Re: [PATCH v20 21/25] x86/cet/shstk: Handle signals for shadow stack

[Kees Cook]

On Wed, Feb 10, 2021 at 09:56:59AM -0800, Yu-cheng Yu wrote:
> To deliver a signal, create a shadow stack restore token and put the token
> and the signal restorer address on the shadow stack.  For sigreturn, verify
> the token and restore from it the shadow stack pointer.
> 
> A shadow stack restore token marks a restore point of the shadow stack.
> The token is distinctively different from any shadow stack address.

How is it different? It seems like it just has the last 2 bits
masked/set?

> In sigreturn, restoring from a token ensures the target address is the
> location pointed by the token.

As in, a token (real stack address with 2-bit mask) is checked against
the real stack address? I don't see a comparison -- it only checks that
it is < TASK_SIZE.

How does cet_restore_signal() figure into this? (As in, the MSR writes?)

-Kees

> Introduce WRUSS, which is a kernel-mode instruction but writes directly to
> user shadow stack.  It is used to construct the user signal stack as
> described above.
> 
> Currently there is no systematic facility for extending a signal context.
> Introduce a signal context extension 'struct sc_ext', which is used to save
> shadow stack restore token address and WAIT_ENDBR status.  WAIT_ENDBR will
> be introduced later in the Indirect Branch Tracking (IBT) series, but add
> that into sc_ext now to keep the struct stable in case the IBT series is
> applied later.
> 
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
> ---
>  arch/x86/ia32/ia32_signal.c            |  17 +++
>  arch/x86/include/asm/cet.h             |   8 ++
>  arch/x86/include/asm/fpu/internal.h    |  10 ++
>  arch/x86/include/asm/special_insns.h   |  32 ++++++
>  arch/x86/include/uapi/asm/sigcontext.h |   9 ++
>  arch/x86/kernel/cet.c                  | 152 +++++++++++++++++++++++++
>  arch/x86/kernel/fpu/signal.c           | 100 ++++++++++++++++
>  arch/x86/kernel/signal.c               |  10 ++
>  8 files changed, 338 insertions(+)
> 
> diff --git a/arch/x86/ia32/ia32_signal.c b/arch/x86/ia32/ia32_signal.c
> index 5e3d9b7fd5fb..aee3e367e184 100644
> --- a/arch/x86/ia32/ia32_signal.c
> +++ b/arch/x86/ia32/ia32_signal.c
> @@ -35,6 +35,7 @@
>  #include <asm/sigframe.h>
>  #include <asm/sighandling.h>
>  #include <asm/smap.h>
> +#include <asm/cet.h>
>  
>  static inline void reload_segments(struct sigcontext_32 *sc)
>  {
> @@ -205,6 +206,7 @@ static void __user *get_sigframe(struct ksignal *ksig, struct pt_regs *regs,
>  				 void __user **fpstate)
>  {
>  	unsigned long sp, fx_aligned, math_size;
> +	void __user *restorer = NULL;
>  
>  	/* Default to using normal stack */
>  	sp = regs->sp;
> @@ -218,8 +220,23 @@ static void __user *get_sigframe(struct ksignal *ksig, struct pt_regs *regs,
>  		 ksig->ka.sa.sa_restorer)
>  		sp = (unsigned long) ksig->ka.sa.sa_restorer;
>  
> +	if (ksig->ka.sa.sa_flags & SA_RESTORER) {
> +		restorer = ksig->ka.sa.sa_restorer;
> +	} else if (current->mm->context.vdso) {
> +		if (ksig->ka.sa.sa_flags & SA_SIGINFO)
> +			restorer = current->mm->context.vdso +
> +				vdso_image_32.sym___kernel_rt_sigreturn;
> +		else
> +			restorer = current->mm->context.vdso +
> +				vdso_image_32.sym___kernel_sigreturn;
> +	}
> +
>  	sp = fpu__alloc_mathframe(sp, 1, &fx_aligned, &math_size);
