From: Jiri Slaby <jslaby@suse.cz>

Date: Mon, 19 Oct 2020 10:55:17 +0200

Subject: vt: keyboard, extend func_buf_lock to readers

Git-commit: 82e61c3909db51d91b9d3e2071557b6435018b80

Patch-mainline: 5.10-rc2

References: bnc#1177766 CVE-2020-25656

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.

Cc: <stable@vger.kernel.org>

Reported-by: Minh Yuan <yuanmingbuaa@gmail.com>

Signed-off-by: Jiri Slaby <jslaby@suse.cz>

Link: https://lore.kernel.org/r/20201019085517.10176-2-jslaby@suse.cz

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---

 drivers/tty/vt/keyboard.c |   17 +++++++++++++----

 1 file changed, 13 insertions(+), 4 deletions(-)

--- a/drivers/tty/vt/keyboard.c

+++ b/drivers/tty/vt/keyboard.c

@@ -713,8 +713,13 @@ static void k_fn(struct vc_data *vc, uns

 		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);

 }

@@ -1960,7 +1965,7 @@ out:

 #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;

@@ -1992,10 +1997,14 @@ int vt_do_kdgkb_ioctl(int cmd, struct kb

 	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;

 	}