Re: [PATCH v4] KEYS: encrypted: fix key instantiation with user-provided data

[Nikolaus Voss]

On Thu, 13 Oct 2022, Mimi Zohar wrote:
> On Thu, 2022-10-13 at 08:39 +0200, Nikolaus Voss wrote:
> > Commit cd3bc044af48 ("KEYS: encrypted: Instantiate key with user-provided
> > decrypted data") added key instantiation with user provided decrypted data.
> > The user data is hex-ascii-encoded but was just memcpy'ed to the binary buffer.
> > Fix this to use hex2bin instead.
> >
> > Old keys created from user provided decrypted data saved with "keyctl pipe"
> > are still valid, however if the key is recreated from decrypted data the
> > old key must be converted to the correct format. This can be done with a
> > small shell script, e.g.:
> >
> > BROKENKEY=abcdefABCDEF1234567890aaaaaaaaaa
> > NEWKEY=$(echo -ne $BROKENKEY | xxd -p -c32)
> > keyctl add user masterkey "$(cat masterkey.bin)" @u
> > keyctl add encrypted testkey "new user:masterkey 32 $NEWKEY" @u
> >
> > It is encouraged to switch to a new key because the effective key size
> > of the old keys is only half of the specified size.
>
> Both the old and new decrypted data size is 32 bytes.  Is the above
> statement necessary, especially since the Documentation example does
> the equivalent?

The old key has the same byte size but all bytes must be within the 
hex-ascíi range of characters, otherwise it is refused by the kernel.
So if you wanted a 32 bytes key you get 16 effective bytes for the key. 
In the above example the string size of the $BROKENKEY is 32, while 
the string size of the $NEWKEY is 64.

If you do

$ echo $NEWKEY
6162636465664142434445463132333435363738393061616161616161616161

for the example, the range problem is obvious, so $NEWKEY is still broken. 
That's why it should only be used to recover data which should be 
reencypted with a new key. If you count exactly, the effective key size is 
_slightly_ longer than half of the specified size, but it is still a 
severe security problem.

> > The corresponding test for the Linux Test Project ltp has also been
> > fixed (see link below).
>
> The LTP patch still needs to be revised, but the "Link" is a reference
> to the discussion.  Is the above statement necessary?

As long as the patch is not accepted the discussion is helpful. But feel 
free to delete it upon integration ;-).

> > Fixes: cd3bc044af48 ("KEYS: encrypted: Instantiate key with user-provided decrypted data")
> > Cc: stable <stable@xxxxxxxxxx>
> > Link: https://lore.kernel.org/ltp/20221006081709.92303897@xxxxxxxxxxxxxxxxxxx/
> > Signed-off-by: Nikolaus Voss <nikolaus.voss@xxxxxxxxxxxxxxx>
>
> Otherwise,
>
> Reviewed-by: Mimi Zohar <zohar@xxxxxxxxxxxxx>

Thanks Mimi!