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@xxxxxxxxxx> 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
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