linux-kernel - Re: [PULL] vhost: cleanups and fixes

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Message-ID: <20181102131334-mutt-send-email-mst@kernel.org>
Date:   Fri, 2 Nov 2018 13:21:06 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     mark.rutland@....com, Kees Cook <keescook@...omium.org>,
        kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        bijan.mottahedeh@...cle.com, gedwards@....com, joe@...ches.com,
        lenaic@...ard.fr, liang.z.li@...el.com, mhocko@...nel.org,
        mhocko@...e.com, stefanha@...hat.com, wei.w.wang@...el.com,
        jasowang@...hat.com
Subject: Re: [PULL] vhost: cleanups and fixes

On Fri, Nov 02, 2018 at 10:10:45AM -0700, Linus Torvalds wrote:
> On Fri, Nov 2, 2018 at 9:59 AM Michael S. Tsirkin <mst@...hat.com> wrote:
> >
> > Just for completeness I'd like to point out for vhost the copies are
> > done from the kernel thread.  So yes we can switch to copy_to/from_user
> > but for e.g. 32-bit userspace running on top of a 64 bit kernel it is
> > IIUC not sufficient - we must *also* do access_ok checks on control path
> > when addresses are passed to the kernel and when current points to the
> > correct task struct.
> 
> Don't you take over the VM with "use_mm()" when you do the copies?

Yes we do.

> So
> yes, it's a kernel thread, but it has a user VM, and though that
> should have the user limits.
> 
> No?
> 
>           Linus

Here's what I meant: we have

#define access_ok(type, addr, size)                                     \
({                                                                      \
        WARN_ON_IN_IRQ();                                               \
        likely(!__range_not_ok(addr, size, user_addr_max()));           \
})

and

#define user_addr_max() (current->thread.addr_limit.seg)

it seems that it depends on current not on the active mm.

get_user and friends are similar:

ENTRY(__get_user_1)
        mov PER_CPU_VAR(current_task), %_ASM_DX
        cmp TASK_addr_limit(%_ASM_DX),%_ASM_AX
        jae bad_get_user
        sbb %_ASM_DX, %_ASM_DX          /* array_index_mask_nospec() */
        and %_ASM_DX, %_ASM_AX
        ASM_STAC
1:      movzbl (%_ASM_AX),%edx
        xor %eax,%eax
        ASM_CLAC
        ret
ENDPROC(__get_user_1)
EXPORT_SYMBOL(__get_user_1)

-- 
MST