Commit 82e61c39 authored Oct 19, 2020 by Jiri Slaby Committed by Greg Kroah-Hartman Oct 28, 2020

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 46ca3f73 (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>

parent 6ca03f90

drivers/tty/vt/keyboard.c

+13 −4

Original line number	Diff line number	Diff line
		@@ -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 @@ 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;
		@@ -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;
		}

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