LinuxLists.cc - [PATCH] riscv: entry: set a0 prior to syscall

2023-07-11 07:02:19

Subject: [PATCH] riscv: entry: set a0 prior to syscall_handler

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. We got same result with 9c2598d43510 ("riscv: entry: Save a0
prior syscall_enter_from_user_mode()").

Compared with x86 and loongarch's implementation of this part of the
function, we think that regs->a0 = -ENOSYS should be advanced before
syscall_handler to fix this problem. We have written the following patch,
which can fix this problem after testing. But we don't know enough about
this part of the code to explain the root cause. Hope someone can find
a reasonable explanation. And we'd like to reword this commit message
according to the explanation in v2

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]>
---
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

2023-07-13 00:41:02

by Guo Ren

[permalink] [raw]

Subject: Re: [PATCH] riscv: entry: set a0 prior to syscall_handler

On Tue, Jul 11, 2023 at 2:22 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. We got same result with 9c2598d43510 ("riscv: entry: Save a0
> prior syscall_enter_from_user_mode()").
>
> Compared with x86 and loongarch's implementation of this part of the
> function, we think that regs->a0 = -ENOSYS should be advanced before
> syscall_handler to fix this problem. We have written the following patch,
> which can fix this problem after testing. But we don't know enough about
> this part of the code to explain the root cause. Hope someone can find
> a reasonable explanation. And we'd like to reword this commit message
> according to the explanation in v2
>
> 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]>
> ---
> 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;
Oh, no. You destroyed the a0 for syscall_handler, right? It's not
reasonable. Let's see which syscall_handler needs a0=-ENOSYS.

Could you give out more detail on your test case?

>
> 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
>

--
Best Regards
Guo Ren

2023-07-13 05:38:28

by Celeste Liu

[permalink] [raw]

Subject: Re: [PATCH] riscv: entry: set a0 prior to syscall_handler

On 2023/7/13 08:00, Guo Ren wrote:
> On Tue, Jul 11, 2023 at 2:22 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. We got same result with 9c2598d43510 ("riscv: entry: Save a0
>> prior syscall_enter_from_user_mode()").
>>
>> Compared with x86 and loongarch's implementation of this part of the
>> function, we think that regs->a0 = -ENOSYS should be advanced before
>> syscall_handler to fix this problem. We have written the following patch,
>> which can fix this problem after testing. But we don't know enough about
>> this part of the code to explain the root cause. Hope someone can find
>> a reasonable explanation. And we'd like to reword this commit message
>> according to the explanation in v2
>>
>> 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]>
>> ---
>> 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;
> Oh, no. You destroyed the a0 for syscall_handler, right? It's not
> reasonable. Let's see which syscall_handler needs a0=-ENOSYS.
>
> Could you give out more detail on your test case?
>

Our test case is here:

int main() {
scmp_filter_ctx ctx;
ctx = seccomp_init(SCMP_ACT_ALLOW);

seccomp_rule_add_exact(ctx, SCMP_ACT_ERRNO(EAFNOSUPPORT), SCMP_SYS(socket), 2,
SCMP_CMP(0, SCMP_CMP_EQ, AF_NETLINK),
SCMP_CMP(2, SCMP_CMP_EQ, NETLINK_AUDIT));

if (seccomp_load(ctx) < 0) {
perror("seccomp_load");
exit(EXIT_FAILURE);
}

int sock_fd1 = socket(AF_NETLINK, SOCK_RAW, NETLINK_GENERIC);
if (sock_fd1 < 0) {
printf("1st socket syscall failed with return value %d and errno %d (%s), which is unexpected\n",
sock_fd1, errno, strerror(errno));
seccomp_release(ctx);
return 1;
}
printf("1st socket created successfully, as expected.\n");

int sock_fd2 = socket(AF_NETLINK, SOCK_RAW, NETLINK_AUDIT);
if (sock_fd2 < 0) {
printf("2nd socket syscall failed with return value %d and errno %d (%s).\n", sock_fd2, errno,
strerror(errno));

if (errno == EAFNOSUPPORT) {
printf("2nd socket syscall failed with EAFNOSUPPORT, as expected.\n");
seccomp_release(ctx);
return 0;
} else {
printf("2nd socket syscall failed with unexpected errno, which is unexpected.\n");
seccomp_release(ctx);
return 1;
}
}
printf("2nd socket created successfully, which is unexpected.\n");
seccomp_release(ctx);

return 2;
}

>>
>> 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
>>
>
>

2023-07-13 15:34:48

by Celeste Liu

[permalink] [raw]

Subject: Re: [PATCH] riscv: entry: set a0 prior to syscall_handler

2023-07-13 17:10:25

by Guo Ren

[permalink] [raw]

Subject: Re: [PATCH] riscv: entry: set a0 prior to syscall_handler

On Thu, Jul 13, 2023 at 11:28 AM Celeste Liu <[email protected]> wrote:
>
>
> On 2023/7/13 08:00, Guo Ren wrote:
> > On Tue, Jul 11, 2023 at 2:22 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. We got same result with 9c2598d43510 ("riscv: entry: Save a0
> >> prior syscall_enter_from_user_mode()").
> >>
> >> Compared with x86 and loongarch's implementation of this part of the
> >> function, we think that regs->a0 = -ENOSYS should be advanced before
> >> syscall_handler to fix this problem. We have written the following patch,
> >> which can fix this problem after testing. But we don't know enough about
> >> this part of the code to explain the root cause. Hope someone can find
> >> a reasonable explanation. And we'd like to reword this commit message
> >> according to the explanation in v2
> >>
> >> 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]>
> >> ---
> >> 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;
> > Oh, no. You destroyed the a0 for syscall_handler, right? It's not
> > reasonable. Let's see which syscall_handler needs a0=-ENOSYS.
>
> syscall_handler always use orig_a0, not a0.
> And I have a mistake in original email, corret one is
> syscall_enter_from_user_mode not syscall_handler.
I misunderstood. Yes, a0 would be replaced by orig_a0:
syscall_enter_from_user_mode_work -> syscall_rollback

If the syscall was denied by syscall_enter_from_user_mode(), the
return number is forced to be -ENOSYS. Maybe regs->a0 has already been
updated by SYSCALL_WORK_SECCOMP. eg:

__seccomp_filter() {
...
case SECCOMP_RET_TRAP:
/* Show the handler the original registers. */
syscall_rollback(current, current_pt_regs());
/* Let the filter pass back 16 bits of data. */
force_sig_seccomp(this_syscall, data, false);
goto skip;

>
> > Could you give out more detail on your test case?
> >
> >>
> >> 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
> >>
> >
> >
>

--
Best Regards
Guo Ren