Re: [PATCH Part1 RFC v4 21/36] x86/head/64: set up a startup %gs for stack protector

[Borislav Petkov]

On Wed, Jul 07, 2021 at 01:14:51PM -0500, Brijesh Singh wrote:
> From: Michael Roth <michael.roth@xxxxxxx>
> 
> As of commit 103a4908ad4d ("x86/head/64: Disable stack protection for
> head$(BITS).o") kernel/head64.c is compiled with -fno-stack-protector
> to allow a call to set_bringup_idt_handler(), which would otherwise
> have stack protection enabled with CONFIG_STACKPROTECTOR_STRONG. While
> sufficient for that case, this will still cause issues if we attempt to

Who's "we"?

Please use passive voice in your text: no "we" or "I", etc.
Personal pronouns are ambiguous in text, especially with so many
parties/companies/etc developing the kernel so let's avoid them please.

> call out to any external functions that were compiled with stack
> protection enabled that in-turn make stack-protected calls, or if the
> exception handlers set up by set_bringup_idt_handler() make calls to
> stack-protected functions.
> 
> Subsequent patches for SEV-SNP CPUID validation support will introduce
> both such cases. Attempting to disable stack protection for everything
> in scope to address that is prohibitive since much of the code, like
> SEV-ES #VC handler, is shared code that remains in use after boot and
> could benefit from having stack protection enabled. Attempting to inline
> calls is brittle and can quickly balloon out to library/helper code
> where that's not really an option.
> 
> Instead, set up %gs to point a buffer that stack protector can use for
> canary values when needed.
> 
> In doing so, it's likely we can stop using -no-stack-protector for
> head64.c, but that hasn't been tested yet, and head32.c would need a
> similar solution to be safe, so that is left as a potential follow-up.

Well, then fix it properly pls. Remove the -no-stack-protector, test it
and send it out, even separately if easier to handle. This version looks
half-baked, just so that it gets you what you need for the SNP stuff but
we don't do half-baked, sorry.

> Signed-off-by: Michael Roth <michael.roth@xxxxxxx>
> Signed-off-by: Brijesh Singh <brijesh.singh@xxxxxxx>
> ---
>  arch/x86/kernel/head64.c | 18 +++++++++++++++++-
>  1 file changed, 17 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
> index f4c3e632345a..8615418f98f1 100644
> --- a/arch/x86/kernel/head64.c
> +++ b/arch/x86/kernel/head64.c
> @@ -74,6 +74,9 @@ static struct desc_struct startup_gdt[GDT_ENTRIES] = {
>  	[GDT_ENTRY_KERNEL_DS]           = GDT_ENTRY_INIT(0xc093, 0, 0xfffff),
>  };
>  
> +/* For use by stack protector code before switching to virtual addresses */
> +static char startup_gs_area[64];

That needs some CONFIG_STACKPROTECTOR ifdeffery around it, below too.

> +
>  /*
>   * Address needs to be set at runtime because it references the startup_gdt
>   * while the kernel still uses a direct mapping.
> @@ -598,6 +601,8 @@ void early_setup_idt(void)
>   */
>  void __head startup_64_setup_env(unsigned long physbase)
>  {
> +	u64 gs_area = (u64)fixup_pointer(startup_gs_area, physbase);
> +
>  	/* Load GDT */
>  	startup_gdt_descr.address = (unsigned long)fixup_pointer(startup_gdt, physbase);
>  	native_load_gdt(&startup_gdt_descr);
> @@ -605,7 +610,18 @@ void __head startup_64_setup_env(unsigned long physbase)
>  	/* New GDT is live - reload data segment registers */
>  	asm volatile("movl %%eax, %%ds\n"
>  		     "movl %%eax, %%ss\n"
> -		     "movl %%eax, %%es\n" : : "a"(__KERNEL_DS) : "memory");
> +		     "movl %%eax, %%es\n"
> +		     "movl %%eax, %%gs\n" : : "a"(__KERNEL_DS) : "memory");
> +
> +	/*
> +	 * GCC stack protection needs a place to store canary values. The
> +	 * default is %gs:0x28, which is what the kernel currently uses.
> +	 * Point GS base to a buffer that can be used for this purpose.
> +	 * Note that newer GCCs now allow this location to be configured,
> +	 * so if we change from the default in the future we need to ensure
> +	 * that this buffer overlaps whatever address ends up being used.
> +	 */
> +	native_wrmsr(MSR_GS_BASE, gs_area, gs_area >> 32);
>  
>  	startup_64_load_idt(physbase);
>  }
> -- 

-- 
Regards/Gruss,
    Boris.

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