Subject: Re: [PULL] vhost: cleanups and fixes

From: "Michael S. Tsirkin" <[email-masked]>
Date: Fri, 2 Nov 2018 13:21:06 -0400

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 <[email-masked]> 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