Re: [PATCH v3 3/7] mm/vmalloc.c: allow vread() to read out vm_map_ram areas

Baoquan He
2023-01-18T02:18:21Z

On 01/16/23 at 04:08pm, Dan Carpenter wrote:
> On Sun, Jan 15, 2023 at 10:08:55PM +0800, Baoquan He wrote:
> > > f181234a5a21fd0 Chen Wandun 2021-09-02 3650 if ((unsigned long)addr + count <= va->va_start)
> > > f181234a5a21fd0 Chen Wandun 2021-09-02 3651 goto finished;
> > > f181234a5a21fd0 Chen Wandun 2021-09-02 3652
> > > f608788cd2d6cae Serapheim Dimitropoulos 2021-04-29 3653 list_for_each_entry_from(va, &vmap_area_list, list) {
> > > e81ce85f960c2e2 Joonsoo Kim 2013-04-29 3654 if (!count)
> > > e81ce85f960c2e2 Joonsoo Kim 2013-04-29 3655 break;
> > > e81ce85f960c2e2 Joonsoo Kim 2013-04-29 3656
> > > 129dbdf298d7383 Baoquan He 2023-01-13 3657 vm = va->vm;
> > > 129dbdf298d7383 Baoquan He 2023-01-13 3658 flags = va->flags & VMAP_FLAGS_MASK;
> > > 129dbdf298d7383 Baoquan He 2023-01-13 3659
> > > 129dbdf298d7383 Baoquan He 2023-01-13 3660 if (!vm && !flags)
> > > ^^^
> > > vm can be NULL if a flag in VMAP_FLAGS_MASK is set.
> > >
> > > e81ce85f960c2e2 Joonsoo Kim 2013-04-29 3661 continue;
> >
> > Right, after the 'continue;' line, only two cases could happen when it
> > comes here. (vm != null) or (vm->flags & VMAP_RAM) is true.
> >
>
> You're saying VMAP_RAM, but strictly speaking the code is checking
> VMAP_FLAGS_MASK and not VMAP_RAM.
>
> +#define VMAP_RAM 0x1 /* indicates vm_map_ram area*/
> +#define VMAP_BLOCK 0x2 /* mark out the vmap_block sub-type*/
> +#define VMAP_FLAGS_MASK 0x3
>
> If we assume that vm is NULL, VMAP_BLOCK is set and VMAP_RAM is clear
> then it would lead to a NULL dereference. There might be reasons why
> that combination is impossible outside the function but we can't tell
> from the information we have here.

VMAP_BLOCK has no chance to be set alone. It has to be set together with
VMAP_RAM if needed.

>
> Which is fine, outside information is a common reason for false
> positives with this check. But I was just concerned about the mix of
> VMAP_FLAGS_MASK and VMAP_RAM.

Thanks, I see your point now, will consider how to improve it.