Re: [PATCH 24/24] x86/syscall/64: Move the checking for sysret to C code

[Nikolay Borisov]

On 31.08.21 г. 20:50, Lai Jiangshan wrote:
> From: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
> 
> Like do_fast_syscall_32() which checks whether it can return to userspace
> via fast instructions before the function returns, do_syscall_64()
> also checks whether it can use sysret to return to userspace before
> do_syscall_64() returns via C code.  And a bunch of ASM code can be removed.
> 
> No functional change intended.
> 
> Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>

<snip>

> +/*
> + * Check if it can use SYSRET.
> + *
> + * Try to use SYSRET instead of IRET if we're returning to
> + * a completely clean 64-bit userspace context.
> + *
> + * Returns 0 to return using IRET or 1 to return using SYSRET.
> + */
> +static __always_inline int can_sysret(struct pt_regs *regs)

nit: Since this is a predicate function why not simply return bool ?

> +{
> +	/* In the Xen PV case we must use iret anyway. */
> +	if (static_cpu_has(X86_FEATURE_XENPV))
> +		return 0;
> +
> +	/* SYSRET requires RCX == RIP && R11 == RFLAGS */
> +	if (regs->ip != regs->cx || regs->flags != regs->r11)
> +		return 0;
> +
> +	/* CS and SS must match SYSRET */
> +	if (regs->cs != __USER_CS || regs->ss != __USER_DS)
> +		return 0;
> +
> +	/*
> +	 * 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 (regs->cx != canonical_address(regs->cx))
> +		return 0;
> +
> +	/*
> +	 * SYSCALL clears RF when it saves RFLAGS in R11 and SYSRET cannot
> +	 * restore RF properly. If the slowpath sets it for whatever reason, we
> +	 * need to restore it correctly.
> +	 *
> +	 * SYSRET can restore TF, but unlike IRET, restoring TF results in a
> +	 * trap from userspace immediately after SYSRET.  This would cause an
> +	 * infinite loop whenever #DB happens with register state that satisfies
> +	 * the opportunistic SYSRET conditions.  For example, single-stepping
> +	 * this user code:
> +	 *
> +	 *           movq	$stuck_here, %rcx
> +	 *           pushfq
> +	 *           popq %r11
> +	 *   stuck_here:
> +	 *
> +	 * would never get past 'stuck_here'.
> +	 */
> +	if (regs->r11 & (X86_EFLAGS_RF | X86_EFLAGS_TF))
> +		return 0;
> +
> +	return 1;
> +}
> +
> +/* Returns 0 to return using IRET or 1 to return using SYSRET. */
> +__visible noinstr int do_syscall_64(struct pt_regs *regs, int nr)

nit: Ditto about bool

>  {
>  	add_random_kstack_offset();
>  	nr = syscall_enter_from_user_mode(regs, nr);
> @@ -84,6 +154,7 @@ __visible noinstr void do_syscall_64(struct pt_regs *regs, int nr)
>  
>  	instrumentation_end();
>  	syscall_exit_to_user_mode(regs);
> +	return can_sysret(regs);
>  }
>  #endif
>  

<snip>