Re: [arm64] kernel BUG at kernel/seccomp.c:1309!

[Gabriel Krisman Bertazi <krisman@collabora.com>]

Jann Horn <jannh@google.com> writes:

> On Mon, Nov 23, 2020 at 2:45 PM Arnd Bergmann <arnd@kernel.org> wrote:
>> On Mon, Nov 23, 2020 at 12:15 PM Naresh Kamboju
>> <naresh.kamboju@linaro.org> wrote:
>> >
>> > While booting arm64 kernel the following kernel BUG noticed on several arm64
>> > devices running linux next 20201123 tag kernel.
>> >
>> >
>> > $ git log --oneline next-20201120..next-20201123 -- kernel/seccomp.c
>> > 5c5c5fa055ea Merge remote-tracking branch 'seccomp/for-next/seccomp'
>> > bce6a8cba7bf Merge branch 'linus'
>> > 7ef95e3dbcee Merge branch 'for-linus/seccomp' into for-next/seccomp
>> > fab686eb0307 seccomp: Remove bogus __user annotations
>> > 0d8315dddd28 seccomp/cache: Report cache data through /proc/pid/seccomp_cache
>> > 8e01b51a31a1 seccomp/cache: Add "emulator" to check if filter is constant allow
>> > f9d480b6ffbe seccomp/cache: Lookup syscall allowlist bitmap for fast path
>> > 23d67a54857a seccomp: Migrate to use SYSCALL_WORK flag
>> >
>> >
>> > Please find these easy steps to reproduce the kernel build and boot.
>>
>> Adding Gabriel Krisman Bertazi to Cc, as the last patch (23d67a54857a) here
>> seems suspicious: it changes
>>
>> diff --git a/include/linux/seccomp.h b/include/linux/seccomp.h
>> index 02aef2844c38..47763f3999f7 100644
>> --- a/include/linux/seccomp.h
>> +++ b/include/linux/seccomp.h
>> @@ -42,7 +42,7 @@ struct seccomp {
>>  extern int __secure_computing(const struct seccomp_data *sd);
>>  static inline int secure_computing(void)
>>  {
>> -       if (unlikely(test_thread_flag(TIF_SECCOMP)))
>> +       if (unlikely(test_syscall_work(SECCOMP)))
>>                 return  __secure_computing(NULL);
>>         return 0;
>>  }
>>
>> which is in the call chain directly before
>>
>> int __secure_computing(const struct seccomp_data *sd)
>> {
>>        int mode = current->seccomp.mode;
>>
>> ...
>>         switch (mode) {
>>         case SECCOMP_MODE_STRICT:
>>                 __secure_computing_strict(this_syscall);  /* may call do_exit */
>>                 return 0;
>>         case SECCOMP_MODE_FILTER:
>>                 return __seccomp_filter(this_syscall, sd, false);
>>         default:
>>                 BUG();
>>         }
>> }
>>
>> Clearly, current->seccomp.mode is set to something other
>> than SECCOMP_MODE_STRICT or SECCOMP_MODE_FILTER
>> while the test_syscall_work(SECCOMP) returns true, and this
>> must have not been the case earlier.
>
> Ah, I think the problem is actually in
> 3136b93c3fb2b7c19e853e049203ff8f2b9dd2cd ("entry: Expose helpers to
> migrate TIF to SYSCALL_WORK flag"). In the !GENERIC_ENTRY case, it
> adds this code:
>
> +#define set_syscall_work(fl)                                           \
> +       set_ti_thread_flag(current_thread_info(), SYSCALL_WORK_##fl)
> +#define test_syscall_work(fl) \
> +       test_ti_thread_flag(current_thread_info(), SYSCALL_WORK_##fl)
> +#define clear_syscall_work(fl) \
> +       clear_ti_thread_flag(current_thread_info(), SYSCALL_WORK_##fl)
> +
> +#define set_task_syscall_work(t, fl) \
> +       set_ti_thread_flag(task_thread_info(t), TIF_##fl)
> +#define test_task_syscall_work(t, fl) \
> +       test_ti_thread_flag(task_thread_info(t), TIF_##fl)
> +#define clear_task_syscall_work(t, fl) \
> +       clear_ti_thread_flag(task_thread_info(t), TIF_##fl)
>
> but the SYSCALL_WORK_FLAGS are not valid on !GENERIC_ENTRY, we'll mix
> up (on arm64) SYSCALL_WORK_BIT_SECCOMP (==0) and TIF_SIGPENDING (==0).
>
> As part of fixing this, it might be a good idea to put "enum
> syscall_work_bit" behind a "#ifdef CONFIG_GENERIC_ENTRY" to avoid
> future accidents like this?

Hi Jan, Arnd,

That is correct.  This is a copy pasta mistake.  My apologies.  I didn't
have a !GENERIC_ENTRY device to test, but just the ifdef would have
caught it.

-- 
Gabriel Krisman Bertazi