Date:   Tue, 18 Jul 2023 17:16:41 +0300
From:   Mika Penttilä <mpenttil@...hat.com>
To:     Brian Gerst <brgerst@...il.com>, linux-kernel@...r.kernel.org,
        x86@...nel.org
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH 2/6] x86/entry/64: Convert SYSRET validation tests to C

Hi,


On 18.7.2023 16.44, Brian Gerst wrote:
> Signed-off-by: Brian Gerst <brgerst@...il.com>
> ---
>   arch/x86/entry/common.c        | 50 ++++++++++++++++++++++++++++++-
>   arch/x86/entry/entry_64.S      | 55 ++--------------------------------
>   arch/x86/include/asm/syscall.h |  2 +-
>   3 files changed, 52 insertions(+), 55 deletions(-)
> 
> diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
> index 6c2826417b33..afe79c3f1c5b 100644
> --- a/arch/x86/entry/common.c
> +++ b/arch/x86/entry/common.c
> @@ -70,8 +70,12 @@ static __always_inline bool do_syscall_x32(struct pt_regs *regs, int nr)
>   	return false;
>   }
>   
> -__visible noinstr void do_syscall_64(struct pt_regs *regs, int nr)
> +/* Returns true to return using SYSRET, or false to use IRET */
> +__visible noinstr bool do_syscall_64(struct pt_regs *regs, int nr)
>   {
> +	long rip;
> +	unsigned int shift_rip;
> +
>   	add_random_kstack_offset();
>   	nr = syscall_enter_from_user_mode(regs, nr);
>   
> @@ -84,6 +88,50 @@ __visible noinstr void do_syscall_64(struct pt_regs *regs, int nr)
>   
>   	instrumentation_end();
>   	syscall_exit_to_user_mode(regs);
> +
> +	/*
> +	 * Check that the register state is valid for using SYSRET to exit
> +	 * to userspace.  Otherwise use the slower but fully capable IRET
> +	 * exit path.
> +	 */
> +
> +	/* XEN PV guests always use IRET path */
> +	if (cpu_feature_enabled(X86_FEATURE_XENPV))
> +		return false;
> +
> +	/* SYSRET requires RCX == RIP and R11 == EFLAGS */
> +	if (unlikely(regs->cx != regs->ip || regs->r11 != regs->flags))
> +		return false;
> +
> +	/* CS and SS must match the values set in MSR_STAR */
> +	if (unlikely(regs->cs != __USER_CS || regs->ss != __USER_DS))
> +		return false;
> +
> +	/*
> +	 * On Intel CPUs, SYSRET with non-canonical RCX/RIP will #GP
> +	 * in kernel space.  This essentially lets the user take over
> +	 * the kernel, since userspace controls RSP.
> +	 *
> +	 * Change top bits to match most significant bit (47th or 56th bit
> +	 * depending on paging mode) in the address.
> +	 */
> +	shift_rip = (64 - __VIRTUAL_MASK_SHIFT + 1);

Should this be:

	shift_rip = (64 - __VIRTUAL_MASK_SHIFT - 1);
?

> +	rip = (long) regs->ip;
> +	rip <<= shift_rip;
> +	rip >>= shift_rip;
> +	if (unlikely((unsigned long) rip != regs->ip))
> +		return false;
> +
> +	/*
> +	 * SYSRET cannot restore RF.  It can restore TF, but unlike IRET,
> +	 * restoring TF results in a trap from userspace immediately after
> +	 * SYSRET.
> +	 */
> +	if (unlikely(regs->flags & (X86_EFLAGS_RF | X86_EFLAGS_TF)))
> +		return false;
> +
> +	/* Use SYSRET to exit to userspace */
> +	return true;
>   }
>   #endif
>   
> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> index c01776a51545..b1288e22cae8 100644
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -123,60 +123,9 @@ SYM_INNER_LABEL(entry_SYSCALL_64_after_hwframe, SYM_L_GLOBAL)
>   	 * Try to use SYSRET instead of IRET if we're returning to
>   	 * a completely clean 64-bit userspace context.  If we're not,
>   	 * go to the slow exit path.
> -	 * In the Xen PV case we must use iret anyway.
>   	 */
> -
> -	ALTERNATIVE "", "jmp	swapgs_restore_regs_and_return_to_usermode", \
> -		X86_FEATURE_XENPV
> -
> -	movq	RCX(%rsp), %rcx
> -	movq	RIP(%rsp), %r11
> -
> -	cmpq	%rcx, %r11	/* SYSRET requires RCX == RIP */
> -	jne	swapgs_restore_regs_and_return_to_usermode
> -
> -	/*
> -	 * On Intel CPUs, SYSRET with non-canonical RCX/RIP will #GP
> -	 * in kernel space.  This essentially lets the user take over
> -	 * the kernel, since userspace controls RSP.
> -	 *
> -	 * If width of "canonical tail" ever becomes variable, this will need
> -	 * to be updated to remain correct on both old and new CPUs.
> -	 *
> -	 * Change top bits to match most significant bit (47th or 56th bit
> -	 * depending on paging mode) in the address.
> -	 */
> -#ifdef CONFIG_X86_5LEVEL
> -	ALTERNATIVE "shl $(64 - 48), %rcx; sar $(64 - 48), %rcx", \
> -		"shl $(64 - 57), %rcx; sar $(64 - 57), %rcx", X86_FEATURE_LA57
> -#else
> -	shl	$(64 - (__VIRTUAL_MASK_SHIFT+1)), %rcx
> -	sar	$(64 - (__VIRTUAL_MASK_SHIFT+1)), %rcx
> -#endif
> -
> -	/* If this changed %rcx, it was not canonical */
> -	cmpq	%rcx, %r11
> -	jne	swapgs_restore_regs_and_return_to_usermode
> -
> -	cmpq	$__USER_CS, CS(%rsp)		/* CS must match SYSRET */
> -	jne	swapgs_restore_regs_and_return_to_usermode
> -
> -	movq	R11(%rsp), %r11
> -	cmpq	%r11, EFLAGS(%rsp)		/* R11 == RFLAGS */
> -	jne	swapgs_restore_regs_and_return_to_usermode
> -
> -	/*
> -	 * SYSRET cannot restore RF.  It can restore TF, but unlike IRET,
> -	 * restoring TF results in a trap from userspace immediately after
> -	 * SYSRET.
> -	 */
> -	testq	$(X86_EFLAGS_RF|X86_EFLAGS_TF), %r11
> -	jnz	swapgs_restore_regs_and_return_to_usermode
> -
> -	/* nothing to check for RSP */
> -
> -	cmpq	$__USER_DS, SS(%rsp)		/* SS must match SYSRET */
> -	jne	swapgs_restore_regs_and_return_to_usermode
> +	testb	%al, %al
> +	jz	swapgs_restore_regs_and_return_to_usermode
>   
>   	/*
>   	 * We win! This label is here just for ease of understanding
> diff --git a/arch/x86/include/asm/syscall.h b/arch/x86/include/asm/syscall.h
> index 4fb36fba4b5a..be6c5515e0b9 100644
> --- a/arch/x86/include/asm/syscall.h
> +++ b/arch/x86/include/asm/syscall.h
> @@ -126,7 +126,7 @@ static inline int syscall_get_arch(struct task_struct *task)
>   		? AUDIT_ARCH_I386 : AUDIT_ARCH_X86_64;
>   }
>   
> -void do_syscall_64(struct pt_regs *regs, int nr);
> +bool do_syscall_64(struct pt_regs *regs, int nr);
>   
>   #endif	/* CONFIG_X86_32 */
>   


--Mika

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