[PATCH v3 2/8] media: v4l2-core: explicitly clear ioctl input data

[Arnd Bergmann <arnd@kernel.org>]

From: Arnd Bergmann <arnd@arndb.de>

As seen from a recent syzbot bug report, mistakes in the compat ioctl
implementation can lead to uninitialized kernel stack data getting used
as input for driver ioctl handlers.

The reported bug is now fixed, but it's possible that other related
bugs are still present or get added in the future. As the drivers need
to check user input already, the possible impact is fairly low, but it
might still cause an information leak.

To be on the safe side, always clear the entire ioctl buffer before
calling the conversion handler functions that are meant to initialize
them.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 drivers/media/v4l2-core/v4l2-ioctl.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/media/v4l2-core/v4l2-ioctl.c b/drivers/media/v4l2-core/v4l2-ioctl.c
index 58df927aec7e..f19e56116e53 100644
--- a/drivers/media/v4l2-core/v4l2-ioctl.c
+++ b/drivers/media/v4l2-core/v4l2-ioctl.c
@@ -3124,8 +3124,10 @@ static int video_get_user(void __user *arg, void *parg,
 		if (copy_from_user(parg, (void __user *)arg, n))
 			err = -EFAULT;
 	} else if (in_compat_syscall()) {
+		memset(parg, 0, n);
 		err = v4l2_compat_get_user(arg, parg, cmd);
 	} else {
+		memset(parg, 0, n);
 		switch (cmd) {
 #if !defined(CONFIG_64BIT) && defined(CONFIG_COMPAT_32BIT_TIME)
 		case VIDIOC_QUERYBUF_TIME32:
-- 
2.29.2