[kernel-hardening] Re: [PATCH v3 2/4] x86/syscalls: Specific usage of verify_pre_usermode_state

["H. Peter Anvin" <hpa@zytor.com>]

On 03/14/17 09:29, Thomas Garnier wrote:
> 
> We want to enforce the address limit by default, not only when
> CONFIG_BUG_ON_DATA_CORRUPTION is enabled. I tested this one:
> 
> /* Check user-mode state on fast path return. */
> movq PER_CPU_VAR(current_task), %rax
> btq $63, TASK_addr_limit(%rax)
> jnc 1f
> #ifdef CONFIG_BUG_ON_DATA_CORRUPTION
> call syscall_return_slowpath
> jmp return_from_SYSCALL_64
> #else
> movq $TASK_SIZE_MAX, %rcx
> movq %rcx, TASK_addr_limit(%rax)
> #endif
> 1:
> 
> I saw that syscall_return_slowpath is supposed to be called not jumped
> to. I could just call verify_pre_usermode_state that would be about
> the same.

I wanted to comment on that thing: why on earth isn't
verify_pre_usermode_state() an inline?  Making it an out-of-line
function adds pointless extra overhead to the C code when we are talking
about a few instructions.

Second, you never do a branch-around to handle an exceptional condition
on the fast path: you jump *out of line* to handle the special
condition; a forward branch is preferred since it is slightly more
likely to be predicted not taken.

Now, I finally had a chance to actually look at the full file (I was
preoccupied yesterday), and am a bit disappointed, to say the least.

First of all, the jump target you need is only a handful of instructions
further down the code path; you need to do *exactly* that is done when
the test of _TIF_ALLWORK_MASK right above is tested!  Not only that, but
you already have PER_CPU_VAR(current_task) in %r11 just ready to be
used!  This was all in the three instructions immediately prior to the
code you modified...

So, all you'd need would be:

	movq $TASK_SIZE_MAX, %rcx
	cmpq %rcx, TASK_addr_limit(%r11)
	jne 1f

We even get a short jump instruction!

(Using bt saves one more instruction, but see previous caveats about it.)

	-hpa