Overview

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

vt: selection: Fix unsigned underflow and slab-out-of-bounds read in paste_selection()

In paste_selection(), the loop copies min_t(unsigned int, vc_sel.buf_len - pasted,tty->receive_room) bytes per iteration into tty_ldisc_receive_buf() and increments pasted += count.

Because the selection mutex (vc_sel.lock) is dropped inside the loop Whenever the line discipline buffer fills up and paste_selection() sleeps on tty->write_wait, a concurrent TIOCLINUX (TIOCL_SETSEL) ioctl can replace vc_sel.buffer with a shorter selection and reduce vc_sel.buf_len below pasted.

When paste_selection() resumes, vc_sel.buf_len - pasted underflows as an unsigned integer to a large positive value, causing tty_ldisc_receive_buf(ld, vc_sel.buffer + pasted, NULL, count) to read up to 4094 bytes out-of-bounds past the newly allocated vc_sel.buffer.

Fix this by terminating the loop when pasted >= vc_sel.buf_len.

Kernel stack trace (Linux 7.3.0-rc3):

BUG: KASAN: slab-out-of-bounds in n_tty_receive_buf_common+0xa01/0x1650 Read of size 4094 at addr ffff888101c58010 by task kworker/u17:1/65 Workqueue: events_unbound flush_to_ldisc Call Trace: <TASK> dump_stack_lvl+0x70/0xa0 print_report+0x153/0x4c6 kasan_report+0xf1/0x120 kasan_check_range+0x11c/0x200 __asan_memcpy+0x29/0x70 n_tty_receive_buf_common+0xa01/0x1650 tty_ldisc_receive_buf+0x66/0x110 tty_port_default_receive_buf+0x6b/0xb0 flush_to_ldisc+0x1b4/0x410 process_one_work+0x6ff/0x1110 worker_thread+0x4a8/0xb70 kthread+0x307/0x3e0 ret_from_fork+0x3ed/0x680 </TASK>

Affected

• Linux >= e8c75a30a23c6ba63f4ef6895cbf41fd42f21aa2 < 037d7e3ca5c593092f4b0d471eb869a85cbddde0
• Linux >= e8c75a30a23c6ba63f4ef6895cbf41fd42f21aa2 < 619c6d7dd76fff15a9714fbf02ce299b737f01a2
• Linux >= e8c75a30a23c6ba63f4ef6895cbf41fd42f21aa2 < 39495ef5d6f8019e62d65807f217a7ca8232727a
• Linux 54c522782fd11a720f959ee9cb5a15bb9580343d
• Linux 05c508edc9d0b44e4706d3c2cf14ee12aac210d2
• Linux e5be0e24ffc7f5783a3864b5b958088ed15be9e8
• Linux a4719f6d07b2c63223f7452c435c5f578f105cfe
• Linux b4492f1e7456bd162714c0ec2815c2749d930844
• Linux 9a9c13ad21a8220cb70c9807a920f43520515f94
• Linux 8c368d36878e2fc99285279fc5e14b3d3cb8947c
• Linux >= 3.16.83 < 3.17
• Linux >= 4.4.216 < 4.5
• Linux >= 4.9.216 < 4.10
• Linux >= 4.14.173 < 4.15
• Linux >= 4.19.109 < 4.20
• Linux >= 5.4.25 < 5.5
• Linux >= 5.5.9 < 5.6
• Linux 5.6

Remediation

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