Re: [PATCH] mm/usercopy: Use memory range to be accessed for wraparound check

[isaacm]

On 2018-11-14 03:46, William Kucharski wrote:
> > On Nov 14, 2018, at 4:09 AM, David Laight <David.Laight@xxxxxxxxxx> 
> > wrote:
> >
> > From: William Kucharski
> > > Sent: 14 November 2018 10:35
> > >
> > > > On Nov 13, 2018, at 5:51 PM, Isaac J. Manjarres 
> > > > <isaacm@xxxxxxxxxxxxxx> wrote:
> > > >
> > > > diff --git a/mm/usercopy.c b/mm/usercopy.c
> > > > index 852eb4e..0293645 100644
> > > > --- a/mm/usercopy.c
> > > > +++ b/mm/usercopy.c
> > > > @@ -151,7 +151,7 @@ static inline void check_bogus_address(const 
> > > > unsigned long ptr, unsigned long n,
> > > > 				       bool to_user)
> > > > {
> > > > 	/* Reject if object wraps past end of memory. */
> > > > -	if (ptr + n < ptr)
> > > > +	if (ptr + (n - 1) < ptr)
> > > > 		usercopy_abort("wrapped address", NULL, to_user, 0, ptr + n);
> > >
> > > I'm being paranoid, but is it possible this routine could ever be 
> > > passed "n" set to zero?
> > >
> > > If so, it will erroneously abort indicating a wrapped address as (n - 
> > > 1) wraps to ULONG_MAX.
> > >
> > > Easily fixed via:
> > >
> > > 	if ((n != 0) && (ptr + (n - 1) < ptr))
> >
> > Ugg... you don't want a double test.
> >
> > I'd guess that a length of zero is likely, but a usercopy that 
> > includes
> > the highest address is going to be invalid because it is a kernel 
> > address
> > (on most archs, and probably illegal on others).
> > What you really want to do is add 'ptr + len' and check the carry 
> > flag.
>
> The extra test is only a few extra instructions, but I understand the
> concern. (Though I don't
> know how you'd access the carry flag from C in a machine-independent
> way. Also, for the
> calculation to be correct you still need to check 'ptr + (len - 1)'
> for the wrap.)
>
> You could also theoretically call gcc's __builtin_uadd_overflow() if
> you want to get carried away.
>
> As I mentioned, I was just being paranoid, but the passed zero length
> issue stood out to me.
>
>     William Kucharski

Hi William,

Thank you and David for your feedback. The check_bogus_address() routine 
is only invoked from one place in the kernel, which is 
__check_object_size(). Before invoking check_bogus_address, 
__check_object_size ensures that n is non-zero, so it is not possible to 
call this routine with n being 0. Therefore, we shouldn't run into the 
scenario you described. Also, in the case where we are copying a page's 
contents into a kernel space buffer and will not have that buffer 
interacting with userspace at all, this change to that check should 
still be valid, correct?

Thanks,
Isaac Manjarres