linux-kernel - Re: [syzbot] KASAN: slab-out-of-bounds Write in usb_hcd_poll_rh

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Message-ID: <Yc9odypVqqB2uMm/@rowland.harvard.edu>
Date:   Fri, 31 Dec 2021 15:30:47 -0500
From:   Alan Stern <stern@...land.harvard.edu>
To:     syzbot <syzbot+3ae6a2b06f131ab9849f@...kaller.appspotmail.com>
Cc:     andreyknvl@...gle.com, dvyukov@...gle.com,
        gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org,
        linux-usb@...r.kernel.org, syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] KASAN: slab-out-of-bounds Write in
 usb_hcd_poll_rh_status (2)

On Fri, Dec 31, 2021 at 09:44:06AM -0800, syzbot wrote:
> Hello,
> 
> syzbot has tested the proposed patch but the reproducer is still triggering an issue:
> KASAN: slab-out-of-bounds Write in usb_hcd_poll_rh_status
> 
> vhci_hcd vhci_hcd.0: poll_rh_status: len 2 maxch 0 tblen 1
> ==================================================================
> BUG: KASAN: slab-out-of-bounds in memcpy include/linux/fortify-string.h:225 [inline]
> BUG: KASAN: slab-out-of-bounds in usb_hcd_poll_rh_status+0x5f4/0x780 drivers/usb/core/hcd.c:776
> Write of size 2 at addr ffff88801da403c0 by task syz-executor133/4062

I think I understand the problem.  This patch is intended to fix it.

Alan Stern

#syz test: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/ eec4df26e24e

Index: usb-devel/drivers/usb/core/hcd.c
===================================================================
--- usb-devel.orig/drivers/usb/core/hcd.c
+++ usb-devel/drivers/usb/core/hcd.c
@@ -753,6 +753,7 @@ void usb_hcd_poll_rh_status(struct usb_h
 {
 	struct urb	*urb;
 	int		length;
+	int		status;
 	unsigned long	flags;
 	char		buffer[6];	/* Any root hubs with > 31 ports? */
 
@@ -770,11 +771,17 @@ void usb_hcd_poll_rh_status(struct usb_h
 		if (urb) {
 			clear_bit(HCD_FLAG_POLL_PENDING, &hcd->flags);
 			hcd->status_urb = NULL;
+			if (urb->transfer_buffer_length >= length) {
+				status = 0;
+			} else {
+				status = -EOVERFLOW;
+				length = urb->transfer_buffer_length;
+			}
 			urb->actual_length = length;
 			memcpy(urb->transfer_buffer, buffer, length);
 
 			usb_hcd_unlink_urb_from_ep(hcd, urb);
-			usb_hcd_giveback_urb(hcd, urb, 0);
+			usb_hcd_giveback_urb(hcd, urb, status);
 		} else {
 			length = 0;
 			set_bit(HCD_FLAG_POLL_PENDING, &hcd->flags);