From: Thomas Gleixner <tglx@linutronix.de>
To: Jaccon Bastiaansen <jaccon.bastiaansen@gmail.com>
Cc: x86@kernel.org, mingo@redhat.com,
"H. Peter Anvin" <hpa@zytor.com>,
Peter Zijlstra <peterz@infradead.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
h.zuidam@computer.org, stable@vger.kernel.org
Subject: Re: [RFC]: Possible race condition in kernel futex code
Date: Fri, 9 Oct 2015 10:06:41 +0100 (IST) [thread overview]
Message-ID: <alpine.DEB.2.11.1510090946200.6097@nanos> (raw)
In-Reply-To: <CAGzjT4ez+gWr3BFQsEr-wma+vs6UZNJ+mRARx_BWoAKEJSsN=w@mail.gmail.com>
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
next prev parent reply other threads:[~2015-10-09 9:07 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-10-05 15:11 [RFC]: Possible race condition in kernel futex code Jaccon Bastiaansen
2015-10-09 9:06 ` Thomas Gleixner [this message]
2015-10-09 9:49 ` Hans Zuidam
2015-10-09 10:25 ` Thomas Gleixner
2015-10-09 11:35 ` Peter Zijlstra
2015-10-18 0:23 ` Greg KH
2016-05-15 22:34 ` Ben Hutchings
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