On Mon, Aug 16 2021 at 11:27, Davidlohr Bueso wrote:

On Mon, 16 Aug 2021, Huacai Chen wrote:

quoted

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/ (local)

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