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




On Thu, Jan 11, 2018 at 5:11 PM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> 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.

Oversight, I was focused on the uaccess_begin conversions. Yes, let me
go add ASM_IFENCE after the ASM_STAC in those paths.