Re: [PATCH] bpf: Fix integer overflow in prealloc_elems_and_freelist()

[Tatushiko Yasumatsu]

On Tue, Sep 28, 2021 at 02:29:43PM +0200, Daniel Borkmann wrote:
> On 9/25/21 7:31 AM, Tatsuhiko Yasumatsu wrote:
> > In prealloc_elems_and_freelist(), the multiplication to calculate the
> > size passed to bpf_map_area_alloc() could lead to an integer overflow.
> > As a result, out-of-bounds write could occur in pcpu_freelist_populate()
> > as reported by KASAN:
> >
> > [...]
> > [   16.968613] BUG: KASAN: slab-out-of-bounds in pcpu_freelist_populate+0xd9/0x100
> > [   16.969408] Write of size 8 at addr ffff888104fc6ea0 by task crash/78
> > [   16.970038]
> > [   16.970195] CPU: 0 PID: 78 Comm: crash Not tainted 5.15.0-rc2+ #1
> > [   16.970878] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1ubuntu1.1 04/01/2014
> > [   16.972026] Call Trace:
> > [   16.972306]  dump_stack_lvl+0x34/0x44
> > [   16.972687]  print_address_description.constprop.0+0x21/0x140
> > [   16.973297]  ? pcpu_freelist_populate+0xd9/0x100
> > [   16.973777]  ? pcpu_freelist_populate+0xd9/0x100
> > [   16.974257]  kasan_report.cold+0x7f/0x11b
> > [   16.974681]  ? pcpu_freelist_populate+0xd9/0x100
> > [   16.975190]  pcpu_freelist_populate+0xd9/0x100
> > [   16.975669]  stack_map_alloc+0x209/0x2a0
> > [   16.976106]  __sys_bpf+0xd83/0x2ce0
> > [...]
> >
> > The possibility of this overflow was originally discussed in [0], but
> > was overlooked.
> >
> > Fix the integer overflow by casting one operand to u64.
> >
> > [0] https://lore.kernel.org/bpf/728b238e-a481-eb50-98e9-b0f430ab01e7@xxxxxxxxx/
> >
> > Fixes: 557c0c6e7df8 ("bpf: convert stackmap to pre-allocation")
> > Signed-off-by: Tatsuhiko Yasumatsu <th.yasumatsu@xxxxxxxxx>
> > ---
> >   kernel/bpf/stackmap.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
> > index 09a3fd97d329..8941dc83a769 100644
> > --- a/kernel/bpf/stackmap.c
> > +++ b/kernel/bpf/stackmap.c
> > @@ -66,7 +66,7 @@ static int prealloc_elems_and_freelist(struct bpf_stack_map *smap)
> >     u32 elem_size = sizeof(struct stack_map_bucket) + smap->map.value_size;
>
> Thanks a lot for the fix, Tatsuhiko! Could we just change the above elem_size to u64 instead?

Thank you for your review, Daniel!
Yes, I think it's possible to just change elem_size to u64.

We just have to be careful to cast one operand (smap->map.value_size)
to u64, so that the integer overflow won't happen in 32-bit
architectures.
This is necessary because in 32-bit architectures, the result of
sizeof() is a 32-bit integer.

I will update the patch.

>
> >     int err;
> > -   smap->elems = bpf_map_area_alloc(elem_size * smap->map.max_entries,
> > +   smap->elems = bpf_map_area_alloc((u64)elem_size * smap->map.max_entries,
> >                                      smap->map.numa_node);
> >     if (!smap->elems)
> >             return -ENOMEM;
> >
>
> Best,
> Daniel

Best regards,
Tatsuhiko Yasumatsu