, usama.anjum@collabora.com, 
	rdunlap@infradead.org, jeffxu@google.com, jorgelo@chromium.org, 
	groeck@chromium.org, linux-kernel@vger.kernel.org, 
	linux-kselftest@vger.kernel.org, linux-mm@kvack.org, pedro.falcato@gmail.com, 
	dave.hansen@intel.com, linux-hardening@vger.kernel.org, deraadt@openbsd.org
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On Wed, Jan 24, 2024 at 2:49=E2=80=AFPM Jeff Xu <jeffxu@chromium.org> wrote=
:
>
> On Wed, Jan 24, 2024 at 12:06=E2=80=AFPM Liam R. Howlett
> <Liam.Howlett@oracle.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 are=
a
> > (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 erro=
r
> > 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 b=
y
> > 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 rar=
e
> > 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