Re: [Intel-gfx] [PATCH] drm/i915: use static const array for PICK macro

[Arnd Bergmann <arnd@arndb.de>]

On Tue, Mar 21, 2017 at 12:23 PM, Jani Nikula
<jani.nikula@linux.intel.com> wrote:
> On Tue, 21 Mar 2017, Daniel Vetter <daniel@ffwll.ch> wrote:
>> On Tue, Mar 21, 2017 at 09:44:07AM +0100, Arnd Bergmann wrote:
>>> On Tue, Mar 21, 2017 at 9:26 AM, Jani Nikula
>>> <jani.nikula@linux.intel.com> wrote:
>>> > On Mon, 20 Mar 2017, Arnd Bergmann <arnd@arndb.de> wrote:
>>> >> The varargs macro trick in _PIPE3/_PHY3/_PORT3 was meant as an optimization
>>> >> to shrink the i915 kernel module by around 1000 bytes.
>>> >
>>> > Really, I didn't care one bit about the size shrink, I only cared about
>>> > making it easier and less error prone to increase the number of args in
>>> > a number of places. Maintainability and correctness were the goals. Just
>>> > for the record. ;)
>>>
>>> Ok. My only interest here is the warning about possible stack overflow,
>>> though the fact that KASAN considers the array code to be fragile is
>>> an indication that it is perhaps actually dangerous: if we ever run into
>>> a bug that causes the array index to overflow, we might in theory
>>> have a security bug that lets users access arbitrary kernel pointers.
>>>
>>> While the risk for that actually happening is very low, the original code
>>> was safer in that regard. My patch on top of yours merely turns a
>>> hypothetical arbitrary stack access into an arbitrary .data access,
>>> and I don't even know which one would be worse.
>>
>> Even without these arrays, if userspace could control the index we feed
>> into these you get arbitrary mmio access. Or semi-arbitrary at least.
>>
>> None of these are bugs we should ever let through, and I think with the
>> current code design (where the driver constructs structs that contain the
>> right indizes, and userspace only ever gets to point at these structs
>> using an idr lookup) none of these are likely to happen.
>
> That's all true, but I'm curious if explicit checks would help
> kasan. Something like:
>
> diff --git a/drivers/gpu/drm/i915/i915_reg.h b/drivers/gpu/drm/i915/i915_reg.h
> index 04c8f69fcc62..0ab32a05b5d8 100644
> --- a/drivers/gpu/drm/i915/i915_reg.h
> +++ b/drivers/gpu/drm/i915/i915_reg.h
> @@ -48,7 +48,8 @@ static inline bool i915_mmio_reg_valid(i915_reg_t reg)
>         return !i915_mmio_reg_equal(reg, INVALID_MMIO_REG);
>  }
>
> -#define _PICK(__index, ...) (((const u32 []){ __VA_ARGS__ })[__index])
> +#define _PICK_NARGS(...) ARRAY_SIZE(((const u32 []){ __VA_ARGS__ }))
> +#define _PICK(__index, ...) ((__index) >= 0 && (__index) < _PICK_NARGS(__VA_ARGS__) ? ((const u32 []){ __VA_ARGS__ })[__index] : 0)
>
>  #define _PIPE(pipe, a, b) ((a) + (pipe)*((b)-(a)))
>  #define _MMIO_PIPE(pipe, a, b) _MMIO(_PIPE(pipe, a, b))
>
> ---
>
> Arnd, can you check that with kasan please? (I don't have gcc 7.) For me
> the size diff against current git is
>
>     text           data     bss     dec     hex filename
> -1137236          31211    2948 1171395  11dfc3 drivers/gpu/drm/i915/i915.ko
> +1139702          31211    2948 1173861  11e965 drivers/gpu/drm/i915/i915.ko
>

I just revisited my old patch when I ran into the stack size warning once
more, and realized I had not really answered your question earlier.

I compared your version to what is in 4.15-rc3 now, and to my version,
and confirmed that yours produces the largest code size of the three,
and doesn't address the warnings we get, but does cause additional
warnings ("comparison of constant '3' with boolean expression is always
true"), so that won't get us anywhere. Here are the numbers I get
with gcc-8:

   text    data     bss     dec     hex filename
2500045 486453    6912 2993410 2dad02 i915-kasan-4_14.ko
2488028 497909    6912 2992849 2daad1 i915-kasan-arnd.ko
2508814 486453    6912 3002179 2dcf43 i915-kasan-jani.ko
1639798   63269    4448 1707515 1a0dfb i915-nokasan-4.15.ko
1635284   63269    4448 1703001 19fc59 i915-nokasan-arnd.ko
1648331   63269    4448 1716048 1a2f50 i915-nokasan-jani.ko

I'll resend my old patch with the original description since I can't easily
reproduce it now without your original change, and the code has
changed again in the meantime, so I had to slightly adapt my
patch to still apply.

        Arnd