Re: [PATCH 15/23] x86/fpu: Add sanity checks for XFD

[Mika Penttilä]

On 22.10.2021 1.55, Chang S. Bae wrote:
> From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>
> Add debug functionality to ensure that the XFD MSR is up to date for XSAVE*
> and XRSTOR* operations.
>
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Signed-off-by: Chang S. Bae <chang.seok.bae@xxxxxxxxx>
> ---
> Changes from the tglx tree:
> * Rename the xstate ops validation helper. (Dave Hansen)
> * Update code comments. (Dave Hansen)
> ---
>  arch/x86/kernel/fpu/core.c   |  9 +++---
>  arch/x86/kernel/fpu/signal.c |  6 ++--
>  arch/x86/kernel/fpu/xstate.c | 55 ++++++++++++++++++++++++++++++++++++
>  arch/x86/kernel/fpu/xstate.h | 34 +++++++++++++++++++---
>  4 files changed, 92 insertions(+), 12 deletions(-)
>
> diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
> index e92c9d79441c..f4b02ed47034 100644
> --- a/arch/x86/kernel/fpu/core.c
> +++ b/arch/x86/kernel/fpu/core.c
> @@ -166,7 +166,7 @@ void restore_fpregs_from_fpstate(struct fpstate *fpstate, u64 mask)
>  		 */
>  		mask = fpu_kernel_cfg.max_features & mask;
>  
> -		os_xrstor(&fpstate->regs.xsave, mask);
> +		os_xrstor(fpstate, mask);
>  	} else {
>  		if (use_fxsr())
>  			fxrstor(&fpstate->regs.fxsave);
> @@ -537,7 +537,7 @@ void fpu__drop(struct fpu *fpu)
>  static inline void restore_fpregs_from_init_fpstate(u64 features_mask)
>  {
>  	if (use_xsave())
> -		os_xrstor(&init_fpstate.regs.xsave, features_mask);
> +		os_xrstor(&init_fpstate, features_mask);
>  	else if (use_fxsr())
>  		fxrstor(&init_fpstate.regs.fxsave);
>  	else
> @@ -594,9 +594,8 @@ void fpu__clear_user_states(struct fpu *fpu)
>  	 * corresponding registers.
>  	 */
>  	if (xfeatures_mask_supervisor() &&
> -	    !fpregs_state_valid(fpu, smp_processor_id())) {
> -		os_xrstor(&fpu->fpstate->regs.xsave, xfeatures_mask_supervisor());
> -	}
> +	    !fpregs_state_valid(fpu, smp_processor_id()))
> +		os_xrstor_supervisor(fpu->fpstate);
>  
>  	/* Reset user states in registers. */
>  	restore_fpregs_from_init_fpstate(XFEATURE_MASK_USER_RESTORE);
> diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
> index d128eb581ffc..a937980fd02b 100644
> --- a/arch/x86/kernel/fpu/signal.c
> +++ b/arch/x86/kernel/fpu/signal.c
> @@ -261,7 +261,7 @@ static int __restore_fpregs_from_user(void __user *buf, u64 ufeatures,
>  			ret = fxrstor_from_user_sigframe(buf);
>  
>  		if (!ret && unlikely(init_bv))
> -			os_xrstor(&init_fpstate.regs.xsave, init_bv);
> +			os_xrstor(&init_fpstate, init_bv);
>  		return ret;
>  	} else if (use_fxsr()) {
>  		return fxrstor_from_user_sigframe(buf);
> @@ -322,7 +322,7 @@ static bool restore_fpregs_from_user(void __user *buf, u64 xrestore,
>  	 * been restored from a user buffer directly.
>  	 */
>  	if (test_thread_flag(TIF_NEED_FPU_LOAD) && xfeatures_mask_supervisor())
> -		os_xrstor(&fpu->fpstate->regs.xsave, xfeatures_mask_supervisor());
> +		os_xrstor_supervisor(fpu->fpstate);
>  
>  	fpregs_mark_activate();
>  	fpregs_unlock();
> @@ -432,7 +432,7 @@ static bool __fpu_restore_sig(void __user *buf, void __user *buf_fx,
>  		u64 mask = user_xfeatures | xfeatures_mask_supervisor();
>  
>  		fpregs->xsave.header.xfeatures &= mask;
> -		success = !os_xrstor_safe(&fpregs->xsave,
> +		success = !os_xrstor_safe(fpu->fpstate,
>  					  fpu_kernel_cfg.max_features);
>  	} else {
>  		success = !fxrstor_safe(&fpregs->fxsave);
> diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
> index c178aa91abb3..c2cbe14aaa00 100644
> --- a/arch/x86/kernel/fpu/xstate.c
> +++ b/arch/x86/kernel/fpu/xstate.c
> @@ -1301,6 +1301,61 @@ EXPORT_SYMBOL_GPL(fpstate_clear_xstate_component);
>  #endif
>  
>  #ifdef CONFIG_X86_64
> +
> +#ifdef CONFIG_X86_DEBUG_FPU
> +/*
> + * Ensure that a subsequent XSAVE* or XRSTOR* instruction with RFBM=@mask
> + * can safely operate on the @fpstate buffer.
> + */
> +static bool xstate_op_valid(struct fpstate *fpstate, u64 mask, bool rstor)
> +{
> +	u64 xfd = __this_cpu_read(xfd_state);
> +
> +	if (fpstate->xfd == xfd)
> +		return true;
> +
> +	/* For current's fpstate the XFD state must be correct. */
> +	if (fpstate->xfd == current->thread.fpu.fpstate->xfd)
> +		return false;
> +
Should this return true or is the comment confusing?


