linux-kernel - Re: [PATCH 176/208] x86/alternatives, x86/fpu: Add 'alternatives_patched' debug flag and use it in xsave

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Date:	Wed, 6 May 2015 00:47:59 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	linux-kernel@...r.kernel.org,
	Andy Lutomirski <luto@...capital.net>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Fenghua Yu <fenghua.yu@...el.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 176/208] x86/alternatives, x86/fpu: Add
 'alternatives_patched' debug flag and use it in xsave_state()

On Tue, May 05, 2015 at 07:58:00PM +0200, Ingo Molnar wrote:
> We'd like to use xsave_state() earlier, but its SYSTEM_BOOTING check
> is too imprecise.
> 
> The real condition that xsave_state() would like to check is whether
> alternative XSAVE instructions were patched into the kernel image
> already.
> 
> Add such a (read-mostly) debug flag and use it in xsave_state().
> 
> Cc: Andy Lutomirski <luto@...capital.net>
> Cc: Borislav Petkov <bp@...en8.de>
> Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> Cc: Fenghua Yu <fenghua.yu@...el.com>
> Cc: H. Peter Anvin <hpa@...or.com>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Oleg Nesterov <oleg@...hat.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: Ingo Molnar <mingo@...nel.org>
> ---
>  arch/x86/include/asm/alternative.h | 6 ++++++
>  arch/x86/include/asm/fpu/xstate.h  | 2 +-
>  arch/x86/kernel/alternative.c      | 5 +++++
>  3 files changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/alternative.h b/arch/x86/include/asm/alternative.h
> index ba32af062f61..7bfc85bbb8ff 100644
> --- a/arch/x86/include/asm/alternative.h
> +++ b/arch/x86/include/asm/alternative.h
> @@ -52,6 +52,12 @@ struct alt_instr {
>  	u8  padlen;		/* length of build-time padding */
>  } __packed;
>  
> +/*
> + * Debug flag that can be tested to see whether alternative
> + * instructions were patched in already:
> + */
> +extern int alternatives_patched;

Looks useful...

> +
>  extern void alternative_instructions(void);
>  extern void apply_alternatives(struct alt_instr *start, struct alt_instr *end);
>  
> diff --git a/arch/x86/include/asm/fpu/xstate.h b/arch/x86/include/asm/fpu/xstate.h
> index 31a002ad5aeb..ab2c507b58b6 100644
> --- a/arch/x86/include/asm/fpu/xstate.h
> +++ b/arch/x86/include/asm/fpu/xstate.h
> @@ -119,7 +119,7 @@ static inline int xsave_state(struct xsave_struct *fx)
>  	u32 hmask = mask >> 32;
>  	int err = 0;
>  
> -	WARN_ON(system_state == SYSTEM_BOOTING);
> +	WARN_ON(!alternatives_patched);

Btw, shouldn't we be doing this check in what is called now
copy_kernel_to_xregs() too, i.e., right under this function which is now
called copy_xregs_to_kernel()?

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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