Re: Cramfs: "unable to handle kernel paging request" when reading a file from a fuzzed FS image

Re: Cramfs: "unable to handle kernel paging request" when reading a file from a fuzzed FS image

[Nicolas Pitre]

On Sun, 28 Oct 2018, Anatoly Trosinenko wrote:

> Hello,
> 
> When reading a file from a fuzzed cramfs image, unhandled kernel
> paging request occurs.

Hmmm... It doesn't show up on my test system.

> How to reproduce with kvm-xfstests:
> 1) Checkout the v4.19 tag, copy x86_64-config-4.14 to .config, perform
> `make olddefconfig`
> 2) Enable Cramfs in the config, then compile
> 3) In the `kvm-xfstests shell` perform:
> 
> root@kvm-xfstests:~# mount /vtmp
> root@kvm-xfstests:~# mount /vtmp/cramfs.img /mnt

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.


Nicolas

Re: Cramfs: "unable to handle kernel paging request" when reading a file from a fuzzed FS image

[Anatoly Trosinenko]

> 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").

> 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...

# uname -a
Linux trosinenko-pc 4.18.0-10-generic #11-Ubuntu SMP Thu Oct 11
15:13:55 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux

Best regards
Anatoly
пн, 29 окт. 2018 г. в 6:43, Nicolas Pitre <nicolas.pitre@linaro.org>:
>
> On Sun, 28 Oct 2018, Anatoly Trosinenko wrote:
>
> > Hello,
> >
> > When reading a file from a fuzzed cramfs image, unhandled kernel
> > paging request occurs.
>
> Hmmm... It doesn't show up on my test system.
>
> > How to reproduce with kvm-xfstests:
> > 1) Checkout the v4.19 tag, copy x86_64-config-4.14 to .config, perform
> > `make olddefconfig`
> > 2) Enable Cramfs in the config, then compile
> > 3) In the `kvm-xfstests shell` perform:
> >
> > root@kvm-xfstests:~# mount /vtmp
> > root@kvm-xfstests:~# mount /vtmp/cramfs.img /mnt
>
> 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.
>
>
> Nicolas

Re: Cramfs: "unable to handle kernel paging request" when reading a file from a fuzzed FS image

[Nicolas Pitre]

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 *sb, unsigned int offset,
 			continue;
 		blk_offset = (blocknr - buffer_blocknr[i]) << PAGE_SHIFT;
 		blk_offset += 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

Re: Cramfs: "unable to handle kernel paging request" when reading a file from a fuzzed FS image

[Anatoly Trosinenko]

Tested in fresh torvalds/master branch. Thank you!

Best regards
Anatoly
пн, 29 окт. 2018 г. в 19:03, Nicolas Pitre <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 *sb, unsigned int offset,
>                         continue;
>                 blk_offset = (blocknr - buffer_blocknr[i]) << PAGE_SHIFT;
>                 blk_offset += 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