[PATCH 2/2] Input: zero ff_effect before compat copy in input_ff_effect_from_user

["Iván Ezequiel Rodriguez" <ivanrwcm25@gmail.com>]

In the compat path input_ff_effect_from_user() aliases the caller's
native struct ff_effect with the smaller struct ff_effect_compat and
copies only the compat sized prefix:

	compat_effect = (struct ff_effect_compat *)effect;

	if (copy_from_user(compat_effect, buffer,
			   sizeof(struct ff_effect_compat)))

The tail of the native structure is never written. Callers pass an
uninitialized on-stack object, for example evdev_do_ioctl() for
EVIOCSFF, so those bytes keep their previous stack contents.
input_ff_upload() then stores the full native structure in
ff->effects[id], from where a uinput based force feedback daemon can
read it back via UI_BEGIN_FF_UPLOAD, disclosing kernel stack memory to
userspace.

Zero the effect before the compat copy.

Fixes: 2d56f3a32c0e ("Input: refactor evdev 32bit compat to be shareable with uinput")
Cc: stable@vger.kernel.org
Signed-off-by: Iván Ezequiel Rodriguez <ivanrwcm25@gmail.com>
---
 drivers/input/input-compat.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/input/input-compat.c b/drivers/input/input-compat.c
index a5043193ead8..8860a0730294 100644
--- a/drivers/input/input-compat.c
+++ b/drivers/input/input-compat.c
@@ -76,6 +76,8 @@ int input_ff_effect_from_user(const char __user *buffer, size_t size,
 		 */
 		compat_effect = (struct ff_effect_compat *)effect;
 
+		memset(effect, 0, sizeof(*effect));
+
 		if (copy_from_user(compat_effect, buffer,
 				   sizeof(struct ff_effect_compat)))
 			return -EFAULT;
-- 
2.43.0