Re: [PATCH] bfs: add sanity check at bfs_fill_super().

From: Tigran Aivazian <aivazian.tigran@gmail.com>
To: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc: Dmitry Vyukov <dvyukov@google.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	syzbot <syzbot+71c6b5d68e91149fc8a4@syzkaller.appspotmail.com>,
	syzkaller-bugs <syzkaller-bugs@googlegroups.com>
Subject: Re: [PATCH] bfs: add sanity check at bfs_fill_super().
Date: Thu, 14 Jun 2018 17:15:36 +0100	[thread overview]
Message-ID: <CAK+_RL=3k4LipYPixFf1HsTvHu-ebgZQy5oTJtDwh9NGjrSxig@mail.gmail.com> (raw)
In-Reply-To: <CAK+_RLk3ntTB4aJgT9U3CctoYq581juzu=ZMNosN+pbJ5vBk0g@mail.gmail.com>

On 14 June 2018 at 16:13, Tigran Aivazian <aivazian.tigran@gmail.com> wrote:
> On 14 June 2018 at 14:28, Tetsuo Handa
> <penguin-kernel@i-love.sakura.ne.jp> wrote:
>> What is possible largest value for imap_len ?
>>
>>   info->si_lasti = (le32_to_cpu(bfs_sb->s_start) - BFS_BSIZE) / sizeof(struct bfs_inode) + BFS_ROOT_INO - 1;
>>   imap_len = (info->si_lasti / 8) + 1;
>>   info->si_imap = kzalloc(imap_len, GFP_KERNEL);
>>
>> Since sizeof(struct bfs_inode) is 64 and bfs_sb->s_start is unsigned 32bits integer
>> (where constraints is BFS_BSIZE <= bfs_sb->s_start <= bfs_sb->s_end), theoretically
>> it is possible to assign bfs_sb->s_start > 2GB (apart from whether such value makes
>> sense). Then, isn't it possible that imap_len > 4M and still hit KMALLOC_MAX_SIZE limit?
>
> You are correct, but the proper fix should be to restrict imap_len to
> whatever the maximum value allowed by BFS filesystem layout and reject
> anything beyond it. I will try to remember what it was from the notes
> I made when I wrote BFS back in 1999. Please wait (possibly a few
> days) and I will let you know what those values are.

Actually, a more accurate sanity check for the value of s_start should
be (patch against 4.16.3):

--- fs/bfs/inode.c.0 2018-06-14 16:50:52.136792126 +0100
+++ fs/bfs/inode.c 2018-06-14 16:51:49.344792119 +0100
@@ -350,7 +350,8 @@

  s->s_magic = BFS_MAGIC;

- if (le32_to_cpu(bfs_sb->s_start) > le32_to_cpu(bfs_sb->s_end)) {
+ if (le32_to_cpu(bfs_sb->s_start) > le32_to_cpu(bfs_sb->s_end) ||
+        le32_to_cpu(bfs_sb->s_start) < sizeof(struct bfs_super_block)
+ sizeof(struct bfs_dirent)) {
  printf("Superblock is corrupted\n");
  goto out1;
  }

However, that doesn't address the issue of the _upper_ limit of
s_start, i.e. it can still get (on an invalid image pretending to be
BFS) arbitrarily large and cause the allocation to fail as you
described. I will dig a bit more (in my memories :) and try to come up
with the check which doesn't reject a valid BFS image and at the same
time restricts s_start (or imap_len which ultimately depends on it)
sufficiently to prevent wild kernel memory allocation requests.

Btw, I included in the WikiPedia article "Boot File System" a
reference to the original "BFS kernel support" webpage from those
ancient days: http://ftp.linux.org.uk/pub/linux/iBCS/bfs/
