linux-kernel - Re: [PATCH] futex: Fix fault_in_user

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Date:   Mon, 16 Aug 2021 21:03:18 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Davidlohr Bueso <dave@...olabs.net>,
        Huacai Chen <chenhuacai@...ngson.cn>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Darren Hart <dvhart@...radead.org>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        linux-mips@...r.kernel.org, linux-kernel@...r.kernel.org,
        Xuefeng Li <lixuefeng@...ngson.cn>,
        Huacai Chen <chenhuacai@...il.com>,
        Jiaxun Yang <jiaxun.yang@...goat.com>,
        Hongchen Zhang <zhanghongchen@...ngson.cn>
Subject: Re: [PATCH] futex: Fix fault_in_user_writeable()

On Mon, Aug 16 2021 at 11:27, Davidlohr Bueso wrote:
> On Mon, 16 Aug 2021, Huacai Chen wrote:
>
>>fault_in_user_writeable() should verify R/W access but only verify W. In
>>most archs W implies R, but not true in MIPS and LoongArch, so fix it.
>
> Yuck for a find_vma() in futex.c. If this is a problem in MIPS, shouldn't
> the fix be there? Furthermore it's stated that fault_in_user_writeable():
>
> "Fault in user address and verify RW access"

That seems to be wishful thinking given the fact that some architectures
do not imply R for FLAG_FAULT_WRITE.

> And you guys seem to have proposed it already:
>
> https://lore.kernel.org/linux-mips/20200630005845.1239974-1-liulichao@loongson.cn/

That's surely one way to fix that. If that does not work for whatever
reason, then we really don't want this find_vma() hack there, but rather
something like:

    if (IS_ENABLED(CONFIG_ARCH_USER_FAULT_VOODOO) && get_user(&tmp, uaddr))
	return -EFAULT;

Thanks,

        tglx