USB: EHCI: Improve port index sanitizing

Now that Kees Cook has added a definition for HCS_N_PORTS_MAX in
commit 72dd1843232c ("USB: EHCI: Add register array bounds to HCS
ports"), the code in ehci_hub_control() which sanitizes port index
values can be improved a little.

The idea behind this change is that it prevents a possible
out-of-bounds pointer computation, which the compiler might be able to
detect since the port_status[] array now has a fixed length rather
than a variable length.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Link: https://lore.kernel.org/r/20211002190217.GA537967@rowland.harvard.edu
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/usb/host/ehci-hub.c b/drivers/usb/host/ehci-hub.c
index c4f6a25..efe30e3 100644 (file)
--- a/drivers/usb/host/ehci-hub.c
+++ b/drivers/usb/host/ehci-hub.c
@@ -745,12 +745,13 @@ int ehci_hub_control(
        unsigned        selector;
 
        /*
-        * Avoid underflow while calculating (wIndex & 0xff) - 1.
-        * The compiler might deduce that wIndex can never be 0 and then
-        * optimize away the tests for !wIndex below.
+        * Avoid out-of-bounds values while calculating the port index
+        * from wIndex.  The compiler doesn't like pointers to invalid
+        * addresses, even if they are never used.
         */
-       temp = wIndex & 0xff;
-       temp -= (temp > 0);
+       temp = (wIndex - 1) & 0xff;
+       if (temp >= HCS_N_PORTS_MAX)
+               temp = 0;
        status_reg = &ehci->regs->port_status[temp];
        hostpc_reg = &ehci->regs->hostpc[temp];