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

[Alan Stern]

On Tue, 27 Dec 2016, Felipe Balbi wrote:

> 
> Hi,
> 
> Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> writes:
> > 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) {
> >
> 
> I already have a patch from Greg for this. See [1]
> 
> [1] https://git.kernel.org/cgit/linux/kernel/git/balbi/usb.git/commit/?id=230bc0cb8ff222d9f0fbbd93a80393140b39481f

The two patches fix different problems.  My patch goes on the pathway
where dev->state > STATE_DEV_OPENED in dev_config(), and Greg's patch
handles the case where it is <=.

Alan Stern