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 commit 9c2598d43510 ("riscv: entry:
Save a0 prior syscall_enter_from_user_mode()").
After analysing code, we think that regs->a0 = -ENOSYS should only be executed
when syscall != -1 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
executed. 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 <[email protected]>
Co-developed-by: Ruizhe Pan <[email protected]>
Signed-off-by: Ruizhe Pan <[email protected]>
Co-developed-by: Shiqi Zhang <[email protected]>
Signed-off-by: Shiqi Zhang <[email protected]>
Signed-off-by: Celeste Liu <[email protected]>
Tested-by: Felix Yan <[email protected]>
---
v2 -> v3: use if-statement instead of set default value,
clarify the type of syscall
v1 -> v2: added explanation on why always got ENOSYS
arch/riscv/kernel/traps.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)
diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
index f910dfccbf5d2..5cef728745420 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);
--
2.41.0
Celeste Liu <[email protected]> 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 commit 9c2598d43510 ("riscv: entry:
> Save a0 prior syscall_enter_from_user_mode()").
>
> After analysing code, we think that regs->a0 = -ENOSYS should only be executed
> when syscall != -1 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
> executed. 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 <[email protected]>
> Co-developed-by: Ruizhe Pan <[email protected]>
> Signed-off-by: Ruizhe Pan <[email protected]>
> Co-developed-by: Shiqi Zhang <[email protected]>
> Signed-off-by: Shiqi Zhang <[email protected]>
> Signed-off-by: Celeste Liu <[email protected]>
> Tested-by: Felix Yan <[email protected]>
Reviewed-by: Björn Töpel <[email protected]>
On Wed, Jul 19, 2023 at 5:01 AM Celeste Liu <[email protected]> wrote:
>
> 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 commit 9c2598d43510 ("riscv: entry:
> Save a0 prior syscall_enter_from_user_mode()").
>
> After analysing code, we think that regs->a0 = -ENOSYS should only be executed
> when syscall != -1 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
> executed. 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 <[email protected]>
> Co-developed-by: Ruizhe Pan <[email protected]>
> Signed-off-by: Ruizhe Pan <[email protected]>
> Co-developed-by: Shiqi Zhang <[email protected]>
> Signed-off-by: Shiqi Zhang <[email protected]>
> Signed-off-by: Celeste Liu <[email protected]>
> Tested-by: Felix Yan <[email protected]>
> ---
>
> v2 -> v3: use if-statement instead of set default value,
> clarify the type of syscall
> v1 -> v2: added explanation on why always got ENOSYS
>
> arch/riscv/kernel/traps.c | 6 +++++-
> 1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
> index f910dfccbf5d2..5cef728745420 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)
Maybe we should define an explicit macro for this ERRNO in
__seccomp_filter, and this style obeys the coding convention.
For this patch:
Reviewed-by: Guo Ren <[email protected]>
Cc: loongarch guy, please check loongarch's code. :)
> regs->a0 = -ENOSYS;
>
> syscall_exit_to_user_mode(regs);
> --
> 2.41.0
>
--
Best Regards
Guo Ren
On Jul 19 2023, Celeste Liu wrote:
> @@ -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)
You can also use syscall != -1UL or even syscall != -1.
--
Andreas Schwab, SUSE Labs, [email protected]
GPG Key fingerprint = 0196 BAD8 1CE9 1970 F4BE 1748 E4D4 88E3 0EEA B9D7
"And now for something completely different."
...
> > + /*
> > + * 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)
If you leave 'syscall' signed and write:
if (syscall >= 0 && syscall < NR_syscalls)
the compiler will use a single unsigned compare.
There is no need to 'optimise' it yourself.
David
-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)
Andreas Schwab <[email protected]> writes:
> On Jul 19 2023, Celeste Liu wrote:
>
>> @@ -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)
>
> You can also use syscall != -1UL or even syscall != -1.
The former is indeed better for the eyes! :-) The latter will get a
-Wsign-compare warning, no?
Björn
On July 20, 2023 12:28:47 AM GMT+08:00, "Björn Töpel" <[email protected]> wrote:
>Andreas Schwab <[email protected]> writes:
>
>> On Jul 19 2023, Celeste Liu wrote:
>>
>>> @@ -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)
>>
>> You can also use syscall != -1UL or even syscall != -1.
>
>The former is indeed better for the eyes! :-) The latter will get a
>-Wsign-compare warning, no?
>
>
>Björn
Well, that's true. And I just found out that by C standards, converting
ulong to long is implementation-defined behavior, unlike long to ulong
which is well-defined. So it is really better than (long)syscall != -1L.
Celeste Liu <[email protected]> writes:
> On July 20, 2023 12:28:47 AM GMT+08:00, "Björn Töpel" <[email protected]> wrote:
>>Andreas Schwab <[email protected]> writes:
>>
>>> On Jul 19 2023, Celeste Liu wrote:
>>>
>>>> @@ -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)
>>>
>>> You can also use syscall != -1UL or even syscall != -1.
>>
>>The former is indeed better for the eyes! :-) The latter will get a
>>-Wsign-compare warning, no?
>>
>>
>>Björn
>
> Well, that's true. And I just found out that by C standards, converting
> ulong to long is implementation-defined behavior, unlike long to ulong
> which is well-defined. So it is really better than (long)syscall != -1L.
If you're respinning, I suggest you use David's suggestion:
* Remove the comment I suggest you to add
* Use (signed) long
* Add syscall >= 0 &&
* else if (syscall != -1)
Which is the least amount of surprises IMO.
On July 20, 2023 5:08:37 PM GMT+08:00, "Björn Töpel" <[email protected]> wrote:
>Celeste Liu <[email protected]> writes:
>
>> On July 20, 2023 12:28:47 AM GMT+08:00, "Björn Töpel" <[email protected]> wrote:
>>>Andreas Schwab <[email protected]> writes:
>>>
>>>> On Jul 19 2023, Celeste Liu wrote:
>>>>
>>>>> @@ -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)
>>>>
>>>> You can also use syscall != -1UL or even syscall != -1.
>>>
>>>The former is indeed better for the eyes! :-) The latter will get a
>>>-Wsign-compare warning, no?
>>>
>>>
>>>Björn
>>
>> Well, that's true. And I just found out that by C standards, converting
>> ulong to long is implementation-defined behavior, unlike long to ulong
>> which is well-defined. So it is really better than (long)syscall != -1L.
>
>If you're respinning, I suggest you use David's suggestion:
> * Remove the comment I suggest you to add
> * Use (signed) long
> * Add syscall >= 0 &&
> * else if (syscall != -1)
>
>Which is the least amount of surprises IMO.
v4 has sent