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

[Jani Nikula <jani.nikula@linux.intel.com>]

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

BR,
Jani.


-- 
Jani Nikula, Intel Open Source Technology Center