[PATCH] arm64: kpti: Update arm64_kernel_use_ng_mappings() when forced on

[PATCH] arm64: kpti: Update arm64_kernel_use_ng_mappings() when forced on

[James Morse]

Since commit b89d82ef01b3 ("arm64: kpti: Avoid rewriting early page
tables when KASLR is enabled"), a kernel built with CONFIG_RANDOMIZE_BASE
can decide early whether to use non-global mappings by checking the
kaslr_offset().

A kernel built without CONFIG_RANDOMIZE_BASE, instead checks the
cpufeature static-key.

This leaves a gap where CONFIG_RANDOMIZE_BASE was enabled, no
kaslr seed was provided, but kpti was forced on using the cmdline
option.

When the decision is made late, kpti_install_ng_mappings() will re-write
the page tables, but arm64_kernel_use_ng_mappings()'s value does not
change as it only tests the cpufeature static-key if
CONFIG_RANDOMIZE_BASE is disabled.
This function influences PROT_DEFAULT via PTE_MAYBE_NG, and causes
pgattr_change_is_safe() to catch nG->G transitions when the unchanged
PROT_DEFAULT is used as part of PAGE_KERNEL_RO:
[    1.942255] alternatives: patching kernel code
[    1.998288] ------------[ cut here ]------------
[    2.000693] kernel BUG at arch/arm64/mm/mmu.c:165!
[    2.019215] Internal error: Oops - BUG: 0 [#1] PREEMPT SMP
[    2.020257] Modules linked in:
[    2.020807] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.0.0-rc2 #51
[    2.021917] Hardware name: linux,dummy-virt (DT)
[    2.022790] pstate: 40000005 (nZcv daif -PAN -UAO)
[    2.023742] pc : __create_pgd_mapping+0x508/0x6d0
[    2.024671] lr : __create_pgd_mapping+0x500/0x6d0

[    2.058059] Process swapper/0 (pid: 1, stack limit = 0x(____ptrval____))
[    2.059369] Call trace:
[    2.059845]  __create_pgd_mapping+0x508/0x6d0
[    2.060684]  update_mapping_prot+0x48/0xd0
[    2.061477]  mark_linear_text_alias_ro+0xdc/0xe4
[    2.070502]  smp_cpus_done+0x90/0x98
[    2.071216]  smp_init+0x100/0x114
[    2.071878]  kernel_init_freeable+0xd4/0x220
[    2.072750]  kernel_init+0x10/0x100
[    2.073455]  ret_from_fork+0x10/0x18

[    2.075414] ---[ end trace 3572f3a7782292de ]---
[    2.076389] Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b

If arm64_kernel_unmapped_at_el0() is true, arm64_kernel_use_ng_mappings()
should also be true.

Signed-off-by: James Morse <james.morse@arm.com>
CC: Ard Biesheuvel <ard.biesheuvel@linaro.org>
CC: John Garry <john.garry@huawei.com>
CC: Will Deacon <will.deacon@arm.com>

---
 arch/arm64/include/asm/mmu.h | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/mmu.h b/arch/arm64/include/asm/mmu.h
index ac352accb3d9..3e8063f4f9d3 100644
--- a/arch/arm64/include/asm/mmu.h
+++ b/arch/arm64/include/asm/mmu.h
@@ -60,8 +60,11 @@ static inline bool arm64_kernel_use_ng_mappings(void)
 	 * later determine that kpti is required, then
 	 * kpti_install_ng_mappings() will make them non-global.
 	 */
+	if (arm64_kernel_unmapped_at_el0())
+		return true;
+
 	if (!IS_ENABLED(CONFIG_RANDOMIZE_BASE))
-		return arm64_kernel_unmapped_at_el0();
+		return false;
 
 	/*
 	 * KASLR is enabled so we're going to be enabling kpti on non-broken
-- 
2.20.1


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Re: [PATCH] arm64: kpti: Update arm64_kernel_use_ng_mappings() when forced on

[John Garry]

On 15/01/2019 18:49, James Morse wrote:
> Since commit b89d82ef01b3 ("arm64: kpti: Avoid rewriting early page

About b89d82ef01b3, I got an *unconfirmed* (emphasis on this, I don't 
want to cry wolf) report yesterday that the symptom I saw (boot delay) 
has been seen on 5.0-rc2 on our D06 board. I could not see it.

