Re: [Question] About SECCOMP issue for ILP32

[Yury Norov]

On Mon, Aug 31, 2020 at 5:48 AM Xiongfeng Wang
<wangxiongfeng2@xxxxxxxxxx> wrote:
>
> Hi Yury,
>

Hi Xiongfeng,

[restore CC list]

Haven't seen this before. What kernel / glibc / ltp do you use?

> We were testing the ILP32 feature and came accross a problem. Very apperaciate
> it if you could give us some help !
>
> We compile the LTP testsuite with '-mabi=ilp32' and run it on a machine with
> kernel and glibc applied with ILP32 patches. But we failed on one testcase,
> prctl04. It print the following error info.
> 'prctl04.c:199: FAIL: SECCOMP_MODE_STRICT doesn't permit read(2) write(2) and
> _exit(2)'
>
> The testcase is like below, syscall 'prctl' followed by a syscall 'write'.
> prctl(PR_SET_SECCOMP, SECCOMP_MODE_STRICT);
> SAFE_WRITE(1, fd, "a", 1);
>
> When we execute syscall 'write', we receive a SIGKILL. It's not as expected.
> We track the kernel and found out it is because we failed the syscall_whitelist
> check in '__secure_computing_strict'. Because flag 'TIF_32BIT_AARCH64' is set,
> we falls into the 'in_compat_syscall()' branch. We compare the parameter
> 'this_syscall' with return value of 'get_compat_model_syscalls()'
> The syscall number of '__NR_write' for ilp32 application is 64, but it is 4 for
> 'model_syscalls_32' returned from 'get_compat_model_syscalls()'
> So '__secure_computing_strict' retuned with 'do_exit(SIGKILL)'. We have a
> modification like below, but I am not sure if it correct or not.
>
> --- a/kernel/seccomp.c
> +++ b/kernel/seccomp.c
> @@ -618,7 +618,7 @@ static void __secure_computing_strict(int this_syscall)
>  {
>         const int *syscall_whitelist = mode1_syscalls;
>  #ifdef CONFIG_COMPAT
> -       if (in_compat_syscall())
> +       if (is_a32_compat_task())
>                 syscall_whitelist = get_compat_mode1_syscalls();

It calls the arch function from generic code. It may break build for
other arches.
This also looks dangerous because it treats ILP32 execution as non-compat.

The right approach would be implementing arch-specific
get_compat_mode1_syscalls()
in arch/arm64/include/asm/seccomp.h that returns an appropriate table.
Refer MIPS
code for this: arch/mips/include/asm/seccomp.h

Thanks,
Yury

>  #endif
>         do {
>
>
> Thanks,
> Xiongfeng
>