Patch "usb: f_fs: Force Reserved1=1 in OS_DESC_EXT_COMPAT" has been added to the 4.14-stable tree

This is a note to let you know that I've just added the patch titled

usb: f_fs: Force Reserved1=1 in OS_DESC_EXT_COMPAT

to the 4.14-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
usb-f_fs-force-reserved1-1-in-os_desc_ext_compat.patch
and it can be found in the queue-4.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: John Keeping <[email protected]>
Date: Mon, 27 Nov 2017 18:15:40 +0000
Subject: usb: f_fs: Force Reserved1=1 in OS_DESC_EXT_COMPAT

From: John Keeping <[email protected]>

commit a3acc696085e112733d191a77b106e67a4fa110b upstream.

The specification says that the Reserved1 field in OS_DESC_EXT_COMPAT must have the value "1", but when this feature was first implemented we rejected any non-zero values.

This was adjusted to accept all non-zero values (while now rejecting zero) in commit 53642399aa71 ("usb: gadget: f_fs: Fix wrong check on reserved1 of OS_DESC_EXT_COMPAT"), but that breaks any userspace programs that worked previously by returning EINVAL when Reserved1 == 0 which was previously the only value that succeeded!

If we just set the field to "1" ourselves, both old and new userspace programs continue to work correctly and, as a bonus, old programs are now compliant with the specification without having to fix anything themselves.

---
 drivers/usb/gadget/function/f_fs.c |   13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

@@ -2286,9 +2286,18 @@ static int __ffs_data_do_os_desc(enum ff
 		int i;
 
 		if (len < sizeof(*d) ||
-		    d->bFirstInterfaceNumber >= ffs->interfaces_count ||
-		    !d->Reserved1)
+		    d->bFirstInterfaceNumber >= ffs->interfaces_count)
 			return -EINVAL;
+		if (d->Reserved1 != 1) {
+			/*
+			 * According to the spec, Reserved1 must be set to 1
+			 * but older kernels incorrectly rejected non-zero
+			 * values.  We fix it here to avoid returning EINVAL
+			 * in response to values we used to accept.
+			 */
+			pr_debug("usb_ext_compat_desc::Reserved1 forced to 1\n");
+			d->Reserved1 = 1;
+		}
 		for (i = 0; i < ARRAY_SIZE(d->Reserved2); ++i)
 			if (d->Reserved2[i])
 				return -EINVAL;

Patches currently in stable-queue which might be from [email protected] are

queue-4.14/usb-f_fs-force-reserved1-1-in-os_desc_ext_compat.patch