Re: [PATCH v2 6/7] x86: BUG() when uaccess helpers fault on kernel addresses

From: Jann Horn
Date: Mon Aug 27 2018 - 20:51:40 EST

On Mon, Aug 27, 2018 at 8:58 PM Jann Horn <jannh@xxxxxxxxxx> wrote:
> There have been multiple kernel vulnerabilities that permitted userspace to
> pass completely unchecked pointers through to userspace accessors:
> - the waitid() bug - commit 96ca579a1ecc ("waitid(): Add missing
> access_ok() checks")
> - the sg/bsg read/write APIs
> - the infiniband read/write APIs
> These don't happen all that often, but when they do happen, it is hard to
> test for them properly; and it is probably also hard to discover them with
> fuzzing. Even when an unmapped kernel address is supplied to such buggy
> code, it just returns -EFAULT instead of doing a proper BUG() or at least
> WARN().
> This patch attempts to make such misbehaving code a bit more visible by
> refusing to do a fixup in the pagefault handler code when a userspace
> accessor causes #PF on a kernel address and the current context isn't
> whitelisted.

Kees noticed that when you have vmapped stacks on (I was testing with
KASAN, so I couldn't enable vmapped stacks), exact_copy_from_user() in
copy_mount_options() is incompatible with this patch when used on
kernel pointers. do_mount_root() and devtmpfsd() call kys_mount() with
kernel pointers. So I'll have to either add that to the whitelist
and/or refactor the mount options copy logic first (so that it uses
strncpy_from_user() as long as the data argument isn't binary data).

I've wanted to fiddle with that code anyway - silent truncation at
0xfff bytes isn't very robust, especially when suid binaries call
sys_mount() with a partly user-controlled options string...