[PATCH] fbmem: prevent potential use-after-free issues with console_lock()

[Hang Zhang]

In do_fb_ioctl(), user specified "fb_info" can be freed in the callee
fbcon_get_con2fb_map_ioctl() -> set_con2fb_map() ->
con2fb_release_oldinfo(), this free operation is protected by
console_lock() in fbcon_set_con2fb_map_ioctl(), it also results in
the change of certain states such as "minfo->dead" in matroxfb_remove(),
so that it can be checked to avoid use-after-free before the use sites
(e.g., the check at the beginning of matroxfb_ioctl()). However,
the problem is that the use site is not protected by the same locks
as for the free operation, e.g., "default" case in do_fb_ioctl()
can lead to "matroxfb_ioctl()" but it's not protected by console_lock(),
which can invalidate the aforementioned state set and check in a
concurrent setting.

Prevent the potential use-after-free issues by protecting the "default"
case in do_fb_ioctl() with console_lock(), similarly as for many other
cases like "case FBIOBLANK" and "case FBIOPAN_DISPLAY".

Signed-off-by: Hang Zhang <zh.nvgt at gmail.com>
---
 drivers/video/fbdev/core/fbmem.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
index 1e70d8c67653..8b1a1527d18a 100644
--- a/drivers/video/fbdev/core/fbmem.c
+++ b/drivers/video/fbdev/core/fbmem.c
@@ -1182,6 +1182,7 @@ static long do_fb_ioctl(struct fb_info *info, unsigned int cmd,
 		console_unlock();
 		break;
 	default:
+		console_lock();
 		lock_fb_info(info);
 		fb = info->fbops;
 		if (fb->fb_ioctl)
@@ -1189,6 +1190,7 @@ static long do_fb_ioctl(struct fb_info *info, unsigned int cmd,
 		else
 			ret = -ENOTTY;
 		unlock_fb_info(info);
+		console_unlock();
 	}
 	return ret;
 }
-- 
2.39.0