[kernel-hardening] Re: [PATCH 9/9] mm: SLUB hardened usercopy support

[Michael Ellerman]

Kees Cook <keescook at chromium.org> writes:

> On Fri, Jul 8, 2016 at 1:41 PM, Kees Cook <keescook at chromium.org> wrote:
>> So, as found already, the position in the usercopy check needs to be
>> bumped down by red_left_pad, which is what Michael's fix does, so I'll
>> include it in the next version.
>
> Actually, after some offline chats, I think this is better, since it
> makes sure the ptr doesn't end up somewhere weird before we start the
> calculations. This leaves the pointer as-is, but explicitly handles
> the redzone on the offset instead, with no wrapping, etc:
>
>         /* Find offset within object. */
>         offset = (ptr - page_address(page)) % s->size;
>
> +       /* Adjust for redzone and reject if within the redzone. */
> +       if (s->flags & SLAB_RED_ZONE) {
> +               if (offset < s->red_left_pad)
> +                       return s->name;
> +               offset -= s->red_left_pad;
> +       }
> +
>         /* Allow address range falling entirely within object size. */
>         if (offset <= s->object_size && n <= s->object_size - offset)
>                 return NULL;

That fixes the case for me in kstrndup(), which allows the system to boot.

I then get two hits, which may or may not be valid:

[    2.309556] usercopy: kernel memory overwrite attempt detected to d000000003510028 (kernfs_node_cache) (64 bytes)
[    2.309995] CPU: 7 PID: 2241 Comm: wait-for-root Not tainted 4.7.0-rc3-00099-g97872fc89d41 #64
[    2.310480] Call Trace:
[    2.310556] [c0000001f4773bf0] [c0000000009bdbe8] dump_stack+0xb0/0xf0 (unreliable)
[    2.311016] [c0000001f4773c30] [c00000000029cf44] __check_object_size+0x74/0x320
[    2.311472] [c0000001f4773cb0] [c00000000005d4d0] copy_from_user+0x60/0xd4
[    2.311873] [c0000001f4773cf0] [c0000000008b38f4] __get_filter+0x74/0x160
[    2.312230] [c0000001f4773d30] [c0000000008b408c] sk_attach_filter+0x2c/0xc0
[    2.312596] [c0000001f4773d60] [c000000000871c34] sock_setsockopt+0x954/0xc00
[    2.313021] [c0000001f4773dd0] [c00000000086ac44] SyS_setsockopt+0x134/0x150
[    2.313380] [c0000001f4773e30] [c000000000009260] system_call+0x38/0x108
[    2.317045] usercopy: kernel memory overwrite attempt detected to d000000003530028 (kernfs_node_cache) (64 bytes)
[    2.317297] CPU: 10 PID: 2242 Comm: wait-for-root Not tainted 4.7.0-rc3-00099-g97872fc89d41 #64
[    2.317475] Call Trace:
[    2.317511] [c0000001f471fbf0] [c0000000009bdbe8] dump_stack+0xb0/0xf0 (unreliable)
[    2.317689] [c0000001f471fc30] [c00000000029cf44] __check_object_size+0x74/0x320
[    2.317861] [c0000001f471fcb0] [c00000000005d4d0] copy_from_user+0x60/0xd4
[    2.318011] [c0000001f471fcf0] [c0000000008b38f4] __get_filter+0x74/0x160
[    2.318165] [c0000001f471fd30] [c0000000008b408c] sk_attach_filter+0x2c/0xc0
[    2.318313] [c0000001f471fd60] [c000000000871c34] sock_setsockopt+0x954/0xc00
[    2.318485] [c0000001f471fdd0] [c00000000086ac44] SyS_setsockopt+0x134/0x150
[    2.318632] [c0000001f471fe30] [c000000000009260] system_call+0x38/0x108


With:

# zgrep SLUB /proc/config.gz
CONFIG_SLUB_DEBUG=y
CONFIG_SLUB=y
CONFIG_SLUB_CPU_PARTIAL=y
CONFIG_SLUB_DEBUG_ON=y
# CONFIG_SLUB_STATS is not set

cheers