>  	*fpstate = (struct _fpstate_32 __user *) sp;
> +
> +	if (save_cet_to_sigframe(1, *fpstate, (unsigned long)restorer))
> +		return (void __user *)-1L;
> +
>  	if (copy_fpstate_to_sigframe(*fpstate, (void __user *)fx_aligned,
>  				     math_size) < 0)
>  		return (void __user *) -1L;
> diff --git a/arch/x86/include/asm/cet.h b/arch/x86/include/asm/cet.h
> index 5750fbcbb952..73435856ce54 100644
> --- a/arch/x86/include/asm/cet.h
> +++ b/arch/x86/include/asm/cet.h
> @@ -6,6 +6,8 @@
>  #include <linux/types.h>
>  
>  struct task_struct;
> +struct sc_ext;
> +
>  /*
>   * Per-thread CET status
>   */
> @@ -18,9 +20,15 @@ struct cet_status {
>  int cet_setup_shstk(void);
>  void cet_disable_shstk(void);
>  void cet_free_shstk(struct task_struct *p);
> +int cet_verify_rstor_token(bool ia32, unsigned long ssp, unsigned long *new_ssp);
> +void cet_restore_signal(struct sc_ext *sc);
> +int cet_setup_signal(bool ia32, unsigned long rstor, struct sc_ext *sc);
>  #else
>  static inline void cet_disable_shstk(void) {}
>  static inline void cet_free_shstk(struct task_struct *p) {}
> +static inline void cet_restore_signal(struct sc_ext *sc) { return; }
> +static inline int cet_setup_signal(bool ia32, unsigned long rstor,
> +				   struct sc_ext *sc) { return -EINVAL; }
>  #endif
>  
>  #endif /* __ASSEMBLY__ */
> diff --git a/arch/x86/include/asm/fpu/internal.h b/arch/x86/include/asm/fpu/internal.h
> index 8d33ad80704f..2c1f59ebe9d8 100644
> --- a/arch/x86/include/asm/fpu/internal.h
> +++ b/arch/x86/include/asm/fpu/internal.h
> @@ -443,6 +443,16 @@ static inline void copy_kernel_to_fpregs(union fpregs_state *fpstate)
>  	__copy_kernel_to_fpregs(fpstate, -1);
>  }
>  
> +#ifdef CONFIG_X86_CET
> +extern int save_cet_to_sigframe(int ia32, void __user *fp,
> +				unsigned long restorer);
> +#else
> +static inline int save_cet_to_sigframe(int ia32, void __user *fp,
> +				       unsigned long restorer)
> +{
> +	return 0;
> +}
> +#endif
>  extern int copy_fpstate_to_sigframe(void __user *buf, void __user *fp, int size);
>  
>  /*
> diff --git a/arch/x86/include/asm/special_insns.h b/arch/x86/include/asm/special_insns.h
> index cc177b4431ae..d979d0deb3ae 100644
> --- a/arch/x86/include/asm/special_insns.h
> +++ b/arch/x86/include/asm/special_insns.h
> @@ -234,6 +234,38 @@ static inline void clwb(volatile void *__p)
>  		: [pax] "a" (p));
>  }
>  
> +#ifdef CONFIG_X86_CET
> +#if defined(CONFIG_IA32_EMULATION) || defined(CONFIG_X86_X32)
> +static inline int write_user_shstk_32(unsigned long addr, unsigned int val)
> +{
> +	asm_volatile_goto("1: wrussd %1, (%0)\n"
> +			  _ASM_EXTABLE(1b, %l[fail])
> +			  :: "r" (addr), "r" (val)
> +			  :: fail);
> +	return 0;
> +fail:
> +	return -EPERM;
> +}
> +#else
> +static inline int write_user_shstk_32(unsigned long addr, unsigned int val)
> +{
> +	WARN_ONCE(1, "%s used but not supported.\n", __func__);
> +	return -EFAULT;
> +}
> +#endif
> +
> +static inline int write_user_shstk_64(unsigned long addr, unsigned long val)
> +{
> +	asm_volatile_goto("1: wrussq %1, (%0)\n"
> +			  _ASM_EXTABLE(1b, %l[fail])
> +			  :: "r" (addr), "r" (val)
> +			  :: fail);
> +	return 0;
> +fail:
> +	return -EPERM;
> +}
> +#endif /* CONFIG_X86_CET */
> +
>  #define nop() asm volatile ("nop")
>  
>  static inline void serialize(void)
> diff --git a/arch/x86/include/uapi/asm/sigcontext.h b/arch/x86/include/uapi/asm/sigcontext.h
