[PATCH v2] riscv: entry: set a0 prior to syscall_enter_from_user_mode

[Celeste Liu]

On 2023/7/19 04:50, Björn Töpel wrote:
> Celeste Liu <coelacanthushex at gmail.com> writes:
> 
>> On 2023/7/19 03:35, Björn Töpel wrote:
>>> Celeste Liu <coelacanthushex at gmail.com> writes:
>>>
>>>> When we test seccomp with 6.4 kernel, we found errno has wrong value.
>>>> If we deny NETLINK_AUDIT with EAFNOSUPPORT, after f0bddf50586d, we will
>>>> get ENOSYS instead. We got same result with 9c2598d43510 ("riscv: entry: Save a0
>>>> prior syscall_enter_from_user_mode()").
>>>>
>>>> After analysing code, we think that regs->a0 = -ENOSYS should be advanced before
>>>> syscall_enter_from_user_mode to fix this problem. In __seccomp_filter, when
>>>> seccomp rejected this syscall with specified errno, they will set a0 to return
>>>> number as syscall ABI, and then return -1. This return number is finally pass
>>>> as return number of syscall_enter_from_user_mode, and then is compared
>>>> with NR_syscalls after converted to ulong (so it will be ULONG_MAX).
>>>> The condition syscall < NR_syscalls will always be false, so regs->a0 = -ENOSYS
>>>> is always executable. It covered a0 set by seccomp, so we always get
>>>> ENOSYS when match seccomp RET_ERRNO rule.
>>>
>>> Isn't something like...
>>>
>>> diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
>>> index f910dfccbf5d..15a8b4898a6c 100644
>>> --- a/arch/riscv/kernel/traps.c
>>> +++ b/arch/riscv/kernel/traps.c
>>> @@ -297,7 +297,7 @@ asmlinkage __visible __trap_section void do_trap_break(struct pt_regs *regs)
>>>  asmlinkage __visible __trap_section void do_trap_ecall_u(struct pt_regs *regs)
>>>  {
>>>  	if (user_mode(regs)) {
>>> -		ulong syscall = regs->a7;
>>> +		long syscall = regs->a7;
>>>  
>>>  		regs->epc += 4;
>>>  		regs->orig_a0 = regs->a0;
>>> @@ -306,7 +306,7 @@ asmlinkage __visible __trap_section void do_trap_ecall_u(struct pt_regs *regs)
>>>  
>>>  		syscall = syscall_enter_from_user_mode(regs, syscall);
>>>  
>>> -		if (syscall < NR_syscalls)
>>> +		if (syscall > -1 && syscall < NR_syscalls)
>>>  			syscall_handler(regs, syscall);
>>>  		else
>>>  			regs->a0 = -ENOSYS;
>>>
>>>
>>> ...easier to read?
>>>
>>>
>>> Björn
>>
>> It seems that your change turn it back to the beginning. If syscall == -1,
>> it is supposed to go neither first nor else branch. It should do NOTHING.
>> However it was still a great idea. It may be better to use a set of if-statement
>> to clarify the logic.
> 
> Ah, gotcha! (Notice that arch/x86/entry/common.c has
> 
>   | 	if (!do_syscall_x64(regs, nr) && !do_syscall_x32(regs, nr) && nr != -1) {
> 
> and in do_syscall_x64()
> 
>   | 	/*
>   | 	 * Convert negative numbers to very high and thus out of range
>   | 	 * numbers for comparisons.
>   | 	 */
>   | 	unsigned int unr = nr;
> 
> 
>> diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
>> index f910dfccbf5d2..d0bd725244594 100644
>> --- a/arch/riscv/kernel/traps.c
>> +++ b/arch/riscv/kernel/traps.c
>> @@ -306,7 +306,9 @@ asmlinkage __visible __trap_section void do_trap_ecall_u(struct pt_regs *regs)
>>  
>>  		syscall = syscall_enter_from_user_mode(regs, syscall);
>>  
>> -		if (syscall < NR_syscalls)
>> +		if (syscall == -1)
>> +			// Do nothing
>> +		else if (syscall < NR_syscalls)
>>  			syscall_handler(regs, syscall);
>>  		else
>>  			regs->a0 = -ENOSYS;
> 
> Maybe something a bit more explicit?
> 
> diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
> index f910dfccbf5d..5cef72874542 100644
> --- a/arch/riscv/kernel/traps.c
> +++ b/arch/riscv/kernel/traps.c
> @@ -297,6 +297,10 @@ asmlinkage __visible __trap_section void do_trap_break(struct pt_regs *regs)
>  asmlinkage __visible __trap_section void do_trap_ecall_u(struct pt_regs *regs)
>  {
>  	if (user_mode(regs)) {
> +		/*
> +		 * Convert negative numbers to very high and thus out of range
> +		 * numbers for comparisons.
> +		 */
>  		ulong syscall = regs->a7;
>  
>  		regs->epc += 4;
> @@ -308,7 +312,7 @@ asmlinkage __visible __trap_section void do_trap_ecall_u(struct pt_regs *regs)
>  
>  		if (syscall < NR_syscalls)
>  			syscall_handler(regs, syscall);
> -		else
> +		else if ((long)syscall != -1L)
>  			regs->a0 = -ENOSYS;
>  
>  		syscall_exit_to_user_mode(regs);
> 
> 
> Björn

Ok, I will send v3.