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

[John Garry <john.garry@huawei.com>]

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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