[PATCH v2 1/2] vt: keyboard, simplify vt_kdgkbsent

[PATCH v2 1/2] vt: keyboard, simplify vt_kdgkbsent

[Jiri Slaby]

Use 'strlen' of the string, add one for NUL terminator and simply do
'copy_to_user' instead of the explicit 'for' loop. This makes the
KDGKBSENT case more compact.

The only thing we need to take care about is NULL 'func_table[i]'. Use
an empty string in that case.

The original check for overflow could never trigger as the func_buf
strings are always shorter or equal to 'struct kbsentry's.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: <stable@vger.kernel.org>
---

[v2] Removed the need for "reorder user buffer handling in
vt_do_kdgkb_ioctl" patch.

 drivers/tty/vt/keyboard.c | 28 +++++++++-------------------
 1 file changed, 9 insertions(+), 19 deletions(-)

diff --git a/drivers/tty/vt/keyboard.c b/drivers/tty/vt/keyboard.c
index 0db53b5b3acf..7bfa95c3252c 100644
--- a/drivers/tty/vt/keyboard.c
+++ b/drivers/tty/vt/keyboard.c
@@ -1995,9 +1995,7 @@ int vt_do_kdsk_ioctl(int cmd, struct kbentry __user *user_kbe, int perm,
 int vt_do_kdgkb_ioctl(int cmd, struct kbsentry __user *user_kdgkb, int perm)
 {
 	struct kbsentry *kbs;
-	char *p;
 	u_char *q;
-	u_char __user *up;
 	int sz, fnw_sz;
 	int delta;
 	char *first_free, *fj, *fnw;
@@ -2023,23 +2021,15 @@ int vt_do_kdgkb_ioctl(int cmd, struct kbsentry __user *user_kdgkb, int perm)
 	i = array_index_nospec(kbs->kb_func, MAX_NR_FUNC);
 
 	switch (cmd) {
-	case KDGKBSENT:
-		sz = sizeof(kbs->kb_string) - 1; /* sz should have been
-						  a struct member */
-		up = user_kdgkb->kb_string;
-		p = func_table[i];
-		if(p)
-			for ( ; *p && sz; p++, sz--)
-				if (put_user(*p, up++)) {
-					ret = -EFAULT;
-					goto reterr;
-				}
-		if (put_user('\0', up)) {
-			ret = -EFAULT;
-			goto reterr;
-		}
-		kfree(kbs);
-		return ((p && *p) ? -EOVERFLOW : 0);
+	case KDGKBSENT: {
+		/* size should have been a struct member */
+		unsigned char *from = func_table[i] ? : "";
+
+		ret = copy_to_user(user_kdgkb->kb_string, from,
+				strlen(from) + 1) ? -EFAULT : 0;
+
+		goto reterr;
+	}
 	case KDSKBSENT:
 		if (!perm) {
 			ret = -EPERM;
-- 
2.28.0

[PATCH 2/2] vt: keyboard, extend func_buf_lock to readers

[Jiri Slaby]

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.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Reported-by: Minh Yuan <yuanmingbuaa@gmail.com>
Cc: <stable@vger.kernel.org>
---

[v2]
 - Removed the need for "reorder user buffer handling in
   vt_do_kdgkb_ioctl" patch.
 - given the above, use kbs->kb_string instead of another kmalloc.

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

diff --git a/drivers/tty/vt/keyboard.c b/drivers/tty/vt/keyboard.c
index 7bfa95c3252c..78acc270e39a 100644
--- a/drivers/tty/vt/keyboard.c
+++ b/drivers/tty/vt/keyboard.c
@@ -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;
 	}
-- 
2.28.0