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

Overview

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().

Affected

• Linux >= 73049768ad57145acd337102c5aa3c788e6642c8 < 1485a3dd97ab1af2605243cabe6b604546d46dd1
• Linux >= 7cc3dd79777f6ae4625ec37e84dd18a26dc88bde < 233a2e190b12884d186027d0b86d43a48799b69c
• Linux >= a287620312dc6dcb9a093417a0e589bf30fcf38a < 9dfcadc5e003fb250dd471db90144bd5366140e9
• Linux >= a287620312dc6dcb9a093417a0e589bf30fcf38a < 226bd5603371cd9f6642cbb9e9a677f445de3530
• Linux 43a6281790273c1b0a9ab76609ff0245b968f1e6
• Linux b6bbb85cf45bf0b070e741997fe0af3a772c5ad5
• Linux ff4806202749a51938236214adc0281481a57366
• Linux 8232fca738011ca2ec865b46ec721d1796dc0580
• Linux 74be188eb2dc1c99d63986167b9a67d415fe7326
• Linux 09a43e81279b8da15526da09877134b8bcf618b0
• Linux >= 6.12.95 < 6.12.113
• Linux >= 6.18.37 < 6.18.56
• Linux >= 5.10.260 < 5.11
• Linux >= 5.15.211 < 5.16
• Linux >= 6.1.177 < 6.2
• Linux >= 6.6.144 < 6.7
• Linux >= 7.0.14 < 7.1
• Linux >= 7.1.2 < 7.2
• Linux 7.2

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.