Re: [PATCH 02/10] x86/cet: Introduce WRUSS instruction

[Balbir Singh]

On Thu, 2018-06-07 at 07:37 -0700, Yu-cheng Yu wrote:
> WRUSS is a new kernel-mode instruction but writes directly
> to user shadow stack memory.  This is used to construct
> a return address on the shadow stack for the signal
> handler.
> 
> This instruction can fault if the user shadow stack is
> invalid shadow stack memory.  In that case, the kernel does
> fixup.
> 
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
> ---
>  arch/x86/include/asm/special_insns.h          | 44 +++++++++++++++++++++++++++
>  arch/x86/lib/x86-opcode-map.txt               |  2 +-
>  arch/x86/mm/fault.c                           | 13 +++++++-
>  tools/objtool/arch/x86/lib/x86-opcode-map.txt |  2 +-
>  4 files changed, 58 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/include/asm/special_insns.h b/arch/x86/include/asm/special_insns.h
> index 317fc59b512c..8ce532fcc171 100644
> --- a/arch/x86/include/asm/special_insns.h
> +++ b/arch/x86/include/asm/special_insns.h
> @@ -237,6 +237,50 @@ static inline void clwb(volatile void *__p)
>  		: [pax] "a" (p));
>  }
>  
> +#ifdef CONFIG_X86_INTEL_CET
> +
> +#if defined(CONFIG_IA32_EMULATION) || defined(CONFIG_X86_X32)
> +static inline int write_user_shstk_32(unsigned long addr, unsigned int val)
> +{
> +	int err;
> +
> +	asm volatile("1:.byte 0x66, 0x0f, 0x38, 0xf5, 0x37\n"

It would nice to use something like ASM_WRUSS/Q like ASM_CLAC/ASM_STAC.
Is the 0x37 spurious? I don't see addr/val being used in the instructions
either.

