Re: [PATCH v3 02/10] certs: Fix blacklisted hexadecimal hash string check

Jarkko Sakkinen
2021-01-21T06:39:48Z

On Wed, Jan 20, 2021 at 12:12:50PM +0100, Mickaël Salaün wrote:
>
> On 20/01/2021 04:43, Jarkko Sakkinen wrote:
> > On Thu, Jan 14, 2021 at 04:19:01PM +0100, Mickaël Salaün wrote:
> >> From: Mickaël Salaün <mic@linux.microsoft.com>
> >>
> >> When looking for a blacklisted hash, bin2hex() is used to transform a
> >> binary hash to an ascii (lowercase) hexadecimal string. This string is
> >> then search for in the description of the keys from the blacklist
> >> keyring. When adding a key to the blacklist keyring,
> >> blacklist_vet_description() checks the hash prefix and the hexadecimal
> >> string, but not that this string is lowercase. It is then valid to set
> >> hashes with uppercase hexadecimal, which will be silently ignored by the
> >> kernel.
> >>
> >> Add an additional check to blacklist_vet_description() to check that
> >> hexadecimal strings are in lowercase.
> >>
> >> Cc: David Woodhouse <dwmw2@infradead.org>
> >> Signed-off-by: Mickaël Salaün <mic@linux.microsoft.com>
> >> Signed-off-by: David Howells <dhowells@redhat.com>
> >> Reviewed-by: Ben Boeckel <mathstuf@gmail.com>
> >> ---
> >>
> >> Changes since v2:
> >> * Cherry-pick v1 patch from
> >> https://lore.kernel.org/lkml/2659836.1607940186@warthog.procyon.org.uk/
> >> to rebase on v5.11-rc3.
> >> * Rearrange Cc order.
> >> ---
> >> certs/blacklist.c | 2 +-
> >> 1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/certs/blacklist.c b/certs/blacklist.c
> >> index 2719fb2fbc1c..a888b934a1cd 100644
> >> --- a/certs/blacklist.c
> >> +++ b/certs/blacklist.c
> >> @@ -37,7 +37,7 @@ static int blacklist_vet_description(const char *desc)
> >> found_colon:
> >> desc++;
> >> for (; *desc; desc++) {
> >> - if (!isxdigit(*desc))
> >> + if (!isxdigit(*desc) || isupper(*desc))
> >> return -EINVAL;
> >> n++;
> >> }
> >> --
> >> 2.30.0
> >>
> >
> > Shouldn't this rather convert the upper case to lower case? I don't like
> > the ABI break that this causes.
>
> It doesn't break the ABI because keys loaded in the blacklist keyring
> can only happen with builtin hashes. Moreover these builtin hashes will
> be checked by patch 10/10 at build time.

Right the patches are just out of order then.

/Jarkko

>
> This patch is also important to remove a false sense of security and
> warns about mis-blacklisted certificates or binaries:
> https://lore.kernel.org/lkml/c9664a67-61b7-6b4a-86d7-5aca9ff06fa5@digikod.net/
>
> Hot-patching keys doesn't seem a good idea, especially when these keys
> are signed. Moreover, it would bring additional complexity and will
> require to change the core of the key management.
>
> >
> > /Jarkko
> >
>