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From: Catalin Marinas <catalin.marinas@arm.com>
To: Ard Biesheuvel <ardb@kernel.org>
Cc: Linux ARM <linux-arm-kernel@lists.infradead.org>,
	Marc Zyngier <maz@kernel.org>, Will Deacon <will@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Anshuman Khandual <anshuman.khandual@arm.com>,
	Quentin Perret <qperret@google.com>,
	Android Kernel Team <kernel-team@android.com>
Subject: Re: [PATCH 2/2] arm64: mm: use XN table mapping attributes for the linear region
Date: Fri, 5 Mar 2021 20:37:27 +0000	[thread overview]
Message-ID: <20210305203727.GA5710@arm.com> (raw)
In-Reply-To: <CAMj1kXGHz9bRneJkkEzirTjAuSsatoLMvc+aAXVxgrXbL2XXZg@mail.gmail.com>

On Fri, Mar 05, 2021 at 08:17:07PM +0100, Ard Biesheuvel wrote:
> On Fri, 5 Mar 2021 at 20:06, Catalin Marinas <catalin.marinas@arm.com> wrote:
> > On Thu, Mar 04, 2021 at 06:11:45PM +0100, Ard Biesheuvel wrote:
> > > The way the arm64 kernel virtual address space is constructed guarantees
> > > that swapper PGD entries are never shared between the linear region on
> > > the one hand, and the vmalloc region on the other, which is where all
> > > kernel text, module text and BPF text mappings reside.
> > >
> > > This means that mappings in the linear region (which never require
> > > executable permissions) never share any table entries at any level with
> > > mappings that do require executable permissions, and so we can set the
> > > table-level PXN/UXN attributes for all table entries that are created
> > > while setting up mappings in the linear region. Since swapper's PGD
> > > level page table is mapped r/o itself, this adds another layer of
> > > robustness to the way the kernel manages its own page tables.
> >
> > In ARMv8.1 the architecture added the possibility of disabling the
> > hierarchical page table permissions (FEAT_HPDS) so that we can use these
> > bits for software.
> 
> Sure, but I don't think there is a shortage of software bits in table
> descriptors, right? And we don't enable the feature in the first
> place.

We are short of software bits but in the *pte*, so disabling the
hierarchical permissions doesn't anyway help. So, ignore me, the patches
are fine ;).

-- 
Catalin

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      reply	other threads:[~2021-03-05 20:39 UTC|newest]

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-03-04 17:11 [PATCH 0/2] arm64: use XN table mappings for the linear region Ard Biesheuvel
2021-03-04 17:11 ` [PATCH 1/2] arm64: mm: add missing P4D definitions and use them consistently Ard Biesheuvel
2021-03-04 17:39   ` Mark Rutland
2021-03-08  9:06   ` Anshuman Khandual
2021-03-08  9:07     ` Ard Biesheuvel
2021-03-04 17:11 ` [PATCH 2/2] arm64: mm: use XN table mapping attributes for the linear region Ard Biesheuvel
2021-03-04 17:39   ` Mark Rutland
2021-03-05  8:13     ` Ard Biesheuvel
2021-03-05 19:06   ` Catalin Marinas
2021-03-05 19:17     ` Ard Biesheuvel
2021-03-05 20:37       ` Catalin Marinas [this message]

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