Re: 3.15: kernel BUG at kernel/auditsc.c:1525!

Andy Lutomirski
2014-06-16T21:55:03Z

On Mon, Jun 16, 2014 at 2:48 PM, H. Peter Anvin <hpa@zytor.com> wrote:
> On 06/16/2014 02:35 PM, Andy Lutomirski wrote:
>>
>> To hpa, etc: It appears that entry_32.S is missing any call to the
>> audit exit hook on the badsys path. If I'm diagnosing this bug report
>> correctly, this causes OOPSes.
>>
>> The the world at large: it's increasingly apparent that no one (except
>> maybe the blackhats) has ever scrutinized the syscall auditing code.
>> This is two old severe bugs in the code that have probably been there
>> for a long time.
>>
>
> Yes, the audit code is a total mess.
>
>> The bad syscall nr paths are their own incomprehensible route
>> through the entry control flow. Rearrange them to work just like
>> syscalls that return -ENOSYS.
>
> I have to admit... it sort of lends itself to a solution like this:
>
> /* For the 64-bit case, analogous code for 32 bits */
> movl $__NR_syscall_max+1,%ecx # *Not* __NR_syscall_max
> cmpq %rcx,%rax
> cmovae %rcx,%rax
> movq %r10,%rcx
> call *sys_call_table(,%rax,8)
>
> ... and having an extra (invalid) system call slot in the syscall table
> beyond the end instead of branching off separately.
>
> (Note: we could use either cmova or cmovae, and either the 32- or 64-bit
> form... the reason why is left as an exercise to the reader.)

For 64-bit, I want to do this instead:

https://git.kernel.org/cgit/linux/kernel/git/luto/linux.git/commit/?h=x86/seccomp-fastpath&id=a5ec2d7af2c54b55fc7201fa662138b53fbbda39

I see no reason why the 64-bit badsys code needs its own code path at
all. I haven't sent it yet because AFAICT it doesn't fix any bug, and
the series it's a part of isn't ready.

I'm also contemplating rewriting the 64-bit syscall entry work path in C.

--Andy