> index 844d60eb1882..cf2d55db3be4 100644
> --- a/arch/x86/include/uapi/asm/sigcontext.h
> +++ b/arch/x86/include/uapi/asm/sigcontext.h
> @@ -196,6 +196,15 @@ struct _xstate {
>  	/* New processor state extensions go here: */
>  };
>  
> +/*
> + * Located at the end of sigcontext->fpstate, aligned to 8.
> + */
> +struct sc_ext {
> +	unsigned long total_size;
> +	unsigned long ssp;
> +	unsigned long wait_endbr;
> +};
> +
>  /*
>   * The 32-bit signal frame:
>   */
> diff --git a/arch/x86/kernel/cet.c b/arch/x86/kernel/cet.c
> index d25a03215984..08e43d9b5176 100644
> --- a/arch/x86/kernel/cet.c
> +++ b/arch/x86/kernel/cet.c
> @@ -19,6 +19,8 @@
>  #include <asm/fpu/xstate.h>
>  #include <asm/fpu/types.h>
>  #include <asm/cet.h>
> +#include <asm/special_insns.h>
> +#include <uapi/asm/sigcontext.h>
>  
>  static void start_update_msrs(void)
>  {
> @@ -72,6 +74,80 @@ static unsigned long alloc_shstk(unsigned long size, int flags)
>  	return addr;
>  }
>  
> +#define TOKEN_MODE_MASK	3UL
> +#define TOKEN_MODE_64	1UL
> +#define IS_TOKEN_64(token) (((token) & TOKEN_MODE_MASK) == TOKEN_MODE_64)
> +#define IS_TOKEN_32(token) (((token) & TOKEN_MODE_MASK) == 0)
> +
> +/*
> + * Verify the restore token at the address of 'ssp' is
> + * valid and then set shadow stack pointer according to the
> + * token.
> + */
> +int cet_verify_rstor_token(bool ia32, unsigned long ssp,
> +			   unsigned long *new_ssp)
> +{
> +	unsigned long token;
> +
> +	*new_ssp = 0;
> +
> +	if (!IS_ALIGNED(ssp, 8))
> +		return -EINVAL;
> +
> +	if (get_user(token, (unsigned long __user *)ssp))
> +		return -EFAULT;
> +
> +	/* Is 64-bit mode flag correct? */
> +	if (!ia32 && !IS_TOKEN_64(token))
> +		return -EINVAL;
> +	else if (ia32 && !IS_TOKEN_32(token))
> +		return -EINVAL;
> +
> +	token &= ~TOKEN_MODE_MASK;
> +
> +	/*
> +	 * Restore address properly aligned?
> +	 */
> +	if ((!ia32 && !IS_ALIGNED(token, 8)) || !IS_ALIGNED(token, 4))
> +		return -EINVAL;
> +
> +	/*
> +	 * Token was placed properly?
> +	 */
> +	if (((ALIGN_DOWN(token, 8) - 8) != ssp) || token >= TASK_SIZE_MAX)
> +		return -EINVAL;
> +
> +	*new_ssp = token;
> +	return 0;
> +}
> +
> +/*
> + * Create a restore token on the shadow stack.
> + * A token is always 8-byte and aligned to 8.
> + */
> +static int create_rstor_token(bool ia32, unsigned long ssp,
> +			      unsigned long *new_ssp)
> +{
> +	unsigned long addr;
> +
> +	*new_ssp = 0;
> +
> +	if ((!ia32 && !IS_ALIGNED(ssp, 8)) || !IS_ALIGNED(ssp, 4))
> +		return -EINVAL;
> +
> +	addr = ALIGN_DOWN(ssp, 8) - 8;
> +
> +	/* Is the token for 64-bit? */
> +	if (!ia32)
> +		ssp |= TOKEN_MODE_64;
> +
> +	if (write_user_shstk_64(addr, ssp))
> +		return -EFAULT;
> +
> +	*new_ssp = addr;
> +	return 0;
> +}
> +
>  int cet_setup_shstk(void)
>  {
>  	unsigned long addr, size;
> @@ -145,3 +221,79 @@ void cet_free_shstk(struct task_struct *tsk)
>  	cet->shstk_base = 0;
>  	cet->shstk_size = 0;
>  }
> +
> +/*
> + * Called from __fpu__restore_sig() and XSAVES buffer is protected by
> + * set_thread_flag(TIF_NEED_FPU_LOAD) in the slow path.
> + */
> +void cet_restore_signal(struct sc_ext *sc_ext)
> +{
> +	struct cet_user_state *cet_user_state;
> +	struct cet_status *cet = &current->thread.cet;
> +	u64 msr_val = 0;
> +
> +	if (!static_cpu_has(X86_FEATURE_SHSTK))
> +		return;
> +
