On Monday 25 May 2009 21:22:06 Trent Piepho wrote:
> On Mon, 25 May 2009, Laurent Pinchart wrote:
> > diff -r e0d881b21bc9 linux/drivers/media/video/v4l2-ioctl.c
> > --- a/linux/drivers/media/video/v4l2-ioctl.c Tue May 19 15:12:17 2009
> > +0200 +++ b/linux/drivers/media/video/v4l2-ioctl.c Sun May 24 18:26:29
> > 2009 +0200 @@ -402,6 +402,10 @@
> > a specific control that caused it. */
> > p->error_idx = p->count;
> > user_ptr = (void __user *)p->controls;
> > + if (p->count > KMALLOC_MAX_SIZE / sizeof(p->controls[0])) {
> > + err = -ENOMEM;
> > + goto out_ext_ctrl;
> > + }
> > if (p->count) {
> > ctrls_size = sizeof(struct v4l2_ext_control) * p->count;
> > /* Note: v4l2_ext_controls fits in sbuf[] so mbuf is
> > still NULL. */
> > @@ -1859,6 +1863,10 @@
> > a specific control that caused it. */
> > p->error_idx = p->count;
> > user_ptr = (void __user *)p->controls;
> > + if (p->count > KMALLOC_MAX_SIZE / sizeof(p->controls[0])) {
> > + err = -ENOMEM;
> > + goto out_ext_ctrl;
> > + }
> > if (p->count) {
> > ctrls_size = sizeof(struct v4l2_ext_control) * p->count;
> > /* Note: v4l2_ext_controls fits in sbuf[] so mbuf is
> > still NULL. */
> >
> > Restricting v4l2_ext_controls::count to values smaller than
> > KMALLOC_MAX_SIZE / sizeof(struct v4l2_ext_control) should be enough, but
> > we might want to restrict the value even further. I'd like opinions on
> > this.
>
> One thing that could be done is to call access_ok() on the range before
> kmalloc'ing a buffer. If p->count is too high, then it's possible that the
> copy_from_user will fail because the process does not have the address
> space to copy.
arch/x86/include/asm/uaccess.h, about access_ok():
* Note that, depending on architecture, this function probably just
* checks that the pointer is in the user space range - after calling
* this function, memory access functions may still return -EFAULT.
I don't think it's worth it. Let's just kmalloc (or kzalloc) and
copy_from_user. If one of them fails we'll return an error.
Could a very large number of control requests be used as a DoS attack vector ?
A userspace application could kmalloc large amounts of memory without any
restriction. Memory would be reclaimed eventually, but after performing a
large number of USB requests, which could take quite a long time.
Best regards,
Laurent Pinchart
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