Please note that this would be same board which we saw this on:
https://lkml.org/lkml/2018/6/20/589

I only witnessed the issue on the predecessor A72-based D05 board.

The reporter is very busy, but we'll keep tabs on it.

Thanks,
John

> tables when KASLR is enabled"), a kernel built with CONFIG_RANDOMIZE_BASE
> can decide early whether to use non-global mappings by checking the
> kaslr_offset().
>
> A kernel built without CONFIG_RANDOMIZE_BASE, instead checks the
> cpufeature static-key.
>
> This leaves a gap where CONFIG_RANDOMIZE_BASE was enabled, no
> kaslr seed was provided, but kpti was forced on using the cmdline
> option.
>
> When the decision is made late, kpti_install_ng_mappings() will re-write
> the page tables, but arm64_kernel_use_ng_mappings()'s value does not
> change as it only tests the cpufeature static-key if
> CONFIG_RANDOMIZE_BASE is disabled.
> This function influences PROT_DEFAULT via PTE_MAYBE_NG, and causes
> pgattr_change_is_safe() to catch nG->G transitions when the unchanged
> PROT_DEFAULT is used as part of PAGE_KERNEL_RO:
> [    1.942255] alternatives: patching kernel code
> [    1.998288] ------------[ cut here ]------------
> [    2.000693] kernel BUG at arch/arm64/mm/mmu.c:165!
> [    2.019215] Internal error: Oops - BUG: 0 [#1] PREEMPT SMP
> [    2.020257] Modules linked in:
> [    2.020807] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.0.0-rc2 #51
> [    2.021917] Hardware name: linux,dummy-virt (DT)
> [    2.022790] pstate: 40000005 (nZcv daif -PAN -UAO)
> [    2.023742] pc : __create_pgd_mapping+0x508/0x6d0
> [    2.024671] lr : __create_pgd_mapping+0x500/0x6d0
>
> [    2.058059] Process swapper/0 (pid: 1, stack limit = 0x(____ptrval____))
> [    2.059369] Call trace:
> [    2.059845]  __create_pgd_mapping+0x508/0x6d0
> [    2.060684]  update_mapping_prot+0x48/0xd0
> [    2.061477]  mark_linear_text_alias_ro+0xdc/0xe4
> [    2.070502]  smp_cpus_done+0x90/0x98
> [    2.071216]  smp_init+0x100/0x114
> [    2.071878]  kernel_init_freeable+0xd4/0x220
> [    2.072750]  kernel_init+0x10/0x100
> [    2.073455]  ret_from_fork+0x10/0x18
>
> [    2.075414] ---[ end trace 3572f3a7782292de ]---
> [    2.076389] Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b
>
> If arm64_kernel_unmapped_at_el0() is true, arm64_kernel_use_ng_mappings()
> should also be true.
>
> Signed-off-by: James Morse <james.morse@arm.com>
> CC: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> CC: John Garry <john.garry@huawei.com>
> CC: Will Deacon <will.deacon@arm.com>
>
> ---
>  arch/arm64/include/asm/mmu.h | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm64/include/asm/mmu.h b/arch/arm64/include/asm/mmu.h
> index ac352accb3d9..3e8063f4f9d3 100644
> --- a/arch/arm64/include/asm/mmu.h
> +++ b/arch/arm64/include/asm/mmu.h
> @@ -60,8 +60,11 @@ static inline bool arm64_kernel_use_ng_mappings(void)
>  	 * later determine that kpti is required, then
>  	 * kpti_install_ng_mappings() will make them non-global.
>  	 */
> +	if (arm64_kernel_unmapped_at_el0())
> +		return true;
> +
>  	if (!IS_ENABLED(CONFIG_RANDOMIZE_BASE))
> -		return arm64_kernel_unmapped_at_el0();
> +		return false;
>
>  	/*
>  	 * KASLR is enabled so we're going to be enabling kpti on non-broken
>



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Re: [PATCH] arm64: kpti: Update arm64_kernel_use_ng_mappings() when forced on

[James Morse]

Hi John,

On 16/01/2019 09:38, John Garry wrote:
> On 15/01/2019 18:49, James Morse wrote:
>> Since commit b89d82ef01b3 ("arm64: kpti: Avoid rewriting early page
> 
> About b89d82ef01b3, I got an *unconfirmed* (emphasis on this, I don't want to
> cry wolf) report yesterday that the symptom I saw (boot delay) has been seen on
> 5.0-rc2 on our D06 board. I could not see it.

