[PATCH] tty: vt: keyboard: add range check to kbs->kb_func index

[Colin King]

From: Colin Ian King <colin.k...@canonical.com>

A value outside the range 0..MAX_NR_FUNC-1 in kbs->kb_func will
cause an array bounds overflow on func_table. Fix this by adding
a range check.

Detected by CoverityScan, CID#401961 ("Untrusted array index read")

Fixes: 079c9534a96d ("vt:tackle kbd_table")
Signed-off-by: Colin Ian King <colin.k...@canonical.com>
---
 drivers/tty/vt/keyboard.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/tty/vt/keyboard.c b/drivers/tty/vt/keyboard.c
index f4166263bb3a..1ecf545a96a8 100644
--- a/drivers/tty/vt/keyboard.c
+++ b/drivers/tty/vt/keyboard.c
@@ -1982,6 +1982,11 @@ int vt_do_kdgkb_ioctl(int cmd, struct kbsentry __user 
*user_kdgkb, int perm)
        kbs->kb_string[sizeof(kbs->kb_string)-1] = '\0';
        i = kbs->kb_func;
 
+       if (i < 0 || i >= MAX_NR_FUNC) {
+               ret = -EINVAL;
+               goto reterr;
+       }
+
        switch (cmd) {
        case KDGKBSENT:
                sz = sizeof(kbs->kb_string) - 1; /* sz should have been
-- 
2.14.1