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Date: Sun, 11 Sep 2016 14:55:12 +0100
From: Ard Biesheuvel <ard.biesheuvel@...aro.org>
To: kernel-hardening@...ts.openwall.com
Cc: Catalin Marinas <catalin.marinas@....com>, Kees Cook <keescook@...omium.org>,
Will Deacon <will.deacon@....com>, AKASHI Takahiro <takahiro.akashi@...aro.org>,
James Morse <james.morse@....com>,
"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>
Subject: Re: Re: [PATCH v2 3/7] arm64: Introduce
uaccess_{disable, enable} functionality based on TTBR0_EL1
On 6 September 2016 at 11:45, Mark Rutland <mark.rutland@....com> wrote:
> On Tue, Sep 06, 2016 at 11:27:42AM +0100, Catalin Marinas wrote:
>> On Mon, Sep 05, 2016 at 06:20:38PM +0100, Mark Rutland wrote:
>> > On Fri, Sep 02, 2016 at 04:02:09PM +0100, Catalin Marinas wrote:
>> > > +static inline void uaccess_ttbr0_enable(void)
>> > > +{
>> > > + unsigned long flags;
>> > > +
>> > > + /*
>> > > + * Disable interrupts to avoid preemption and potential saved
>> > > + * TTBR0_EL1 updates between reading the variable and the MSR.
>> > > + */
>> > > + local_irq_save(flags);
>> > > + write_sysreg(current_thread_info()->ttbr0, ttbr0_el1);
>> > > + isb();
>> > > + local_irq_restore(flags);
>> > > +}
>> >
>> > I don't follow what problem this actually protects us against. In the
>> > case of preemption everything should be saved+restored transparently, or
>> > things would go wrong as soon as we enable IRQs anyway.
>> >
>> > Is this a hold-over from a percpu approach rather than the
>> > current_thread_info() approach?
>>
>> If we get preempted between reading current_thread_info()->ttbr0 and
>> writing TTBR0_EL1, a series of context switches could lead to the update
>> of the ASID part of ttbr0. The actual MSR would store an old ASID in
>> TTBR0_EL1.
>
> Ah! Can you fold something about racing with an ASID update into the
> description?
>
>> > > +#else
>> > > +static inline void uaccess_ttbr0_disable(void)
>> > > +{
>> > > +}
>> > > +
>> > > +static inline void uaccess_ttbr0_enable(void)
>> > > +{
>> > > +}
>> > > +#endif
>> >
>> > I think that it's better to drop the ifdef and add:
>> >
>> > if (!IS_ENABLED(CONFIG_ARM64_TTBR0_PAN))
>> > return;
>> >
>> > ... at the start of each function. GCC should optimize the entire thing
>> > away when not used, but we'll get compiler coverage regardless, and
>> > therefore less breakage. All the symbols we required should exist
>> > regardless.
>>
>> The reason for this is that thread_info.ttbr0 is conditionally defined.
>> I don't think the compiler would ignore it.
>
> Good point; I missed that.
>
> [...]
>
>> > How about something like:
>> >
>> > .macro alternative_endif_else_nop
>> > alternative_else
>> > .rept ((662b-661b) / 4)
>> > nop
>> > .endr
>> > alternative_endif
>> > .endm
>> >
>> > So for the above we could have:
>> >
>> > alternative_if_not ARM64_HAS_PAN
>> > save_and_disable_irq \tmp2
>> > uaccess_ttbr0_enable \tmp1
>> > restore_irq \tmp2
>> > alternative_endif_else_nop
>> >
>> > I'll see about spinning a patch, or discovering why that happens to be
>> > broken.
>>
>> This looks better. Minor comment, I would actually name the ending
>> statement alternative_else_nop_endif to match the order in which you'd
>> normally write them.
>
> Completely agreed. I already made this change locally, immediately after
> sending the suggestion. :)
>
>> > > * tables again to remove any speculatively loaded cache lines.
>> > > */
>> > > mov x0, x25
>> > > - add x1, x26, #SWAPPER_DIR_SIZE
>> > > + add x1, x26, #SWAPPER_DIR_SIZE + RESERVED_TTBR0_SIZE
>> > > dmb sy
>> > > bl __inval_cache_range
>> > >
>> > > diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
>> > > index 659963d40bb4..fe393ccf9352 100644
>> > > --- a/arch/arm64/kernel/vmlinux.lds.S
>> > > +++ b/arch/arm64/kernel/vmlinux.lds.S
>> > > @@ -196,6 +196,11 @@ SECTIONS
>> > > swapper_pg_dir = .;
>> > > . += SWAPPER_DIR_SIZE;
>> > >
>> > > +#ifdef CONFIG_ARM64_TTBR0_PAN
>> > > + reserved_ttbr0 = .;
>> > > + . += PAGE_SIZE;
>> > > +#endif
>> >
>> > Surely RESERVED_TTBR0_SIZE, as elsewhere?
>>
>> I'll try to move it somewhere where it can be included in vmlinux.lds.S
>> (I can probably include cpufeature.h directly).
>
Do we really need another zero page? The ordinary zero page is already
statically allocated these days, so we could simply move it between
idmap_pg_dir[] and swapper_pg_dir[], and get all the changes in the
early boot code for free (given that it covers the range between the
start of idmap_pg_dir[] and the end of swapper_pg_dir[])
That way, we could refer to __pa(empty_zero_page) anywhere by reading
ttbr1_el1 and subtracting PAGE_SIZE
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