You can still cause kpti_install_ng_mappings() to rewrite the page-tables if you
don't have CONFIG_RANDOMIZE_BASE (or there is no seed), so its not enabled
early, and unmap_kernel_at_el0() thinks the CPU is vulnerable, so enables it late.


> Please note that this would be same board which we saw this on:
> https://lkml.org/lkml/2018/6/20/589
> 
> I only witnessed the issue on the predecessor A72-based D05 board.

2a355ec25729 whitelisted the CPUs on this board. Unless you force kpti on, and
disable KASLR you shouldn't see it.


> The reporter is very busy, but we'll keep tabs on it.

Thanks!


James

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Re: [PATCH] arm64: kpti: Update arm64_kernel_use_ng_mappings() when forced on

[Will Deacon]

On Fri, Mar 01, 2019 at 11:18:21AM +0000, John Garry wrote:
> On 16/01/2019 09:38, John Garry wrote:
> > On 15/01/2019 18:49, James Morse wrote:
> > > Since commit b89d82ef01b3 ("arm64: kpti: Avoid rewriting early page
> > 
> > About b89d82ef01b3, I got an *unconfirmed* (emphasis on this, I don't
> > want to cry wolf) report yesterday that the symptom I saw (boot delay)
> > has been seen on 5.0-rc2 on our D06 board. I could not see it.
> > 
> > Please note that this would be same board which we saw this on:
> > https://lkml.org/lkml/2018/6/20/589
> > 
> > I only witnessed the issue on the predecessor A72-based D05 board.
> > 
> > The reporter is very busy, but we'll keep tabs on it.
> > 
> 
> FYI, this just came in this morning:

This isn't completely surprising, because the kernel doesn't know about
the CPU in D06:

> [    0.000000] Booting Linux on physical CPU 0x0000010000 [0x480fd010]

[...]

> [    0.000000] CPU features: detected: Kernel page table isolation (KPTI)

If the CPU is not affected by meltdown, you can add the MIDR above to the
kpti_safe_list[] table. If it *is* affected by meltdown, then I'm afraid
that you're stuck with the delay at boot for !KASLR kernels (which is an
awful lot better than getting pwned by userspace :)

I think D06 is 8.2, so I would've hoped that you'd implemented
AA64PFR0_EL1.CSV3, which would make this MIDR-based whitelisting
unnecessary.

Will

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Re: [PATCH] arm64: kpti: Update arm64_kernel_use_ng_mappings() when forced on

[Ard Biesheuvel]

On Fri, 1 Mar 2019 at 12:35, Will Deacon <will.deacon@arm.com> wrote:
>
> On Fri, Mar 01, 2019 at 11:18:21AM +0000, John Garry wrote:
> > On 16/01/2019 09:38, John Garry wrote:
> > > On 15/01/2019 18:49, James Morse wrote:
> > > > Since commit b89d82ef01b3 ("arm64: kpti: Avoid rewriting early page
> > >
> > > About b89d82ef01b3, I got an *unconfirmed* (emphasis on this, I don't
> > > want to cry wolf) report yesterday that the symptom I saw (boot delay)
> > > has been seen on 5.0-rc2 on our D06 board. I could not see it.
> > >
> > > Please note that this would be same board which we saw this on:
> > > https://lkml.org/lkml/2018/6/20/589
> > >
> > > I only witnessed the issue on the predecessor A72-based D05 board.
> > >
> > > The reporter is very busy, but we'll keep tabs on it.
> > >
> >
> > FYI, this just came in this morning:
>
> This isn't completely surprising, because the kernel doesn't know about
> the CPU in D06:
>
> > [    0.000000] Booting Linux on physical CPU 0x0000010000 [0x480fd010]
>
> [...]
>
> > [    0.000000] CPU features: detected: Kernel page table isolation (KPTI)
>
> If the CPU is not affected by meltdown, you can add the MIDR above to the
> kpti_safe_list[] table. If it *is* affected by meltdown, then I'm afraid
> that you're stuck with the delay at boot for !KASLR kernels (which is an
> awful lot better than getting pwned by userspace :)
>
> I think D06 is 8.2, so I would've hoped that you'd implemented
> AA64PFR0_EL1.CSV3, which would make this MIDR-based whitelisting
> unnecessary.
>

Note that this is also still tied to the new rodata handling. The new
default is rodata=full, and reverting back to rodata=on (which used to
be the default in prior releases) or rodata=off (not recommended)
should get around this as well.

