• David Howells's avatar
    PKCS#7: Appropriately restrict authenticated attributes and content type · 99db4435
    David Howells authored
    A PKCS#7 or CMS message can have per-signature authenticated attributes
    that are digested as a lump and signed by the authorising key for that
    signature.  If such attributes exist, the content digest isn't itself
    signed, but rather it is included in a special authattr which then
    contributes to the signature.
    
    Further, we already require the master message content type to be
    pkcs7_signedData - but there's also a separate content type for the data
    itself within the SignedData object and this must be repeated inside the
    authattrs for each signer [RFC2315 9.2, RFC5652 11.1].
    
    We should really validate the authattrs if they exist or forbid them
    entirely as appropriate.  To this end:
    
     (1) Alter the PKCS#7 parser to reject any message that has more than one
         signature where at least one signature has authattrs and at least one
         that does not.
    
     (2) Validate authattrs if they are present and strongly restrict them.
         Only the following authattrs are permitted and all others are
         rejected:
    
         (a) contentType.  This is checked to be an OID that matches the
         	 content type in the SignedData object.
    
         (b) messageDigest.  This must match the crypto digest of the data.
    
         (c) signingTime.  If present, we check that this is a valid, parseable
         	 UTCTime or GeneralTime and that the date it encodes fits within
         	 the validity window of the matching X.509 cert.
    
         (d) S/MIME capabilities.  We don't check the contents.
    
         (e) Authenticode SP Opus Info.  We don't check the contents.
    
         (f) Authenticode Statement Type.  We don't check the contents.
    
         The message is rejected if (a) or (b) are missing.  If the message is
         an Authenticode type, the message is rejected if (e) is missing; if
         not Authenticode, the message is rejected if (d) - (f) are present.
    
         The S/MIME capabilities authattr (d) unfortunately has to be allowed
         to support kernels already signed by the pesign program.  This only
         affects kexec.  sign-file suppresses them (CMS_NOSMIMECAP).
    
         The message is also rejected if an authattr is given more than once or
         if it contains more than one element in its set of values.
    
     (3) Add a parameter to pkcs7_verify() to select one of the following
         restrictions and pass in the appropriate option from the callers:
    
         (*) VERIFYING_MODULE_SIGNATURE
    
    	 This requires that the SignedData content type be pkcs7-data and
    	 forbids authattrs.  sign-file sets CMS_NOATTR.  We could be more
    	 flexible and permit authattrs optionally, but only permit minimal
    	 content.
    
         (*) VERIFYING_FIRMWARE_SIGNATURE
    
    	 This requires that the SignedData content type be pkcs7-data and
    	 requires authattrs.  In future, this will require an attribute
    	 holding the target firmware name in addition to the minimal set.
    
         (*) VERIFYING_UNSPECIFIED_SIGNATURE
    
    	 This requires that the SignedData content type be pkcs7-data but
    	 allows either no authattrs or only permits the minimal set.
    
         (*) VERIFYING_KEXEC_PE_SIGNATURE
    
    	 This only supports the Authenticode SPC_INDIRECT_DATA content type
    	 and requires at least an SpcSpOpusInfo authattr in addition to the
    	 minimal set.  It also permits an SPC_STATEMENT_TYPE authattr (and
    	 an S/MIME capabilities authattr because the pesign program doesn't
    	 remove these).
    
         (*) VERIFYING_KEY_SIGNATURE
         (*) VERIFYING_KEY_SELF_SIGNATURE
    
    	 These are invalid in this context but are included for later use
    	 when limiting the use of X.509 certs.
    
     (4) The pkcs7_test key type is given a module parameter to select between
         the above options for testing purposes.  For example:
    
    	echo 1 >/sys/module/pkcs7_test_key/parameters/usage
    	keyctl padd pkcs7_test foo @s </tmp/stuff.pkcs7
    
         will attempt to check the signature on stuff.pkcs7 as if it contains a
         firmware blob (1 being VERIFYING_FIRMWARE_SIGNATURE).
    Suggested-by: 's avatarAndy Lutomirski <luto@kernel.org>
    Signed-off-by: 's avatarDavid Howells <dhowells@redhat.com>
    Reviewed-by: 's avatarMarcel Holtmann <marcel@holtmann.org>
    Reviewed-by: 's avatarDavid Woodhouse <David.Woodhouse@intel.com>
    99db4435