Re: [RFC]: Possible race condition in kernel futex code

[Greg KH <greg@kroah.com>]

On Fri, Oct 09, 2015 at 10:06:41AM +0100, Thomas Gleixner wrote:
> On Mon, 5 Oct 2015, Jaccon Bastiaansen wrote:
> > We did some tests with different compilers, kernel versions and kernel
> > configs, with the following results:
> > 
> > Linux 3.12.48, x86_64_defconfig, GCC 4.6.1 :
> > copy_user_generic_unrolled being used, so race condition possible
> > Linux 3.12.48, x86_64_defconfig, GCC 4.9.1 :
> > copy_user_generic_unrolled being used, so race condition possible
> > Linux 4.2.3, x86_64_defconfig, GCC 4.6.1 : 32 bit read being used, no
> > race condition
> > Linux 4.2.3, x86_64_defconfig, GCC 4.9.1 : 32 bit read being used, no
> > race condition
> > 
> > 
> > Our idea to fix this problem is use an explicit 32 bit read in
> > get_futex_value_locked() instead of using the generic function
> > copy_from_user_inatomic() and hoping the compiler uses an atomic
> > access and the right access size.
> 
> You cannot use an explicit 32bit read. We need an access which handles
> the fault gracefully.
> 
> In current mainline this is done proper:
> 
> ret = __copy_from_user_inatomic(dst, src, size = sizeof(u32))
> 
>         __copy_from_user_nocheck(dst, src, size)
> 
>     	       if (!__builtin_constant_p(size))
>                      return copy_user_generic(dst, (__force void *)src, size);
> 	
> 	       size is constant so we end up in the switch case
> 
> 	       switch(size) {
> 	       
> 	       case 4:
> 	       	    __get_user_asm(*(u32 *)dst, (u32 __user *)src,
> 		     		   ret, "l", "k", "=r", 4);
> 		    return ret;
> ....
> 
> In 3.12 this is different:
> 
>    __copy_from_user_inatomic()
> 	copy_user_generic()
> 	    copy_user_generic_unrolled()
> 
> So this is only an issue for kernel versions < 3.13. It was fixed with
> 
> ff47ab4ff3cd: Add 1/2/4/8 byte optimization to 64bit __copy_{from,to}_user_inatomic
> 
> but nobody noticed that the race you described can happen, so it was
> never backported to the stable kernels.
> 
> @stable: Can you please pick up ff47ab4ff3cd plus 
> 
> df90ca969035d x86, sparse: Do not force removal of __user when calling copy_to/from_user_nocheck()
> 
> for stable kernels <= 3.12?
> 
> If that's too much of churn, then I can come up with an explicit fix
> for this. Let me know.

Now applied to 3.10-stable, thanks.

greg k-h