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Date: Wed, 14 Sep 2016 09:52:29 +0100
From: Mark Rutland <mark.rutland@....com>
To: Kees Cook <keescook@...omium.org>
Cc: Catalin Marinas <catalin.marinas@....com>,
"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>,
Will Deacon <will.deacon@....com>,
James Morse <james.morse@....com>,
Ard Biesheuvel <ard.biesheuvel@...aro.org>,
AKASHI Takahiro <takahiro.akashi@...aro.org>,
"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v3 3/7] arm64: Introduce uaccess_{disable,enable}
functionality based on TTBR0_EL1
On Tue, Sep 13, 2016 at 01:45:21PM -0700, Kees Cook wrote:
> On Tue, Sep 13, 2016 at 10:46 AM, Catalin Marinas
> > +static inline bool system_uses_ttbr0_pan(void)
> > +{
> > + return IS_ENABLED(CONFIG_ARM64_SW_TTBR0_PAN) &&
> > + !cpus_have_cap(ARM64_HAS_PAN);
> > +}
> > +
[...]
> > #define __uaccess_enable(alt) \
> > do { \
> > - asm(ALTERNATIVE("nop", SET_PSTATE_PAN(0), alt, \
> > - CONFIG_ARM64_PAN)); \
> > + if (system_uses_ttbr0_pan()) \
> > + uaccess_ttbr0_enable(); \
> > + else \
> > + asm(ALTERNATIVE("nop", SET_PSTATE_PAN(0), alt, \
> > + CONFIG_ARM64_PAN)); \
> > } while (0)
>
> Does this mean that with CONFIG_ARM64_SW_TTBR0_PAN, even with ARMv8.1,
> a cpu capability bitmask check is done each time we go through
> __uaccess_{en,dis}able?
Catalin reworked cpus_have_cap() to use static keys [1], and that's
queued in the arm64 for-next/core branch [2].
So this should expand to a single branch or nop that we patch when we
detect the presence/absence of PAN. There should be no bitmap check.
Thanks,
Mark.
[1] http://lists.infradead.org/pipermail/linux-arm-kernel/2016-September/454025.html
[2] https://git.kernel.org/cgit/linux/kernel/git/arm64/linux.git/log/?h=for-next/core
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