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

[Thomas Gleixner]

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.

Thanks,

	tglx


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