From: Peter Collingbourne <pcc@google.com>
To: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>,
Linux ARM <linux-arm-kernel@lists.infradead.org>,
Daniel Kiss <daniel.kiss@arm.com>
Subject: Re: [PATCH 2/2] arm64: Configure kernel's PTR_AUTH key when it is built with PTR_AUTH.
Date: Tue, 8 Dec 2020 11:33:33 -0800 [thread overview]
Message-ID: <CAMn1gO4LMkaV_wkE4MGo3UVhY+A+O3b5Y_0FkWudda3fbeAzhg@mail.gmail.com> (raw)
In-Reply-To: <20201208110030.GA13960@gaia>
On Tue, Dec 8, 2020 at 3:00 AM Catalin Marinas <catalin.marinas@arm.com> wrote:
>
> On Mon, Dec 07, 2020 at 03:07:07PM -0800, Peter Collingbourne wrote:
> > On Mon, Dec 7, 2020 at 2:46 PM Daniel Kiss <daniel.kiss@arm.com> wrote:
> > > If the kernel is not compiled with CONFIG_ARM64_PTR_AUTH_KERNEL,
> > > then the kernel does not need a key and kernel's key could be disabled.
> > >
> > > Signed-off-by: Daniel Kiss <daniel.kiss@arm.com>
> > > ---
> > > arch/arm64/include/asm/asm_pointer_auth.h | 68 ++++++++++++++++-------
> > > arch/arm64/include/asm/processor.h | 2 +
> > > arch/arm64/kernel/asm-offsets.c | 4 ++
> > > 3 files changed, 55 insertions(+), 19 deletions(-)
> > >
> > > diff --git a/arch/arm64/include/asm/asm_pointer_auth.h b/arch/arm64/include/asm/asm_pointer_auth.h
> > > index 52dead2a8640..af3d16027e8f 100644
> > > --- a/arch/arm64/include/asm/asm_pointer_auth.h
> > > +++ b/arch/arm64/include/asm/asm_pointer_auth.h
> > > @@ -14,6 +14,12 @@
> > > * thread.keys_user.ap*.
> > > */
> > > .macro ptrauth_keys_install_user tsk, tmp1, tmp2, tmp3
> > > +#ifndef CONFIG_ARM64_PTR_AUTH_KERNEL
> > > + /* Reenable A key */
> > > + mrs \tmp1, sctlr_el1
> > > + orr \tmp1, \tmp1, SCTLR_ELx_ENIA
> > > + msr sctlr_el1, \tmp1
> > > +#endif
> >
> > We should avoid an unconditional MSR on exit like this as it is
> > expensive (for my PR_PAC_SET_ENABLED_KEYS series I measured the cost
> > of entry/exit MSR as 43.7ns on Cortex-A75 and 33.0ns on Apple M1). In
> > that series I take care not to touch SCTLR_EL1 unless necessary.
> > Likewise for the MSRs on entry below.
>
> I think that's how Daniel attempted the first (internal) version of
> these patches. In theory you don't need to touch SCTLR_ELx_EN* at all as
> long as the kernel does not use any PAC instructions. However, I was
> a bit concerned about this and thought it's safer if, when
> !CONFIG_ARM64_PTR_AUTH_KERNEL, the EnIA bit is cleared while in the
> kernel.
>
> If we can guarantee that the compiler does not generate any PAC
> instructions (it may assume they are no-ops) and vendor modules don't
> have such instructions either, we may be able to relax this.
The way I see it it isn't too different from the current prohibition
on using IB in the kernel (and to a lesser extent DA/DB/GA since those
can't be accessed from nop-space as far as I'm aware), or NEON
instructions in most parts of the kernel, or the stack protector
cookie when building with -fno-stack-protector etc. i.e. if you do
that then you're breaking the ABI.
Is your concern that distributions may default to enabling
-mbranch-protection which would result in the PAC instructions being
used? To address that I think it is reasonable to expect the compiler
not to use PAC instructions when passing -mbranch-protection=none, and
if the compiler does so then that is a bug in the compiler.
Peter
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next prev parent reply other threads:[~2020-12-08 19:35 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-12-07 22:46 arm64: split ARM64_PTR_AUTH option to userspace and kernel configs Daniel Kiss
2020-12-07 22:46 ` [PATCH 1/2] arm64: Add ARM64_PTR_AUTH_KERNEL config option Daniel Kiss
2020-12-07 22:46 ` [PATCH 2/2] arm64: Configure kernel's PTR_AUTH key when it is built with PTR_AUTH Daniel Kiss
2020-12-07 23:07 ` Peter Collingbourne
2020-12-08 11:00 ` Catalin Marinas
2020-12-08 19:33 ` Peter Collingbourne [this message]
2020-12-09 10:51 ` Will Deacon
2020-12-09 11:56 ` Daniel Kiss
2020-12-18 11:56 ` arm64: split ARM64_PTR_AUTH option to userspace and kernel Daniel Kiss
2020-12-18 11:56 ` [PATCH v2 1/2] arm64: Add ARM64_PTR_AUTH_KERNEL config option Daniel Kiss
2021-01-26 13:27 ` Will Deacon
2021-02-08 14:39 ` Daniel Kiss
2020-12-18 11:56 ` [PATCH v2 2/2] arm64: Do not configure kernel's PTR_AUTH key when it not needed Daniel Kiss
2021-01-26 13:32 ` Will Deacon
2021-01-26 13:17 ` arm64: split ARM64_PTR_AUTH option to userspace and kernel Will Deacon
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