Re: arch/x86/kernel/vsyscall_64.c: overeager NOP of syscalls

Arne Georg Gleditsch
2008-02-22T10:37:28Z

Andi Kleen <ak@muc.de> writes:
> On Wed, Feb 20, 2008 at 02:57:34PM +0100, Arne Georg Gleditsch wrote:
>> Hi,
>>
>> I'm looking at 2.6.25-rc2. vsyscall_sysctl_change contains code to NOP
>> out the actual system call instructions of the vsyscall page when
>> vsyscall64 is enabled. This seems to interact badly with the fallback
>> code in do_vgettimeofday which tries to call gettimeofday if the
>> configured clock source does not support vread. (In effect,
>> gettimeofday() becomes a nop and time() always returns 0. Not very
>> useful.)
>>
>> Is there a good reason to keep this? Aren't the instructions in
>> question avoided (or invoked) according to the vsyscall64 flag by the
>> surrounding logic anyway?
>
> Yes they are. But a system call sequence at a known fixed address
> is potentially useful to exploits. That is why it is nop'ed out when
> it is not needed.

Reasonable enough, as long as it can be determined to be not needed.
Still, isn't the __vsyscall_gtod_data structure part of the same page?
Wouldn't that give you access to any 2-byte opcode you want every 64k
seconds? You'd need to time your attack, of course, but that could be
done prior to actually launching the exploit...

--
Arne.