> +	cet_user_state = get_xsave_addr(&current->thread.fpu.state.xsave,
> +					XFEATURE_CET_USER);
> +	if (!cet_user_state)
> +		return;
> +
> +	if (cet->shstk_size) {
> +		if (test_thread_flag(TIF_NEED_FPU_LOAD))
> +			cet_user_state->user_ssp = sc_ext->ssp;
> +		else
> +			wrmsrl(MSR_IA32_PL3_SSP, sc_ext->ssp);
> +
> +		msr_val |= CET_SHSTK_EN;
> +	}
> +
> +	if (test_thread_flag(TIF_NEED_FPU_LOAD))
> +		cet_user_state->user_cet = msr_val;
> +	else
> +		wrmsrl(MSR_IA32_U_CET, msr_val);
> +}
> +
> +/*
> + * Setup the shadow stack for the signal handler: first,
> + * create a restore token to keep track of the current ssp,
> + * and then the return address of the signal handler.
> + */
> +int cet_setup_signal(bool ia32, unsigned long rstor_addr, struct sc_ext *sc_ext)
> +{
> +	struct cet_status *cet = &current->thread.cet;
> +	unsigned long ssp = 0, new_ssp = 0;
> +	int err;
> +
> +	if (cet->shstk_size) {
> +		if (!rstor_addr)
> +			return -EINVAL;
> +
> +		ssp = cet_get_shstk_addr();
> +		err = create_rstor_token(ia32, ssp, &new_ssp);
> +		if (err)
> +			return err;
> +
> +		if (ia32) {
> +			ssp = new_ssp - sizeof(u32);
> +			err = write_user_shstk_32(ssp, (unsigned int)rstor_addr);
> +		} else {
> +			ssp = new_ssp - sizeof(u64);
> +			err = write_user_shstk_64(ssp, rstor_addr);
> +		}
> +
> +		if (err)
> +			return err;
> +
> +		sc_ext->ssp = new_ssp;
> +	}
> +
> +	if (ssp) {
> +		start_update_msrs();
> +		wrmsrl(MSR_IA32_PL3_SSP, ssp);
> +		end_update_msrs();
> +	}
> +
> +	return 0;
> +}
> diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
> index a4ec65317a7f..270e4649f435 100644
> --- a/arch/x86/kernel/fpu/signal.c
> +++ b/arch/x86/kernel/fpu/signal.c
> @@ -52,6 +52,74 @@ static inline int check_for_xstate(struct fxregs_state __user *buf,
>  	return 0;
>  }
>  
> +#ifdef CONFIG_X86_CET
> +int save_cet_to_sigframe(int ia32, void __user *fp, unsigned long restorer)
> +{
> +	int err = 0;
> +
> +	if (!current->thread.cet.shstk_size)
> +		return 0;
> +
> +	if (fp) {
> +		struct sc_ext ext = {};
> +
> +		err = cet_setup_signal(ia32, restorer, &ext);
> +		if (!err) {
> +			void __user *p = fp;
> +
> +			ext.total_size = sizeof(ext);
> +
> +			if (ia32)
> +				p += sizeof(struct fregs_state);
> +
> +			p += fpu_user_xstate_size + FP_XSTATE_MAGIC2_SIZE;
> +			p = (void __user *)ALIGN((unsigned long)p, 8);
> +
> +			if (copy_to_user(p, &ext, sizeof(ext)))
> +				return -EFAULT;
> +		}
> +	}
> +
> +	return err;
> +}
> +
> +static int get_cet_from_sigframe(int ia32, void __user *fp, struct sc_ext *ext)
> +{
> +	int err = 0;
> +
> +	memset(ext, 0, sizeof(*ext));
> +
> +	if (!current->thread.cet.shstk_size)
> +		return 0;
> +
> +	if (fp) {
> +		void __user *p = fp;
> +
> +		if (ia32)
> +			p += sizeof(struct fregs_state);
> +
> +		p += fpu_user_xstate_size + FP_XSTATE_MAGIC2_SIZE;
> +		p = (void __user *)ALIGN((unsigned long)p, 8);
> +
> +		if (copy_from_user(ext, p, sizeof(*ext)))
> +			return -EFAULT;
> +
> +		if (ext->total_size != sizeof(*ext))
> +			return -EFAULT;
> +
> +		if (current->thread.cet.shstk_size)
> +			err = cet_verify_rstor_token(ia32, ext->ssp, &ext->ssp);
> +	}
> +
> +	return err;
> +}
> +#else
> +static int get_cet_from_sigframe(int ia32, void __user *fp, struct sc_ext *ext)
> +{
> +	return 0;
> +}
> +#endif
> +
>  /*
>   * Signal frame handlers.
>   */
