Re: BUG: bad usercopy in hidraw_ioctl

[Al Viro <viro@zeniv.linux.org.uk>]

On Wed, Aug 07, 2019 at 12:58:21PM -0700, Matthew Wilcox wrote:
> On Wed, Aug 07, 2019 at 12:28:06PM -0700, syzbot wrote:
> > usercopy: Kernel memory exposure attempt detected from wrapped address
> > (offset 0, size 0)!
> > ------------[ cut here ]------------
> > kernel BUG at mm/usercopy.c:98!
> 
> This report is confusing because the arguments to usercopy_abort() are wrong.
> 
>         /* Reject if object wraps past end of memory. */
>         if (ptr + n < ptr)
>                 usercopy_abort("wrapped address", NULL, to_user, 0, ptr + n);
> 
> ptr + n is not 'size', it's what wrapped.  I don't know what 'offset'
> should be set to, but 'size' should be 'n'.  Presumably we don't want to
> report 'ptr' because it'll leak a kernel address ... reporting 'n' will
> leak a range for a kernel address, but I think that's OK?  Admittedly an
> attacker can pass in various values for 'n', but it'll be quite noisy
> and leave a trace in the kernel logs for forensics to find afterwards.
> 
> > Call Trace:
> >  check_bogus_address mm/usercopy.c:151 [inline]
> >  __check_object_size mm/usercopy.c:260 [inline]
> >  __check_object_size.cold+0xb2/0xba mm/usercopy.c:250
> >  check_object_size include/linux/thread_info.h:119 [inline]
> >  check_copy_size include/linux/thread_info.h:150 [inline]
> >  copy_to_user include/linux/uaccess.h:151 [inline]
> >  hidraw_ioctl+0x38c/0xae0 drivers/hid/hidraw.c:392
> 
> The root problem would appear to be:
> 
>                                 else if (copy_to_user(user_arg + offsetof(
>                                         struct hidraw_report_descriptor,
>                                         value[0]),
>                                         dev->hid->rdesc,
>                                         min(dev->hid->rsize, len)))
> 
> That 'min' should surely be a 'max'?

Surely not.  ->rsize is the amount of data available to copy out; len
is the size of buffer supplied by userland to copy into.

BTW, why is it playing those games with offsetof, anyway?  What's wrong
with
	struct hidraw_report_descriptor __user *p = user_arg;
	...
	get_user(&p->size)
	...
	copy_to_user(p->value, ...)