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

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

[Celeste Liu]

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.

Fixes: f0bddf50586d ("riscv: entry: Convert to generic entry")
Reported-by: Felix Yan <felixonmars@archlinux.org>
Co-developed-by: Ruizhe Pan <c141028@gmail.com>
Signed-off-by: Ruizhe Pan <c141028@gmail.com>
Co-developed-by: Shiqi Zhang <shiqi@isrc.iscas.ac.cn>
Signed-off-by: Shiqi Zhang <shiqi@isrc.iscas.ac.cn>
Signed-off-by: Celeste Liu <CoelacanthusHex@gmail.com>
Tested-by: Felix Yan <felixonmars@archlinux.org>
---

v1 -> v2: added explanation on why always got ENOSYS

 arch/riscv/kernel/traps.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
index f910dfccbf5d2..ccadb5ffd063c 100644
--- a/arch/riscv/kernel/traps.c
+++ b/arch/riscv/kernel/traps.c
@@ -301,6 +301,7 @@ asmlinkage __visible __trap_section void do_trap_ecall_u(struct pt_regs *regs)
 
 		regs->epc += 4;
 		regs->orig_a0 = regs->a0;
+		regs->a0 = -ENOSYS;
 
 		riscv_v_vstate_discard(regs);
 
@@ -308,8 +309,6 @@ asmlinkage __visible __trap_section void do_trap_ecall_u(struct pt_regs *regs)
 
 		if (syscall < NR_syscalls)
 			syscall_handler(regs, syscall);
-		else
-			regs->a0 = -ENOSYS;
 
 		syscall_exit_to_user_mode(regs);
 	} else {
-- 
2.41.0


_______________________________________________
linux-riscv mailing list
linux-riscv@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-riscv

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

[Björn Töpel]

Celeste Liu <coelacanthushex@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

_______________________________________________
linux-riscv mailing list
linux-riscv@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-riscv

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

[Celeste Liu]

On 2023/7/19 03:35, Björn Töpel wrote:
> Celeste Liu <coelacanthushex@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.

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;


_______________________________________________
linux-riscv mailing list
linux-riscv@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-riscv

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

[Celeste Liu]

On 2023/7/19 04:35, Celeste Liu wrote:
> 
> On 2023/7/19 03:35, Björn Töpel wrote:
>> Celeste Liu <coelacanthushex@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.
> 
> 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;
> 

Oh, it's better to reorder to:

diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
index f910dfccbf5d2..932814e34ac37 100644
--- a/arch/riscv/kernel/traps.c
+++ b/arch/riscv/kernel/traps.c
@@ -308,7 +308,7 @@ asmlinkage __visible __trap_section void do_trap_ecall_u(struct pt_regs *regs)
 
 		if (syscall < NR_syscalls)
 			syscall_handler(regs, syscall);
-		else
+		else if (syscall != -1)
 			regs->a0 = -ENOSYS;
 
 		syscall_exit_to_user_mode(regs);


I will send v3 later.

_______________________________________________
linux-riscv mailing list
linux-riscv@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-riscv

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

[Björn Töpel]

Celeste Liu <coelacanthushex@gmail.com> writes:

> On 2023/7/19 03:35, Björn Töpel wrote:
>> Celeste Liu <coelacanthushex@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

_______________________________________________
linux-riscv mailing list
linux-riscv@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-riscv

Re: [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@gmail.com> writes:
> 
>> On 2023/7/19 03:35, Björn Töpel wrote:
>>> Celeste Liu <coelacanthushex@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.

_______________________________________________
linux-riscv mailing list
linux-riscv@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-riscv