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

[Björn Töpel]

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