Re: [PATCH 01/17] x86, fpu: wrap get_xsave_addr() to make it safer

Andy Lutomirski
2015-03-24T22:29:06Z

On Tue, Mar 24, 2015 at 3:20 PM, Dave Hansen <dave.hansen@intel.com> wrote:
> From: Dave Hansen <dave.hansen@linux.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 to a helper.
>
> Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: Rik van Riel <riel@redhat.com>
> Cc: Suresh Siddha <sbsiddha@gmail.com>
> Cc: Andy Lutomirski <luto@amacapital.net>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: "H. Peter Anvin" <hpa@zytor.com>
> Cc: Fenghua Yu <fenghua.yu@intel.com>
> Cc: the arch/x86 maintainers <x86@kernel.org>
> ---
> arch/x86/include/asm/xsave.h | 1 +
> arch/x86/kernel/xsave.c | 32 ++++++++++++++++++++++++++++++++
> 2 files changed, 33 insertions(+)
>
> diff --git a/arch/x86/include/asm/xsave.h b/arch/x86/include/asm/xsave.h
> index c9a6d68..86b58fb 100644
> --- a/arch/x86/include/asm/xsave.h
> +++ b/arch/x86/include/asm/xsave.h
> @@ -252,6 +252,7 @@ static inline int xrestore_user(struct xsave_struct __user *buf, u64 mask)
> }
>
> 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 --git a/arch/x86/kernel/xsave.c b/arch/x86/kernel/xsave.c
> index 34f66e5..9919e7e 100644
> --- a/arch/x86/kernel/xsave.c
> +++ b/arch/x86/kernel/xsave.c
> @@ -740,3 +740,35 @@ void *get_xsave_addr(struct xsave_struct *xsave, int xstate)
> 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,
> + * 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;
> +
> + unlazy_fpu(tsk);
> + xstate = tsk->thread.fpu.state;
> + /*
> + * This might be unallocated if the FPU
> + * was never in use.
> + */
> + if (!xstate)
> + return NULL;
> +
> + return get_xsave_addr(&xstate->xsave, xsave_field);
> +}
> +EXPORT_SYMBOL_GPL(tsk_get_xsave_field);
> --
> 1.9.1
>

I have one objection to get_xsave_addr and tsk_get_xsave_field: I
think that get_xsave_addr should be called either
get_xsave_addr_for_read or get_xsave_addr_for_write, depending on
which of those it does.

Your function appears to be getting it for write (I assume that's what
the unlazy_fpu is for), so I'd rather have it called
tsk_get_xsave_field_for_write or something like that.

--Andy

--
Andy Lutomirski
AMA Capital Management, LLC