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.