FAILED: patch "[PATCH] vt: selection: Fix unsigned underflow and slab-out-of-bounds" failed to apply to 6.6-stable tree

The patch below does not apply to the 6.6-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.6.y
git checkout FETCH_HEAD
git cherry-pick -x 39495ef5d6f8019e62d65807f217a7ca8232727a
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026100408-grimy-repressed-6259@gregkh' --subject-prefix 'PATCH 6.6.y' 'HEAD^..'

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 39495ef5d6f8019e62d65807f217a7ca8232727a Mon Sep 17 00:00:00 2001
From: Hui Peng <[email protected]>
Date: Sat, 19 Sep 2026 11:00:41 +0000
Subject: [PATCH] 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>
 ==================================================================

index 13f4e48b4142..f1a3bc5b5fad 100644
@@ -434,8 +434,8 @@ int paste_selection(struct tty_struct *tty)
 			bps = NULL;
 		}
 
-		count = vc_sel.buf_len - pasted;
-		if (count) {
+		if (vc_sel.buf_len > pasted) {
+			count = vc_sel.buf_len - pasted;
 			pasted += tty_ldisc_receive_buf(ld, vc_sel.buffer + pasted,
 							NULL, count);
 			if (vc_sel.buf_len > pasted)