> +	/*
> +	 * XRSTOR(S) from init_fpstate are always correct as it will just
> +	 * bring all components into init state and not read from the
> +	 * buffer. XSAVE(S) raises #PF after init.
> +	 */
> +	if (fpstate == &init_fpstate)
> +		return rstor;
> +
> +	/*
> +	 * XSAVE(S): clone(), fpu_swap_kvm_fpu()
> +	 * XRSTORS(S): fpu_swap_kvm_fpu()
> +	 */
> +
> +	/*
> +	 * No XSAVE/XRSTOR instructions (except XSAVE itself) touch
> +	 * the buffer area for XFD-disabled state components.
> +	 */
> +	mask &= ~xfd;
> +
> +	/*
> +	 * Remove features which are valid in fpstate. They
> +	 * have space allocated in fpstate.
> +	 */
> +	mask &= ~fpstate->xfeatures;
> +
> +	/*
> +	 * Any remaining state components in 'mask' might be written
> +	 * by XSAVE/XRSTOR. Fail validation it found.
> +	 */
> +	return !mask;
> +}
> +
> +void xfd_validate_state(struct fpstate *fpstate, u64 mask, bool rstor)
> +{
> +	WARN_ON_ONCE(!xstate_op_valid(fpstate, mask, rstor));
> +}
> +#endif /* CONFIG_X86_DEBUG_FPU */
> +
>  static int validate_sigaltstack(unsigned int usize)
>  {
>  	struct task_struct *thread, *leader = current->group_leader;
> diff --git a/arch/x86/kernel/fpu/xstate.h b/arch/x86/kernel/fpu/xstate.h
> index 32a4dee4de3b..29024244965b 100644
> --- a/arch/x86/kernel/fpu/xstate.h
> +++ b/arch/x86/kernel/fpu/xstate.h
> @@ -130,6 +130,12 @@ static inline u64 xfeatures_mask_independent(void)
>  		     : "D" (st), "m" (*st), "a" (lmask), "d" (hmask)	\
>  		     : "memory")
>  
> +#if defined(CONFIG_X86_64) && defined(CONFIG_X86_DEBUG_FPU)
> +extern void xfd_validate_state(struct fpstate *fpstate, u64 mask, bool rstor);
> +#else
> +static inline void xfd_validate_state(struct fpstate *fpstate, u64 mask, bool rstor) { }
> +#endif
> +
>  /*
>   * Save processor xstate to xsave area.
>   *
> @@ -144,6 +150,7 @@ static inline void os_xsave(struct fpstate *fpstate)
>  	int err;
>  
>  	WARN_ON_FPU(!alternatives_patched);
> +	xfd_validate_state(fpstate, mask, false);
>  
>  	XSTATE_XSAVE(&fpstate->regs.xsave, lmask, hmask, err);
>  
> @@ -156,12 +163,23 @@ static inline void os_xsave(struct fpstate *fpstate)
>   *
>   * Uses XRSTORS when XSAVES is used, XRSTOR otherwise.
>   */
> -static inline void os_xrstor(struct xregs_state *xstate, u64 mask)
> +static inline void os_xrstor(struct fpstate *fpstate, u64 mask)
> +{
> +	u32 lmask = mask;
> +	u32 hmask = mask >> 32;
> +
> +	xfd_validate_state(fpstate, mask, true);
> +	XSTATE_XRESTORE(&fpstate->regs.xsave, lmask, hmask);
> +}
> +
> +/* Restore of supervisor state. Does not require XFD */
> +static inline void os_xrstor_supervisor(struct fpstate *fpstate)
>  {
> +	u64 mask = xfeatures_mask_supervisor();
>  	u32 lmask = mask;
>  	u32 hmask = mask >> 32;
>  
> -	XSTATE_XRESTORE(xstate, lmask, hmask);
> +	XSTATE_XRESTORE(&fpstate->regs.xsave, lmask, hmask);
>  }
>  
>  /*
> @@ -184,11 +202,14 @@ static inline int xsave_to_user_sigframe(struct xregs_state __user *buf)
>  	 * internally, e.g. PKRU. That's user space ABI and also required
>  	 * to allow the signal handler to modify PKRU.
>  	 */
> -	u64 mask = current->thread.fpu.fpstate->user_xfeatures;
> +	struct fpstate *fpstate = current->thread.fpu.fpstate;
> +	u64 mask = fpstate->user_xfeatures;
>  	u32 lmask = mask;
>  	u32 hmask = mask >> 32;
>  	int err;
>  
> +	xfd_validate_state(fpstate, mask, false);
> +
>  	stac();
>  	XSTATE_OP(XSAVE, buf, lmask, hmask, err);
>  	clac();
> @@ -206,6 +227,8 @@ static inline int xrstor_from_user_sigframe(struct xregs_state __user *buf, u64
>  	u32 hmask = mask >> 32;
>  	int err;
>  
> +	xfd_validate_state(current->thread.fpu.fpstate, mask, true);
> +
>  	stac();
>  	XSTATE_OP(XRSTOR, xstate, lmask, hmask, err);
>  	clac();
> @@ -217,12 +240,15 @@ static inline int xrstor_from_user_sigframe(struct xregs_state __user *buf, u64
>   * Restore xstate from kernel space xsave area, return an error code instead of
>   * an exception.
>   */
> -static inline int os_xrstor_safe(struct xregs_state *xstate, u64 mask)
> +static inline int os_xrstor_safe(struct fpstate *fpstate, u64 mask)
>  {
> +	struct xregs_state *xstate = &fpstate->regs.xsave;
>  	u32 lmask = mask;
>  	u32 hmask = mask >> 32;
>  	int err;
>  
> +	/* Must enforce XFD update here */
> +
>  	if (cpu_feature_enabled(X86_FEATURE_XSAVES))
>  		XSTATE_OP(XRSTORS, xstate, lmask, hmask, err);
>  	else