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Re: [PATCH] arm64: kpti: Update arm64_kernel_use_ng_mappings() when forced on

[Will Deacon]

On Fri, Mar 01, 2019 at 12:43:01PM +0100, Ard Biesheuvel wrote:
> On Fri, 1 Mar 2019 at 12:35, Will Deacon <will.deacon@arm.com> wrote:
> >
> > On Fri, Mar 01, 2019 at 11:18:21AM +0000, John Garry wrote:
> > > On 16/01/2019 09:38, John Garry wrote:
> > > > On 15/01/2019 18:49, James Morse wrote:
> > > > > Since commit b89d82ef01b3 ("arm64: kpti: Avoid rewriting early page
> > > >
> > > > About b89d82ef01b3, I got an *unconfirmed* (emphasis on this, I don't
> > > > want to cry wolf) report yesterday that the symptom I saw (boot delay)
> > > > has been seen on 5.0-rc2 on our D06 board. I could not see it.
> > > >
> > > > Please note that this would be same board which we saw this on:
> > > > https://lkml.org/lkml/2018/6/20/589
> > > >
> > > > I only witnessed the issue on the predecessor A72-based D05 board.
> > > >
> > > > The reporter is very busy, but we'll keep tabs on it.
> > > >
> > >
> > > FYI, this just came in this morning:
> >
> > This isn't completely surprising, because the kernel doesn't know about
> > the CPU in D06:
> >
> > > [    0.000000] Booting Linux on physical CPU 0x0000010000 [0x480fd010]
> >
> > [...]
> >
> > > [    0.000000] CPU features: detected: Kernel page table isolation (KPTI)
> >
> > If the CPU is not affected by meltdown, you can add the MIDR above to the
> > kpti_safe_list[] table. If it *is* affected by meltdown, then I'm afraid
> > that you're stuck with the delay at boot for !KASLR kernels (which is an
> > awful lot better than getting pwned by userspace :)
> >
> > I think D06 is 8.2, so I would've hoped that you'd implemented
> > AA64PFR0_EL1.CSV3, which would make this MIDR-based whitelisting
> > unnecessary.
> >
> 
> Note that this is also still tied to the new rodata handling. The new
> default is rodata=full, and reverting back to rodata=on (which used to
> be the default in prior releases) or rodata=off (not recommended)
> should get around this as well.

Or, somewhat perversely, enabling CONFIG_RANDOMIZE_BASE and passing a
valid kaslr seed. Performance /and/ security!

Will

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Re: [PATCH] arm64: kpti: Update arm64_kernel_use_ng_mappings() when forced on

[Ard Biesheuvel]

