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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Thu, 11 Jan 2018 17:11:20 -0800
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Subject: Re: [PATCH v2 08/19] x86: use __uaccess_begin_nospec and ASM_IFENCE
 in get_user paths
To: Dan Williams <dan.j.williams@intel.com>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-arch@vger.kernel.org, Andi Kleen <ak@linux.intel.com>,
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        "the arch/x86 maintainers" <x86@kernel.org>,
        Ingo Molnar <mingo@redhat.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
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This patch doesn't affect arch/x86/lib/getuser.S, which I find surprising.

Of all the user access functions, I actually think that get_user() is
the one most likely to have the result then used speculatively as an
index (the required second dependent read to actually leak data).

I do *not* see people doing "copy_from_user()" and then somehow using
the thing as an index to another array. I mean, it can happen (copy a
structure, use a member in that structure), but it doesn't seem to be
the most likely thing.

The most likely thing would seem to be some random ioctl() do a
"get_user()" to get an index, and then using that index. That would
seem to be one of the easier ways to perhaps get that kind of kernel
spectre attack.

Adding the ASM_IFENCE to __get_user_X() in arch/x86/lib/getuser.S
would seem to go naturally together with the copy_user_64.S changes in
this patch.

Is there some reason __get_user_X() was overlooked? Those are _the_
most common user accessor functions that do the address limit
checking.

            Linus