patch "vt: keyboard, extend func_buf_lock to readers" added to tty-linus

This is a note to let you know that I've just added the patch titled

vt: keyboard, extend func_buf_lock to readers

to my tty git tree which can be found at
git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/tty.git
in the tty-linus branch.

The patch will show up in the next release of the linux-next tree (usually sometime within the next 24 hours during the week.)

The patch will hopefully also be merged in Linus's tree for the next -rc kernel release.

If you have any questions about this process, please let me know.

From 82e61c3909db51d91b9d3e2071557b6435018b80 Mon Sep 17 00:00:00 2001
From: Jiri Slaby <[email protected]>
Date: Mon, 19 Oct 2020 10:55:17 +0200
Subject: vt: keyboard, extend func_buf_lock to readers

Both read-side users of func_table/func_buf need locking. Without that, one can easily confuse the code by repeatedly setting altering strings like:
while (1)

	for (a = 0; a < 2; a++) {
		struct kbsentry kbs = {};
		strcpy((char *)kbs.kb_string, a ? ".\n" : "88888\n");
		ioctl(fd, KDSKBSENT, &kbs);
	}

When that program runs, one can get unexpected output by holding F1 (note the unxpected period on the last line):
.
88888
.8888

So protect all accesses to 'func_table' (and func_buf) by preexisting 'func_buf_lock'.

It is easy in 'k_fn' handler as 'puts_queue' is expected not to sleep. On the other hand, KDGKBSENT needs a local (atomic) copy of the string because copy_to_user can sleep. Use already allocated, but unused 'kbs->kb_string' for that purpose.

Note that the program above needs at least CAP_SYS_TTY_CONFIG.

This depends on the previous patch and on the func_buf_lock lock added in commit 46ca3f735f34 (tty/vt: fix write/write race in ioctl(KDSKBSENT) handler) in 5.2.

Likely fixes CVE-2020-25656.

---
 drivers/tty/vt/keyboard.c | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

index 7bfa95c3252c..78acc270e39a 100644
@@ -743,8 +743,13 @@ static void k_fn(struct vc_data *vc, unsigned char value, char up_flag)
 		return;
 
 	if ((unsigned)value < ARRAY_SIZE(func_table)) {
+		unsigned long flags;
+
+		spin_lock_irqsave(&func_buf_lock, flags);
 		if (func_table[value])
 			puts_queue(vc, func_table[value]);
+		spin_unlock_irqrestore(&func_buf_lock, flags);
+
 	} else
 		pr_err("k_fn called with value=%d\n", value);
 }

@@ -1991,7 +1996,7 @@ int vt_do_kdsk_ioctl(int cmd, struct kbentry __user *user_kbe, int perm,
 #undef s
 #undef v
 
-/* FIXME: This one needs untangling and locking */
+/* FIXME: This one needs untangling */
 int vt_do_kdgkb_ioctl(int cmd, struct kbsentry __user *user_kdgkb, int perm)
 {
 	struct kbsentry *kbs;

@@ -2023,10 +2028,14 @@ int vt_do_kdgkb_ioctl(int cmd, struct kbsentry __user *user_kdgkb, int perm)
 	switch (cmd) {
 	case KDGKBSENT: {
 		/* size should have been a struct member */
-		unsigned char *from = func_table[i] ? : "";
+		ssize_t len = sizeof(user_kdgkb->kb_string);
+
+		spin_lock_irqsave(&func_buf_lock, flags);
+		len = strlcpy(kbs->kb_string, func_table[i] ? : "", len);
+		spin_unlock_irqrestore(&func_buf_lock, flags);
 
-		ret = copy_to_user(user_kdgkb->kb_string, from,
-				strlen(from) + 1) ? -EFAULT : 0;
+		ret = copy_to_user(user_kdgkb->kb_string, kbs->kb_string,
+				len + 1) ? -EFAULT : 0;
 
 		goto reterr;
 	}

2.29.1