> +		     "xor %[err],%[err]\n"
> +		     "2:\n"
> +		     ".section .fixup,\"ax\"\n"
> +		     "3: mov $-1,%[err]; jmp 2b\n"
> +		     ".previous\n"
> +		     _ASM_EXTABLE(1b, 3b)
> +		: [err] "=a" (err)
> +		: [val] "S" (val), [addr] "D" (addr)
> +		: "memory");
> +	return err;
> +}
> +#else
> +static inline int write_user_shstk_32(unsigned long addr, unsigned int val)
> +{
> +	return 0;
> +}
> +#endif
> +
> +static inline int write_user_shstk_64(unsigned long addr, unsigned long val)
> +{
> +	int err;
> +
> +	asm volatile("1:.byte 0x66, 0x48, 0x0f, 0x38, 0xf5, 0x37\n"
> +		     "xor %[err],%[err]\n"
> +		     "2:\n"
> +		     ".section .fixup,\"ax\"\n"
> +		     "3: mov $-1,%[err]; jmp 2b\n"
> +		     ".previous\n"
> +		     _ASM_EXTABLE(1b, 3b)
> +		: [err] "=a" (err)
> +		: [val] "S" (val), [addr] "D" (addr)
> +		: "memory");
> +	return err;
> +}
> +#endif /* CONFIG_X86_INTEL_CET */
> +
>  #define nop() asm volatile ("nop")
>  
>  
> diff --git a/arch/x86/lib/x86-opcode-map.txt b/arch/x86/lib/x86-opcode-map.txt
> index e0b85930dd77..72bb7c48a7df 100644
> --- a/arch/x86/lib/x86-opcode-map.txt
> +++ b/arch/x86/lib/x86-opcode-map.txt
> @@ -789,7 +789,7 @@ f0: MOVBE Gy,My | MOVBE Gw,Mw (66) | CRC32 Gd,Eb (F2) | CRC32 Gd,Eb (66&F2)
>  f1: MOVBE My,Gy | MOVBE Mw,Gw (66) | CRC32 Gd,Ey (F2) | CRC32 Gd,Ew (66&F2)
>  f2: ANDN Gy,By,Ey (v)
>  f3: Grp17 (1A)
> -f5: BZHI Gy,Ey,By (v) | PEXT Gy,By,Ey (F3),(v) | PDEP Gy,By,Ey (F2),(v)
> +f5: BZHI Gy,Ey,By (v) | PEXT Gy,By,Ey (F3),(v) | PDEP Gy,By,Ey (F2),(v) | WRUSS Pq,Qq (66),REX.W
>  f6: ADCX Gy,Ey (66) | ADOX Gy,Ey (F3) | MULX By,Gy,rDX,Ey (F2),(v)
>  f7: BEXTR Gy,Ey,By (v) | SHLX Gy,Ey,By (66),(v) | SARX Gy,Ey,By (F3),(v) | SHRX Gy,Ey,By (F2),(v)
>  EndTable
> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index 2b3b9170109c..f157338862f8 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -640,6 +640,17 @@ static int is_f00f_bug(struct pt_regs *regs, unsigned long address)
>  	return 0;
>  }
>  
> +/*
> + * WRUSS is a kernel instrcution and but writes to user
> + * shadow stack memory.  When a fault occurs, both
> + * X86_PF_USER and X86_PF_SHSTK are set.
> + */
> +static int is_wruss(struct pt_regs *regs, unsigned long error_code)
> +{
> +	return (((error_code & (X86_PF_USER | X86_PF_SHSTK)) ==
> +		(X86_PF_USER | X86_PF_SHSTK)) && !user_mode(regs));
> +}
> +
>  static const char nx_warning[] = KERN_CRIT
>  "kernel tried to execute NX-protected page - exploit attempt? (uid: %d)\n";
>  static const char smep_warning[] = KERN_CRIT
> @@ -851,7 +862,7 @@ __bad_area_nosemaphore(struct pt_regs *regs, unsigned long error_code,
>  	struct task_struct *tsk = current;
>  
>  	/* User mode accesses just cause a SIGSEGV */
> -	if (error_code & X86_PF_USER) {
> +	if ((error_code & X86_PF_USER) && !is_wruss(regs, error_code)) {
>  		/*
>  		 * It's possible to have interrupts off here:
>  		 */
> diff --git a/tools/objtool/arch/x86/lib/x86-opcode-map.txt b/tools/objtool/arch/x86/lib/x86-opcode-map.txt
> index e0b85930dd77..72bb7c48a7df 100644
> --- a/tools/objtool/arch/x86/lib/x86-opcode-map.txt
> +++ b/tools/objtool/arch/x86/lib/x86-opcode-map.txt
> @@ -789,7 +789,7 @@ f0: MOVBE Gy,My | MOVBE Gw,Mw (66) | CRC32 Gd,Eb (F2) | CRC32 Gd,Eb (66&F2)
>  f1: MOVBE My,Gy | MOVBE Mw,Gw (66) | CRC32 Gd,Ey (F2) | CRC32 Gd,Ew (66&F2)
>  f2: ANDN Gy,By,Ey (v)
>  f3: Grp17 (1A)
> -f5: BZHI Gy,Ey,By (v) | PEXT Gy,By,Ey (F3),(v) | PDEP Gy,By,Ey (F2),(v)
> +f5: BZHI Gy,Ey,By (v) | PEXT Gy,By,Ey (F3),(v) | PDEP Gy,By,Ey (F2),(v) | WRUSS Pq,Qq (66),REX.W
>  f6: ADCX Gy,Ey (66) | ADOX Gy,Ey (F3) | MULX By,Gy,rDX,Ey (F2),(v)
>  f7: BEXTR Gy,Ey,By (v) | SHLX Gy,Ey,By (66),(v) | SARX Gy,Ey,By (F3),(v) | SHRX Gy,Ey,By (F2),(v)
>  EndTable

Balbir Singh.