Re: [PATCH v2 24/36] x86/entry/compat: Implement opportunistic SYSRETL for compat syscalls

[Borislav Petkov <bp@alien8.de>]

On Mon, Oct 05, 2015 at 05:48:12PM -0700, Andy Lutomirski wrote:
> If CS, SS and IP are as expected and FLAGS is compatible with SYSRETL,
> then return from fast compat syscalls (both SYSCALL and SYSENTER) using
> SYSRETL.
> 
> Unlike native 64-bit opportunistic SYSRET, this is not invisible to
> user code: RCX and R8-R15 end up in a different state than shown
> saved in pt_regs.  To compensate, we only do this when returning to
> the vDSO fast syscall return path.  This won't interfere with
> syscall restart, as we won't use SYSRETL when returning to the INT80
> restart instruction.
> 
> Signed-off-by: Andy Lutomirski <luto@kernel.org>
> ---
>  arch/x86/entry/common.c          | 23 +++++++++++++++++++---
>  arch/x86/entry/entry_64_compat.S | 42 ++++++++++++++++++++++++++++++++++++++--
>  2 files changed, 60 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
> index 1b2606edc621..88dc5ba14d47 100644
> --- a/arch/x86/entry/common.c
> +++ b/arch/x86/entry/common.c
> @@ -363,7 +363,8 @@ __visible void do_int80_syscall_32(struct pt_regs *regs)
>  	syscall_return_slowpath(regs);
>  }
>  
> -__visible void do_fast_syscall_32(struct pt_regs *regs)
> +/* Returns 0 to return using IRET or 1 to return using SYSRETL. */
> +__visible long do_fast_syscall_32(struct pt_regs *regs)
>  {
>  	/*
>  	 * Called using the internal vDSO SYSENTER/SYSCALL32 calling
> @@ -395,12 +396,28 @@ __visible void do_fast_syscall_32(struct pt_regs *regs)
>  		enter_from_user_mode();
>  #endif
>  		prepare_exit_to_usermode(regs);
> -		return;
> +		return 0;	/* Keep it simple: use IRET. */
>  	}
>  	local_irq_disable();
>  
>  	/* Now this is just like a normal syscall. */
>  	do_int80_syscall_32(regs);
> -	return;
> +
> +#ifdef CONFIG_X86_64
> +	/*
> +	 * Opportunistic SYSRETL: if possible, try to return using SYSRETL.
> +	 * SYSRETL is available on all 64-bit CPUs, so we don't need to
> +	 * bother with SYSEXIT.
> +	 *
> +	 * Unlike 64-bit opportunistic SYSRET, we can't check that CX == IP,
> +	 * because the ECX fixup above will ensure that this is essentially
> +	 * never the case.
> +	 */
> +	return regs->cs == __USER32_CS && regs->ss == __USER_DS &&
> +		regs->ip == landing_pad &&
> +		(regs->flags & (X86_EFLAGS_RF | X86_EFLAGS_TF)) == 0;

This could've used some readability massaging:

	return	  regs->cs == __USER32_CS &&
		  regs->ss == __USER_DS &&
		  regs->ip == landing_pad &&
		!(regs->flags & (X86_EFLAGS_RF | X86_EFLAGS_TF));

Although I'm not crazy about that one either. This expression is simply not
parseable at a glance.

Why not write it like the 64-bit opportunistic SYSRET:

	if (regs->cs != __USER32_CS)
		return 0;

	if (regs->ss != __USER_DS)
		return 0;

and so on...

asm is almost the same, except in the "after" case, the CALL to
syscall_trace_enter is after the last JMP.

before:

.L270:
        xorl    %eax, %eax      # D.34060
        cmpq    $35, 136(%rbx)  #, regs_12(D)->cs
        je      .L296   #,
.L263:
        movq    -32(%rbp), %rbx #,
        movq    -24(%rbp), %r12 #,
        movq    -16(%rbp), %r13 #,
        movq    -8(%rbp), %r14  #,
        leave
        ret
.L291:
        movq    %rbx, %rdi      # regs,
        call    syscall_trace_enter     #
        jmp     .L265   #
.L296:
        cmpq    $43, 160(%rbx)  #, regs_12(D)->ss
        jne     .L263   #,
        cmpq    128(%rbx), %r12 # regs_12(D)->ip, landing_pad
        jne     .L263   #,
        xorl    %eax, %eax      # D.34060
        testq   $65792, 144(%rbx)       #, regs_12(D)->flags
        sete    %al     #, D.34060
        jmp     .L263   #


after:


.L271:
        cmpq    $35, 136(%rbx)  #, regs_11(D)->cs
        je      .L293   #,
.L286:
        xorl    %eax, %eax      # D.34060
.L264:
        movq    -32(%rbp), %rbx #,
        movq    -24(%rbp), %r12 #,
        movq    -16(%rbp), %r13 #,
        movq    -8(%rbp), %r14  #,
        leave
        ret
.L293:
        cmpq    $43, 160(%rbx)  #, regs_11(D)->ss
        jne     .L286   #,
        cmpq    128(%rbx), %r12 # regs_11(D)->ip, landing_pad
        jne     .L286   #,
        xorl    %eax, %eax      # D.34060
        testq   $65792, 144(%rbx)       #, regs_11(D)->flags
        sete    %al     #, D.34060
        jmp     .L264   #

-- 
Regards/Gruss,
    Boris.

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