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

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The reload of 'vc' added by commit 8fb9ea65c9 ("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 a287620312 ("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().

Fixes: a287620312 ("vc_screen: fix null-ptr-deref in vcs_notifier() during concurrent vcs_write")
Cc: stable@kernel.org
Signed-off-by: Yi Yang <yiyang13@huawei.com>
Link: https://patch.msgid.link/20260901113131.2760010-1-yiyang13@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

...

This commit is contained in:

Yi Yang 2026-09-01 11:31:31 +00:00 • committed by Greg Kroah-Hartman

parent d9feaa9332

commit 226bd56033

1 changed files with 12 additions and 11 deletions

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23

drivers/tty/vt/vc_screen.c

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