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

[Jarkko Sakkinen]

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@xxxxxxxxxxxxxxxxxxx>
> >>
> >> 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@xxxxxxxxxxxxx>
> >> Signed-off-by: Mickaël Salaün <mic@xxxxxxxxxxxxxxxxxxx>
> >> Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
> >> Reviewed-by: Ben Boeckel <mathstuf@xxxxxxxxx>
> >> ---
> >>
> >> Changes since v2:
> >> * Cherry-pick v1 patch from
> >>   https://lore.kernel.org/lkml/2659836.1607940186@xxxxxxxxxxxxxxxxxxxxxx/
> >>   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@xxxxxxxxxxx/
> 
> 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
> > 
>