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From:   Anatoly Trosinenko <anatoly.trosinenko@gmail.com>
Date:   Wed, 31 Oct 2018 16:06:53 +0300
Message-ID: <CAE5jQCc+LjJOSxAvTLaf1Fh_pRKgTgeS0=kZKgBjBPg1n2B7_w@mail.gmail.com>
Subject: Re: Cramfs: "unable to handle kernel paging request" when reading a
 file from a fuzzed FS image
To:     Nicolas Pitre <nicolas.pitre@linaro.org>
Cc:     linux-kernel@vger.kernel.org
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Tested in fresh torvalds/master branch. Thank you!

Best regards
Anatoly
=D0=BF=D0=BD, 29 =D0=BE=D0=BA=D1=82. 2018 =D0=B3. =D0=B2 19:03, Nicolas Pit=
re <nicolas.pitre@linaro.org>:
>
> On Mon, 29 Oct 2018, Anatoly Trosinenko wrote:
>
> > > How do I populate /vtmp? Mine is empty at this point. I imagine I
> > > should put the cramfs image somewhere on the host, but I'm not that
> > > familiar withkvm.
> >
> > Oops, forgot to say, it is the /tmp/kvm-xfstests-$USER directory on
> > the host (it will be created when you first launch kvm-xfstests and it
> > is "live", i.e. like NFS, not like "pack to ext4 image then boot and
> > mount").
>
> OK, I reproduced it. The fix is as follows:
>
> diff --git a/fs/cramfs/inode.c b/fs/cramfs/inode.c
> index f408994fc6..6e000392e4 100644
> --- a/fs/cramfs/inode.c
> +++ b/fs/cramfs/inode.c
> @@ -202,7 +202,8 @@ static void *cramfs_blkdev_read(struct super_block *s=
b, unsigned int offset,
>                         continue;
>                 blk_offset =3D (blocknr - buffer_blocknr[i]) << PAGE_SHIF=
T;
>                 blk_offset +=3D offset;
> -               if (blk_offset + len > BUFFER_SIZE)
> +               if (blk_offset > BUFFER_SIZE ||
> +                   blk_offset + len > BUFFER_SIZE)
>                         continue;
>                 return read_buffers[i] + blk_offset;
>         }
>
> User space will get a bunch of zeroes rather than an explicit error in
> this case. There is just so many ways to corrupt a cramfs image without
> detecting it afterwards that I don't think it is worth doing more than
> making sure the system won't be compromized.
>
> > > Hmmm... It doesn't show up on my test system.
> >
> > Mounted it on my host Ubuntu 18.10 amd64, executed `cat /mnt/xyz` and
> > it was "Killed". Maybe it is something freshly added or
> > arch-dependent...
>
> It actually depends on whether there is something mapped immediately
> next to the cramfs cache buffer.
>
> In any case, this is a nice catch. Thank you for reporting it.
>
>
> Nicolas