On Fri, 1 Mar 2019 at 12:45, Will Deacon <will.deacon@arm.com> wrote:
>
> On Fri, Mar 01, 2019 at 12:43:01PM +0100, Ard Biesheuvel wrote:
> > On Fri, 1 Mar 2019 at 12:35, Will Deacon <will.deacon@arm.com> wrote:
> > >
> > > On Fri, Mar 01, 2019 at 11:18:21AM +0000, John Garry wrote:
> > > > On 16/01/2019 09:38, John Garry wrote:
> > > > > On 15/01/2019 18:49, James Morse wrote:
> > > > > > Since commit b89d82ef01b3 ("arm64: kpti: Avoid rewriting early page
> > > > >
> > > > > About b89d82ef01b3, I got an *unconfirmed* (emphasis on this, I don't
> > > > > want to cry wolf) report yesterday that the symptom I saw (boot delay)
> > > > > has been seen on 5.0-rc2 on our D06 board. I could not see it.
> > > > >
> > > > > Please note that this would be same board which we saw this on:
> > > > > https://lkml.org/lkml/2018/6/20/589
> > > > >
> > > > > I only witnessed the issue on the predecessor A72-based D05 board.
> > > > >
> > > > > The reporter is very busy, but we'll keep tabs on it.
> > > > >
> > > >
> > > > FYI, this just came in this morning:
> > >
> > > This isn't completely surprising, because the kernel doesn't know about
> > > the CPU in D06:
> > >
> > > > [    0.000000] Booting Linux on physical CPU 0x0000010000 [0x480fd010]
> > >
> > > [...]
> > >
> > > > [    0.000000] CPU features: detected: Kernel page table isolation (KPTI)
> > >
> > > If the CPU is not affected by meltdown, you can add the MIDR above to the
> > > kpti_safe_list[] table. If it *is* affected by meltdown, then I'm afraid
> > > that you're stuck with the delay at boot for !KASLR kernels (which is an
> > > awful lot better than getting pwned by userspace :)
> > >
> > > I think D06 is 8.2, so I would've hoped that you'd implemented
> > > AA64PFR0_EL1.CSV3, which would make this MIDR-based whitelisting
> > > unnecessary.
> > >
> >
> > Note that this is also still tied to the new rodata handling. The new
> > default is rodata=full, and reverting back to rodata=on (which used to
> > be the default in prior releases) or rodata=off (not recommended)
> > should get around this as well.
>
> Or, somewhat perversely, enabling CONFIG_RANDOMIZE_BASE and passing a
> valid kaslr seed. Performance /and/ security!
>

Yeah but unfortunately, it seems the UEFI on that box does not
implement EFI_RNG_PROTOCOL.

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Re: [PATCH] arm64: kpti: Update arm64_kernel_use_ng_mappings() when forced on

[Hanjun Guo]

On 2019/3/1 19:46, Ard Biesheuvel wrote:
> On Fri, 1 Mar 2019 at 12:45, Will Deacon <will.deacon@arm.com> wrote:
>>
>> On Fri, Mar 01, 2019 at 12:43:01PM +0100, Ard Biesheuvel wrote:
>>> On Fri, 1 Mar 2019 at 12:35, Will Deacon <will.deacon@arm.com> wrote:
>>>>
>>>> On Fri, Mar 01, 2019 at 11:18:21AM +0000, John Garry wrote:
>>>>> On 16/01/2019 09:38, John Garry wrote:
>>>>>> On 15/01/2019 18:49, James Morse wrote:
>>>>>>> Since commit b89d82ef01b3 ("arm64: kpti: Avoid rewriting early page
>>>>>>
>>>>>> About b89d82ef01b3, I got an *unconfirmed* (emphasis on this, I don't
>>>>>> want to cry wolf) report yesterday that the symptom I saw (boot delay)
>>>>>> has been seen on 5.0-rc2 on our D06 board. I could not see it.
>>>>>>
>>>>>> Please note that this would be same board which we saw this on:
>>>>>> https://lkml.org/lkml/2018/6/20/589
>>>>>>
>>>>>> I only witnessed the issue on the predecessor A72-based D05 board.
>>>>>>
>>>>>> The reporter is very busy, but we'll keep tabs on it.
>>>>>>
>>>>>
>>>>> FYI, this just came in this morning:
>>>>
>>>> This isn't completely surprising, because the kernel doesn't know about
>>>> the CPU in D06:
>>>>
>>>>> [    0.000000] Booting Linux on physical CPU 0x0000010000 [0x480fd010]
>>>>
>>>> [...]
>>>>
>>>>> [    0.000000] CPU features: detected: Kernel page table isolation (KPTI)
>>>>
>>>> If the CPU is not affected by meltdown, you can add the MIDR above to the
>>>> kpti_safe_list[] table. If it *is* affected by meltdown, then I'm afraid
>>>> that you're stuck with the delay at boot for !KASLR kernels (which is an
>>>> awful lot better than getting pwned by userspace :)
>>>>
>>>> I think D06 is 8.2, so I would've hoped that you'd implemented
>>>> AA64PFR0_EL1.CSV3, which would make this MIDR-based whitelisting
>>>> unnecessary.

D06 has the function of AA64PFR0_EL1.CSV3 but didn't set the bit, and
it's not affected by meltdown...

