[SRU Bionic 0/1] UBUNTU: SAUCE: fbdev: remove redundant lock_fb_info

[Cengiz Can]

BugLink: https://bugs.launchpad.net/bugs/1990690

SRU Justification:

[Impact]
One of the fixing commits for CVE-2021-33655, commit 159a96b199b4
("fbcon: Prevent that screen size is smaller than font size")
introduced a redundant lock_fb_info line into the ioctl flow in fbmem.c.

Users belonging to video group may trigger a deadlock and potentially
lock the system.

```
============================================
WARNING: possible recursive locking detected
4.15.0-195-generic #206 Not tainted
--------------------------------------------
refresh/1248 is trying to acquire lock:
  (&fb_info->lock){+.+.}, at: [<000000004c154cfe>] lock_fb_info+0x1d/0x40

but task is already holding lock:
  (&fb_info->lock){+.+.}, at: [<000000004c154cfe>] lock_fb_info+0x1d/0x40

other info that might help us debug this:
  Possible unsafe locking scenario:
        CPU0
        ----
   lock(&fb_info->lock);
   lock(&fb_info->lock);

*** DEADLOCK ***
  May be due to missing lock nesting notation
 2 locks held by refresh/1248:
  #0: (console_lock){+.+.}, at: [<000000008000aa2b>] do_fb_ioctl+0x435/0x5e0
  #1: (&fb_info->lock){+.+.}, at: [<000000004c154cfe>] lock_fb_info+0x1d/0x40
```

[Test case]
Ran a sample framebuffer userspace test which calls FBIOPUT_VSCREENINFO.

Verified that deadlock is producible.

After removing redundant lock_fb_info, same test program no longer
causes a deadlock.

[Potential regressions]
There are no new potential regressions.

Cengiz Can (1):
  UBUNTU: SAUCE: fbdev: remove redundant lock_fb_info

 drivers/video/fbdev/core/fbmem.c | 1 -
 1 file changed, 1 deletion(-)

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2.34.1