From: Jiri Slaby <jslaby@xxxxxxx>
commit 82e61c3909db51d91b9d3e2071557b6435018b80 upstream.
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@xxxxxxxxxxxxxxx>
Reported-by: Minh Yuan <yuanmingbuaa@xxxxxxxxx>
Signed-off-by: Jiri Slaby <jslaby@xxxxxxx>
Link: https://lore.kernel.org/r/20201019085517.10176-2-jslaby@xxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
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
@@ -742,8 +742,13 @@ 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);
}
@@ -1990,7 +1995,7 @@ 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;
@@ -2022,10 +2027,14 @@ 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;
}
