Re: net/gadget: slab-out-of-bounds write in dev_config

[Alan Stern]

On Tue, 6 Dec 2016, Andrey Konovalov wrote:

> Hi!
> 
> I've got the following error report while running the syzkaller fuzzer.
> 
> ep0_write() doesn't check the length, so a user can cause an
> out-of-bounds with both size and data controlled.
> There's a comment which says "IN DATA+STATUS caller makes len <=
> wLength". While I'm not exactly sure what that means, the length seems
> to be passed unmodified directly from dev_config().

You're right about the comment being misleading.  It looks like 
somebody forgot to actually do the check.

> This doesn't seem to be a critical security issue since gadgetfs can't
> be mounted from a user namespace.
> 
> On commit 3c49de52d5647cda8b42c4255cf8a29d1e22eff5 (Dec 2).
> 
> ==================================================================
> BUG: KASAN: slab-out-of-bounds in dev_config+0x86f/0x1190 at addr
> ffff88003c47e160
> Write of size 65537 by task syz-executor0/6356
> CPU: 3 PID: 6356 Comm: syz-executor0 Not tainted 4.9.0-rc7+ #19
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
>  ffff88003c107ad8 ffffffff81f96aba ffffffff3dc11ef0 1ffff10007820eee
>  ffffed0007820ee6 ffff88003dc11f00 0000000041b58ab3 ffffffff8598b4c8
>  ffffffff81f96828 ffffffff813fb4a0 ffff88003b6eadc0 ffff88003c107738
> Call Trace:
>  [<     inline     >] __dump_stack lib/dump_stack.c:15
>  [<ffffffff81f96aba>] dump_stack+0x292/0x398 lib/dump_stack.c:51
>  [<ffffffff817e4dec>] kasan_object_err+0x1c/0x70 mm/kasan/report.c:159
>  [<     inline     >] print_address_description mm/kasan/report.c:197
>  [<ffffffff817e5080>] kasan_report_error+0x1f0/0x4e0 mm/kasan/report.c:286
>  [<ffffffff817e5705>] kasan_report+0x35/0x40 mm/kasan/report.c:306
>  [<     inline     >] check_memory_region_inline mm/kasan/kasan.c:308
>  [<ffffffff817e3fb9>] check_memory_region+0x139/0x190 mm/kasan/kasan.c:315
>  [<ffffffff817e4044>] kasan_check_write+0x14/0x20 mm/kasan/kasan.c:326
>  [<     inline     >] copy_from_user arch/x86/include/asm/uaccess.h:689
>  [<     inline     >] ep0_write drivers/usb/gadget/legacy/inode.c:1135
>  [<ffffffff83228caf>] dev_config+0x86f/0x1190
> drivers/usb/gadget/legacy/inode.c:1759
>  [<ffffffff817fdd55>] __vfs_write+0x5d5/0x760 fs/read_write.c:510
>  [<ffffffff817ff650>] vfs_write+0x170/0x4e0 fs/read_write.c:560
>  [<     inline     >] SYSC_write fs/read_write.c:607
>  [<ffffffff81803a5b>] SyS_write+0xfb/0x230 fs/read_write.c:599
>  [<ffffffff84f47ec1>] entry_SYSCALL_64_fastpath+0x1f/0xc2

How does this patch work out?

Alan Stern



Index: usb-4.x/drivers/usb/gadget/legacy/inode.c
===================================================================
--- usb-4.x.orig/drivers/usb/gadget/legacy/inode.c
+++ usb-4.x/drivers/usb/gadget/legacy/inode.c
@@ -1126,7 +1126,7 @@ ep0_write (struct file *fd, const char _
 	/* data and/or status stage for control request */
 	} else if (dev->state == STATE_DEV_SETUP) {
 
-		/* IN DATA+STATUS caller makes len <= wLength */
+		len = min(len, (size_t) dev->setup_wLength);
 		if (dev->setup_in) {
 			retval = setup_req (dev->gadget->ep0, dev->req, len);
 			if (retval == 0) {