>>>>
>>>
>>> Note that this is also still tied to the new rodata handling. The new
>>> default is rodata=full, and reverting back to rodata=on (which used to
>>> be the default in prior releases) or rodata=off (not recommended)
>>> should get around this as well.
>>
>> Or, somewhat perversely, enabling CONFIG_RANDOMIZE_BASE and passing a
>> valid kaslr seed. Performance /and/ security!
>>
> 
> Yeah but unfortunately, it seems the UEFI on that box does not
> implement EFI_RNG_PROTOCOL.

Eagle eye :) So if we implement EFI_RNG_PROTOCOL (will take some time) in
firmware and enable CONFIG_RANDOMIZE_BASE, the issue will gone, right?

Thanks
Hanjun



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Re: [PATCH] arm64: kpti: Update arm64_kernel_use_ng_mappings() when forced on

[Will Deacon]

On Fri, Mar 01, 2019 at 08:59:19PM +0800, Hanjun Guo wrote:
> On 2019/3/1 19:46, Ard Biesheuvel wrote:
> > On Fri, 1 Mar 2019 at 12:45, Will Deacon <will.deacon@arm.com> wrote:
> >>
> >> On Fri, Mar 01, 2019 at 12:43:01PM +0100, Ard Biesheuvel wrote:
> >>> On Fri, 1 Mar 2019 at 12:35, Will Deacon <will.deacon@arm.com> wrote:
> >>>>
> >>>> On Fri, Mar 01, 2019 at 11:18:21AM +0000, John Garry wrote:
> >>>>> On 16/01/2019 09:38, John Garry wrote:
> >>>>>> On 15/01/2019 18:49, James Morse wrote:
> >>>>>>> Since commit b89d82ef01b3 ("arm64: kpti: Avoid rewriting early page
> >>>>>>
> >>>>>> About b89d82ef01b3, I got an *unconfirmed* (emphasis on this, I don't
> >>>>>> want to cry wolf) report yesterday that the symptom I saw (boot delay)
> >>>>>> has been seen on 5.0-rc2 on our D06 board. I could not see it.
> >>>>>>
> >>>>>> Please note that this would be same board which we saw this on:
> >>>>>> https://lkml.org/lkml/2018/6/20/589
> >>>>>>
> >>>>>> I only witnessed the issue on the predecessor A72-based D05 board.
> >>>>>>
> >>>>>> The reporter is very busy, but we'll keep tabs on it.
> >>>>>>
> >>>>>
> >>>>> FYI, this just came in this morning:
> >>>>
> >>>> This isn't completely surprising, because the kernel doesn't know about
> >>>> the CPU in D06:
> >>>>
> >>>>> [    0.000000] Booting Linux on physical CPU 0x0000010000 [0x480fd010]
> >>>>
> >>>> [...]
> >>>>
> >>>>> [    0.000000] CPU features: detected: Kernel page table isolation (KPTI)
> >>>>
> >>>> If the CPU is not affected by meltdown, you can add the MIDR above to the
> >>>> kpti_safe_list[] table. If it *is* affected by meltdown, then I'm afraid
> >>>> that you're stuck with the delay at boot for !KASLR kernels (which is an
> >>>> awful lot better than getting pwned by userspace :)
> >>>>
> >>>> I think D06 is 8.2, so I would've hoped that you'd implemented
> >>>> AA64PFR0_EL1.CSV3, which would make this MIDR-based whitelisting
> >>>> unnecessary.
> 
> D06 has the function of AA64PFR0_EL1.CSV3 but didn't set the bit, and
> it's not affected by meltdown...
> 
> >>>>
> >>>
> >>> Note that this is also still tied to the new rodata handling. The new
> >>> default is rodata=full, and reverting back to rodata=on (which used to
> >>> be the default in prior releases) or rodata=off (not recommended)
> >>> should get around this as well.
> >>
> >> Or, somewhat perversely, enabling CONFIG_RANDOMIZE_BASE and passing a
> >> valid kaslr seed. Performance /and/ security!
> >>
> > 
> > Yeah but unfortunately, it seems the UEFI on that box does not
> > implement EFI_RNG_PROTOCOL.
> 
> Eagle eye :) So if we implement EFI_RNG_PROTOCOL (will take some time) in
> firmware and enable CONFIG_RANDOMIZE_BASE, the issue will gone, right?

Yup, but you can also update the MIDR whitelist since you're not affected by
meltdown.

