linux-kernel - Re: [PATCH 01/17] x86, fpu: wrap get_xsave

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Date:	Fri, 27 Mar 2015 16:15:55 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Dave Hansen <dave@...1.net>
Cc:	linux-kernel@...r.kernel.org, x86@...nel.org, tglx@...utronix.de,
	dave.hansen@...ux.intel.com, oleg@...hat.com, riel@...hat.com,
	sbsiddha@...il.com, luto@...capital.net, mingo@...hat.com,
	hpa@...or.com, fenghua.yu@...el.com
Subject: Re: [PATCH 01/17] x86, fpu: wrap get_xsave_addr() to make it safer

On Thu, Mar 26, 2015 at 11:33:34AM -0700, Dave Hansen wrote:
> 
> From: Dave Hansen <dave.hansen@...ux.intel.com>
> 
> The MPX code appears to be saving off the FPU in an unsafe
> way.   It does not disable preemption or ensure that the
> FPU state has been allocated.
> 
> This patch introduces a new helper which will do both of
> those things internally.
> 
> Note that this requires a patch from Oleg in order to work
> properly.  It is currently in tip/x86/fpu.
> 
> > commit f893959b0898bd876673adbeb6798bdf25c034d7
> > Author: Oleg Nesterov <oleg@...hat.com>
> > Date:   Fri Mar 13 18:30:30 2015 +0100
> >
> >    x86/fpu: Don't abuse drop_init_fpu() in flush_thread()
> 
> Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>
> Cc: Oleg Nesterov <oleg@...hat.com>
> Cc: bp@...en8.de
> Cc: Rik van Riel <riel@...hat.com>
> Cc: Suresh Siddha <sbsiddha@...il.com>
> Cc: Andy Lutomirski <luto@...capital.net>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: "H. Peter Anvin" <hpa@...or.com>
> Cc: Fenghua Yu <fenghua.yu@...el.com>
> Cc: the arch/x86 maintainers <x86@...nel.org>
> ---
> 
>  b/arch/x86/include/asm/xsave.h |    1 +
>  b/arch/x86/kernel/xsave.c      |   32 ++++++++++++++++++++++++++++++++
>  2 files changed, 33 insertions(+)
> 
> diff -puN arch/x86/include/asm/xsave.h~tsk_get_xsave_addr arch/x86/include/asm/xsave.h
> --- a/arch/x86/include/asm/xsave.h~tsk_get_xsave_addr	2015-03-26 11:27:04.738204327 -0700
> +++ b/arch/x86/include/asm/xsave.h	2015-03-26 11:27:04.743204552 -0700
> @@ -252,6 +252,7 @@ static inline int xrestore_user(struct x
>  }
>  
>  void *get_xsave_addr(struct xsave_struct *xsave, int xstate);
> +void *tsk_get_xsave_field(struct task_struct *tsk, int xstate_field);
>  void setup_xstate_comp(void);
>  
>  #endif
> diff -puN arch/x86/kernel/xsave.c~tsk_get_xsave_addr arch/x86/kernel/xsave.c
> --- a/arch/x86/kernel/xsave.c~tsk_get_xsave_addr	2015-03-26 11:27:04.740204417 -0700
> +++ b/arch/x86/kernel/xsave.c	2015-03-26 11:27:04.744204597 -0700
> @@ -740,3 +740,35 @@ void *get_xsave_addr(struct xsave_struct
>  	return (void *)xsave + xstate_comp_offsets[feature];
>  }
>  EXPORT_SYMBOL_GPL(get_xsave_addr);
> +
> +/*
> + * This wraps up the common operations that need to occur when retrieving
> + * data from an xsave struct.  It first ensures that the task was actually
> + * using the FPU and retrieves the data in to a buffer.  It then calculates
> + * the offset of the requested field in the buffer.
> + *
> + * This function is safe to call whether the FPU is in use or not.
> + *
> + * Inputs:
> + *	tsk: the task from which we are fetching xsave state
> + *	xstate: state which is defined in xsave.h (e.g. XSTATE_FP, XSTATE_SSE,

I think you mean @xsave_field here?

> + *	etc.)
> + * Output:
> + *	address of the state in the xsave area.
> + */
> +void *tsk_get_xsave_field(struct task_struct *tsk, int xsave_field)
> +{
> +	union thread_xstate *xstate;
> +
> +	if (!used_math())
> +		return NULL;
> +	/*
> +	 * unlazy_fpu() is poorly named and will actually
> +	 * save the xstate off in to the memory buffer.
> +	 */
> +	unlazy_fpu(tsk);
> +	xstate = tsk->thread.fpu.state;
> +
> +	return get_xsave_addr(&xstate->xsave, xsave_field);
> +}
> +EXPORT_SYMBOL_GPL(tsk_get_xsave_field);

I'm not sure we want to export this to modules... and MPX cannot be
built as a module either. So why export it?

-- 
Regards/Gruss,
    Boris.

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