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Message-ID: <20d31ed9-be3d-dca6-ceef-ced35f80d131@nextfour.com>
Date: Mon, 25 Oct 2021 11:33:05 +0300
From: Mika Penttilä <mika.penttila@...tfour.com>
To: "Chang S. Bae" <chang.seok.bae@...el.com>,
linux-kernel@...r.kernel.org
Cc: x86@...nel.org, tglx@...utronix.de, dave.hansen@...ux.intel.com,
arjan@...ux.intel.com, ravi.v.shankar@...el.com
Subject: Re: [PATCH 15/23] x86/fpu: Add sanity checks for XFD
On 22.10.2021 1.55, Chang S. Bae wrote:
> From: Thomas Gleixner <tglx@...utronix.de>
>
> Add debug functionality to ensure that the XFD MSR is up to date for XSAVE*
> and XRSTOR* operations.
>
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: Chang S. Bae <chang.seok.bae@...el.com>
> ---
> 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=@...k
> + * 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
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