Will

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Re: [PATCH] arm64: kpti: Update arm64_kernel_use_ng_mappings() when forced on

[Hanjun Guo]

On 2019/3/1 21:02, Will Deacon wrote:
> On Fri, Mar 01, 2019 at 08:59:19PM +0800, Hanjun Guo wrote:
>> On 2019/3/1 19:46, Ard Biesheuvel wrote:
>>> On Fri, 1 Mar 2019 at 12:45, Will Deacon <will.deacon@arm.com> wrote:
>>>>
>>>> On Fri, Mar 01, 2019 at 12:43:01PM +0100, Ard Biesheuvel wrote:
>>>>> On Fri, 1 Mar 2019 at 12:35, Will Deacon <will.deacon@arm.com> wrote:
>>>>>>
>>>>>> On Fri, Mar 01, 2019 at 11:18:21AM +0000, John Garry wrote:
>>>>>>> On 16/01/2019 09:38, John Garry wrote:
>>>>>>>> On 15/01/2019 18:49, James Morse wrote:
>>>>>>>>> Since commit b89d82ef01b3 ("arm64: kpti: Avoid rewriting early page
>>>>>>>>
>>>>>>>> About b89d82ef01b3, I got an *unconfirmed* (emphasis on this, I don't
>>>>>>>> want to cry wolf) report yesterday that the symptom I saw (boot delay)
>>>>>>>> has been seen on 5.0-rc2 on our D06 board. I could not see it.
>>>>>>>>
>>>>>>>> Please note that this would be same board which we saw this on:
>>>>>>>> https://lkml.org/lkml/2018/6/20/589
>>>>>>>>
>>>>>>>> I only witnessed the issue on the predecessor A72-based D05 board.
>>>>>>>>
>>>>>>>> The reporter is very busy, but we'll keep tabs on it.
>>>>>>>>
>>>>>>>
>>>>>>> FYI, this just came in this morning:
>>>>>>
>>>>>> This isn't completely surprising, because the kernel doesn't know about
>>>>>> the CPU in D06:
>>>>>>
>>>>>>> [    0.000000] Booting Linux on physical CPU 0x0000010000 [0x480fd010]
>>>>>>
>>>>>> [...]
>>>>>>
>>>>>>> [    0.000000] CPU features: detected: Kernel page table isolation (KPTI)
>>>>>>
>>>>>> If the CPU is not affected by meltdown, you can add the MIDR above to the
>>>>>> kpti_safe_list[] table. If it *is* affected by meltdown, then I'm afraid
>>>>>> that you're stuck with the delay at boot for !KASLR kernels (which is an
>>>>>> awful lot better than getting pwned by userspace :)
>>>>>>
>>>>>> I think D06 is 8.2, so I would've hoped that you'd implemented
>>>>>> AA64PFR0_EL1.CSV3, which would make this MIDR-based whitelisting
>>>>>> unnecessary.
>>
>> D06 has the function of AA64PFR0_EL1.CSV3 but didn't set the bit, and
>> it's not affected by meltdown...
>>
>>>>>>
>>>>>
>>>>> Note that this is also still tied to the new rodata handling. The new
>>>>> default is rodata=full, and reverting back to rodata=on (which used to
>>>>> be the default in prior releases) or rodata=off (not recommended)
>>>>> should get around this as well.
>>>>
>>>> Or, somewhat perversely, enabling CONFIG_RANDOMIZE_BASE and passing a
>>>> valid kaslr seed. Performance /and/ security!
>>>>
>>>
>>> Yeah but unfortunately, it seems the UEFI on that box does not
>>> implement EFI_RNG_PROTOCOL.
>>
>> Eagle eye :) So if we implement EFI_RNG_PROTOCOL (will take some time) in
>> firmware and enable CONFIG_RANDOMIZE_BASE, the issue will gone, right?
> 
> Yup, but you can also update the MIDR whitelist since you're not affected by
> meltdown.

Thanks for the guidance, Performance /and/ security is what I prefer :)

I take a look at the code, the logic in unmap_kernel_at_el0() shows that
even I add D06's CPU in kpti_safe_list[] table, I can still enable
CONFIG_RANDOMIZE_BASE + UEFI EFI_RNG_PROTOCOL to get performance and
security.

So it would be useful to prepare a patch to add D06 CPU's MIDR to
kpti_safe_list[] table, to prevent some mis-use case such as above.

Thanks
Hanjun


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