> @@ -295,6 +363,7 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
>  	struct task_struct *tsk = current;
>  	struct fpu *fpu = &tsk->thread.fpu;
>  	struct user_i387_ia32_struct env;
> +	struct sc_ext sc_ext;
>  	u64 user_xfeatures = 0;
>  	int fx_only = 0;
>  	int ret = 0;
> @@ -335,6 +404,10 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
>  	if ((unsigned long)buf_fx % 64)
>  		fx_only = 1;
>  
> +	ret = get_cet_from_sigframe(ia32_fxstate, buf, &sc_ext);
> +	if (ret)
> +		return ret;
> +
>  	if (!ia32_fxstate) {
>  		/*
>  		 * Attempt to restore the FPU registers directly from user
> @@ -349,6 +422,8 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
>  		pagefault_enable();
>  		if (!ret) {
>  
> +			cet_restore_signal(&sc_ext);
> +
>  			/*
>  			 * Restore supervisor states: previous context switch
>  			 * etc has done XSAVES and saved the supervisor states
> @@ -423,6 +498,8 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
>  		if (unlikely(init_bv))
>  			copy_kernel_to_xregs(&init_fpstate.xsave, init_bv);
>  
> +		cet_restore_signal(&sc_ext);
> +
>  		/*
>  		 * Restore previously saved supervisor xstates along with
>  		 * copied-in user xstates.
> @@ -491,12 +568,35 @@ int fpu__restore_sig(void __user *buf, int ia32_frame)
>  	return __fpu__restore_sig(buf, buf_fx, size);
>  }
>  
> +#ifdef CONFIG_X86_CET
> +static unsigned long fpu__alloc_sigcontext_ext(unsigned long sp)
> +{
> +	struct cet_status *cet = &current->thread.cet;
> +
> +	/*
> +	 * sigcontext_ext is at: fpu + fpu_user_xstate_size +
> +	 * FP_XSTATE_MAGIC2_SIZE, then aligned to 8.
> +	 */
> +	if (cet->shstk_size)
> +		sp -= (sizeof(struct sc_ext) + 8);
> +
> +	return sp;
> +}
> +#else
> +static unsigned long fpu__alloc_sigcontext_ext(unsigned long sp)
> +{
> +	return sp;
> +}
> +#endif
> +
>  unsigned long
>  fpu__alloc_mathframe(unsigned long sp, int ia32_frame,
>  		     unsigned long *buf_fx, unsigned long *size)
>  {
>  	unsigned long frame_size = xstate_sigframe_size();
>  
> +	sp = fpu__alloc_sigcontext_ext(sp);
> +
>  	*buf_fx = sp = round_down(sp - frame_size, 64);
>  	if (ia32_frame && use_fxsr()) {
>  		frame_size += sizeof(struct fregs_state);
> diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
> index ea794a083c44..1807379f1d86 100644
> --- a/arch/x86/kernel/signal.c
> +++ b/arch/x86/kernel/signal.c
> @@ -46,6 +46,7 @@
>  #include <asm/syscall.h>
>  #include <asm/sigframe.h>
>  #include <asm/signal.h>
> +#include <asm/cet.h>
>  
>  #ifdef CONFIG_X86_64
>  /*
> @@ -239,6 +240,9 @@ get_sigframe(struct k_sigaction *ka, struct pt_regs *regs, size_t frame_size,
>  	unsigned long buf_fx = 0;
>  	int onsigstack = on_sig_stack(sp);
>  	int ret;
> +#ifdef CONFIG_X86_64
> +	void __user *restorer = NULL;
> +#endif
>  
>  	/* redzone */
>  	if (IS_ENABLED(CONFIG_X86_64))
> @@ -270,6 +274,12 @@ get_sigframe(struct k_sigaction *ka, struct pt_regs *regs, size_t frame_size,
>  	if (onsigstack && !likely(on_sig_stack(sp)))
>  		return (void __user *)-1L;
>  
> +#ifdef CONFIG_X86_64
> +	if (ka->sa.sa_flags & SA_RESTORER)
> +		restorer = ka->sa.sa_restorer;
> +	ret = save_cet_to_sigframe(0, *fpstate, (unsigned long)restorer);
> +#endif
> +
>  	/* save i387 and extended state */
>  	ret = copy_fpstate_to_sigframe(*fpstate, (void __user *)buf_fx, math_size);
>  	if (ret < 0)
> -- 
> 2.21.0
> 
> 

-- 
Kees Cook