Re: [PATCH] stackdepot: fix mempolicy use-after-free

[Dmitry Vyukov]

On Fri, Aug 19, 2016 at 3:41 AM, Vegard Nossum <vegard.nossum@xxxxxxxxxx> wrote:
> This patch fixes the following:
>
>     BUG: KASAN: use-after-free in alloc_pages_current+0x363/0x370 at addr ffff88010b48102c
>     Read of size 2 by task trinity-c2/15425
>     CPU: 0 PID: 15425 Comm: trinity-c2 Not tainted 4.8.0-rc2+ #140
>     Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.9.3-0-ge2fc41e-prebuilt.qemu-proje
>     ct.org 04/01/2014
>      ffff88010b481040 ffff88010b557650 ffffffff81f08d11 ffff88011a40d380
>      ffff88010b481028 ffff88010b557678 ffffffff815dac7c ffff88010b557708
>      ffff88010b481028 ffff88011a40d380 ffff88010b5576f8 ffffffff815daf15
>     Call Trace:
>      [<ffffffff81f08d11>] dump_stack+0x65/0x84
>      [<ffffffff815dac7c>] kasan_object_err+0x1c/0x70
>      [<ffffffff815daf15>] kasan_report_error+0x1f5/0x4c0
>      [<ffffffff815db2fe>] __asan_report_load2_noabort+0x3e/0x40
>      [<ffffffff815cb903>] alloc_pages_current+0x363/0x370    <---- use-after-free
>      [<ffffffff81fa9954>] depot_save_stack+0x3f4/0x490
>      [<ffffffff815d9bb5>] save_stack+0xb5/0xd0
>      [<ffffffff815da211>] kasan_slab_free+0x71/0xb0
>      [<ffffffff815d6643>] kmem_cache_free+0xa3/0x290
>      [<ffffffff815c8149>] __mpol_put+0x19/0x20               <---- free
>      [<ffffffff81260635>] do_exit+0x1515/0x2b70
>      [<ffffffff81261dc4>] do_group_exit+0xf4/0x2f0
>      [<ffffffff81281c5d>] get_signal+0x53d/0x1120
>      [<ffffffff8119e993>] do_signal+0x83/0x1e20
>      [<ffffffff810027af>] exit_to_usermode_loop+0xaf/0x140
>      [<ffffffff810051e4>] syscall_return_slowpath+0x144/0x170
>      [<ffffffff83ae406f>] ret_from_fork+0x2f/0x40
>     Read of size 2 by task trinity-c2/15425
>
> The problem is that we may be calling alloc_pages() in a code path where
> current->mempolicy has already been freed.
>
> By passing __GFP_THISNODE we will always use default_mempolicy (which
> cannot be freed).
>
> Link: https://lkml.org/lkml/2016/7/29/277
> Link: https://github.com/google/kasan/issues/35
> Signed-off-by: Vegard Nossum <vegard.nossum@xxxxxxxxxx>
> ---
>  lib/stackdepot.c | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/lib/stackdepot.c b/lib/stackdepot.c
> index 60f77f1..da015cd 100644
> --- a/lib/stackdepot.c
> +++ b/lib/stackdepot.c
> @@ -243,6 +243,12 @@ depot_stack_handle_t depot_save_stack(struct stack_trace *trace,
>                 alloc_flags &= ~GFP_ZONEMASK;
>                 alloc_flags &= (GFP_ATOMIC | GFP_KERNEL);
>                 alloc_flags |= __GFP_NOWARN;
> +               /*
> +                * Avoid using current->mempolicy which may already have
> +                * been freed -- we may be in the process of saving the
> +                * stack for exactly that __mpol_put() call.
> +                */
> +               alloc_flags |= __GFP_THISNODE;
>                 page = alloc_pages(alloc_flags, STACK_ALLOC_ORDER);
>                 if (page)
>                         prealloc = page_address(page);
> --
> 1.9.1
>

Acked-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>

Thanks!