Re: [PATCH v2 08/19] x86: use __uaccess_begin_nospec and ASM_IFENCE in get_user paths

[Linus Torvalds]

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