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

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Date:   Fri, 10 Sep 2021 15:30:19 +0800
From:   Lai Jiangshan <laijs@...ux.alibaba.com>
To:     Nikolay Borisov <n.borisov.lkml@...il.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org
Cc:     Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 24/24] x86/syscall/64: Move the checking for sysret to C
 code



On 2021/9/10 15:20, Nikolay Borisov wrote:
> 
> 
> On 31.08.21 г. 20:50, Lai Jiangshan wrote:
>> From: Lai Jiangshan <laijs@...ux.alibaba.com>
>>
>> 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@...ux.alibaba.com>
> 
> <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 ?

I don't have any preference.

The choice came from my limitation of the needed knowledge.

I followed the design of do_fast_syscall_32() which returns a 4-byte word
to indicate if it can fast return to userspace,  and I know how to test the
result in ASM for a 4-byte word.  If it was a bool, I don't know how to
test the result in ASM.

> 
>> +{
>> +	/* 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>
>