In the Linux kernel, the following vulnerability has been resolved: vc_screen: reload vc pointer before if (ret) in vcs_write() to avoid UA

In the Linux kernel, the following vulnerability has been resolved:

vc_screen: reload vc pointer before if (ret) in vcs_write() to avoid UAF

The reload of 'vc' added by commit 8fb9ea65c9d1 ("vc_screen: reload load
of struct vc_data pointer in vcs_write() to avoid UAF") sits after the
'if (ret)' block, so the copy-failure break path exits the loop without
reloading vc. If the vc was kfree()'d via vc_port_destruct during the
unlocked copy_from_user() window, the post-loop
'if (written && vc) vcs_scr_updated(vc)' is reached with a stale
non-NULL vc. The '&& vc' guard from commit a287620312dc ("vc_screen:
fix null-ptr-deref in vcs_notifier() during concurrent vcs_write") only
handles the NULL case, not this stale-non-NULL case; vcs_notifier() then
reads param->vc->vc_num from freed memory:

BUG: KASAN: slab-use-after-free in vcs_notifier+0x7c/0xd0
Read of size 2 at addr ffff888007149190
Call Trace:
vcs_notifier+0x7c/0xd0
atomic_notifier_call_chain+0x70/0xa0
vcs_scr_updated+0x77/0xa0
vcs_write+0x71b/0x7e0
Allocated by task: vc_allocate -> con_install -> tty_open
Freed by task: kfree <- vt_ioctl (VT_DISALLOCATE -> vc_port_destruct)

Move the reload to immediately after console_lock(), before 'if (ret)',
so every break path below passes a fresh vc to the post-loop
vcs_scr_updated().