linux-kernel - Re: [RFC PATCH v2 3/6] x86/cpu: Allow caps to be set arbitrarily early

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAMzpN2jTPFTA9NhXfsrTZ0-prUH-1DbZ5ewt92BZSMqBCZyskw@mail.gmail.com>
Date: Tue, 13 May 2025 14:37:34 -0400
From: Brian Gerst <brgerst@...il.com>
To: Ard Biesheuvel <ardb+git@...gle.com>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org, 
	Ard Biesheuvel <ardb@...nel.org>, Ingo Molnar <mingo@...nel.org>, 
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC PATCH v2 3/6] x86/cpu: Allow caps to be set arbitrarily early

On Tue, May 13, 2025 at 7:40 AM Ard Biesheuvel <ardb+git@...gle.com> wrote:
>
> From: Ard Biesheuvel <ardb@...nel.org>
>
> cpu_feature_enabled() uses a ternary alternative, where the late variant
> is based on code patching and the early variant accesses the capability
> field in boot_cpu_data directly.
>
> This allows cpu_feature_enabled() to be called quite early, but it still
> requires that the CPU feature detection code runs before being able to
> rely on the return value of cpu_feature_enabled().
>
> This is a problem for the implementation of pgtable_l5_enabled(), which
> is based on cpu_feature_enabled(X86_FEATURE_5LEVEL_PAGING), and may be
> called extremely early. Currently, there is a hacky workaround where
> some source files that may execute before (but also after) CPU feature
> detection have a different version of pgtable_l5_enabled(), based on the
> USE_EARLY_PGTABLE_L5 preprocessor macro.
>
> Instead, let's make it possible to set CPU feature arbitrarily early, so
> that the X86_FEATURE_5LEVEL_PAGING capability can be set before even
> entering C code.
>
> This involves relying on static initialization of boot_cpu_data and the
> cpu_caps_set/cpu_caps_cleared arrays, so they all need to reside in
> .data. This ensures that they won't be cleared along with the rest of
> BSS.
>
> Note that forcing a capability involves setting it in both
> boot_cpu_data.x86_capability[] and cpu_caps_set[].
>
> Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
> ---
>  arch/x86/kernel/cpu/common.c | 10 ++++------
>  1 file changed, 4 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index bbec5c4cd8ed..aaa6d9e51ef1 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -704,8 +704,8 @@ static const char *table_lookup_model(struct cpuinfo_x86 *c)
>  }
>
>  /* Aligned to unsigned long to avoid split lock in atomic bitmap ops */
> -__u32 cpu_caps_cleared[NCAPINTS + NBUGINTS] __aligned(sizeof(unsigned long));
> -__u32 cpu_caps_set[NCAPINTS + NBUGINTS] __aligned(sizeof(unsigned long));
> +__u32 __read_mostly cpu_caps_cleared[NCAPINTS + NBUGINTS] __aligned(sizeof(unsigned long));
> +__u32 __read_mostly cpu_caps_set[NCAPINTS + NBUGINTS] __aligned(sizeof(unsigned long));

Is there any scenario where capabilities are changed after boot?  If
not, this could possibly be __ro_after_init.

>  #ifdef CONFIG_X86_32
>  /* The 32-bit entry code needs to find cpu_entry_area. */
> @@ -1628,9 +1628,6 @@ static void __init cpu_parse_early_param(void)
>   */
>  static void __init early_identify_cpu(struct cpuinfo_x86 *c)
>  {
> -       memset(&c->x86_capability, 0, sizeof(c->x86_capability));
> -       c->extended_cpuid_level = 0;
> -
>         if (!have_cpuid_p())
>                 identify_cpu_without_cpuid(c);
>
> @@ -1842,7 +1839,8 @@ static void identify_cpu(struct cpuinfo_x86 *c)
>         c->x86_virt_bits = 32;
>  #endif
>         c->x86_cache_alignment = c->x86_clflush_size;
> -       memset(&c->x86_capability, 0, sizeof(c->x86_capability));
> +       if (c != &boot_cpu_data)
> +               memset(&c->x86_capability, 0, sizeof(c->x86_capability));

You can move the clearing of the capabilities to the caller
(identify_secondary_cpu()) instead.


Brian Gerst