pefile: Strip the wrapper off of the cert data block

The certificate data block in a PE binary has a wrapper around the PKCS#7
signature we actually want to get at.  Strip this off and check that we've got
something that appears to be a PKCS#7 signature.

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Vivek Goyal <vgoyal@redhat.com>
Reviewed-by: Kees Cook <keescook@chromium.org>

1 files changed, 71 insertions, 0 deletions

diff --git a/crypto/asymmetric_keys/verify_pefile.c b/crypto/asymmetric_keys/verify_pefile.c
index aec7c509404e..2f5268cb843d 100644
--- a/crypto/asymmetric_keys/verify_pefile.c
+++ b/crypto/asymmetric_keys/verify_pefile.c
@@ -15,6 +15,7 @@
 #include <linux/slab.h>
 #include <linux/err.h>
 #include <linux/pe.h>
+#include <linux/asn1.h>
 #include <crypto/pkcs7.h>
 #include <crypto/hash.h>
 #include "verify_pefile.h"
@@ -118,6 +119,72 @@ static int pefile_parse_binary(const void *pebuf, unsigned int pelen,
         return 0;
 }
 
+/*
+ * Check and strip the PE wrapper from around the signature and check that the
+ * remnant looks something like PKCS#7.
+ */
+static int pefile_strip_sig_wrapper(const void *pebuf,
+                                    struct pefile_context *ctx)
+{
+        struct win_certificate wrapper;
+        const u8 *pkcs7;
+
+        if (ctx->sig_len < sizeof(wrapper)) {
+                pr_debug("Signature wrapper too short\n");
+                return -ELIBBAD;
+        }
+
+        memcpy(&wrapper, pebuf + ctx->sig_offset, sizeof(wrapper));
+        pr_debug("sig wrapper = { %x, %x, %x }\n",
+                 wrapper.length, wrapper.revision, wrapper.cert_type);
+
+        /* Both pesign and sbsign round up the length of certificate table
+         * (in optional header data directories) to 8 byte alignment.
+         */
+        if (round_up(wrapper.length, 8) != ctx->sig_len) {
+                pr_debug("Signature wrapper len wrong\n");
+                return -ELIBBAD;
+        }
+        if (wrapper.revision != WIN_CERT_REVISION_2_0) {
+                pr_debug("Signature is not revision 2.0\n");
+                return -ENOTSUPP;
+        }
+        if (wrapper.cert_type != WIN_CERT_TYPE_PKCS_SIGNED_DATA) {
+                pr_debug("Signature certificate type is not PKCS\n");
+                return -ENOTSUPP;
+        }
+
+        /* Looks like actual pkcs signature length is in wrapper->length.
+         * size obtained from data dir entries lists the total size of
+         * certificate table which is also aligned to octawrod boundary.
+         *
+         * So set signature length field appropriately.
+         */
+        ctx->sig_len = wrapper.length;
+        ctx->sig_offset += sizeof(wrapper);
+        ctx->sig_len -= sizeof(wrapper);
+        if (ctx->sig_len == 0) {
+                pr_debug("Signature data missing\n");
+                return -EKEYREJECTED;
+        }
+
+        /* What's left should a PKCS#7 cert */
+        pkcs7 = pebuf + ctx->sig_offset;
+        if (pkcs7[0] == (ASN1_CONS_BIT | ASN1_SEQ)) {
+                if (pkcs7[1] == 0x82 &&
+                    pkcs7[2] == (((ctx->sig_len - 4) >> 8) & 0xff) &&
+                    pkcs7[3] ==  ((ctx->sig_len - 4)       & 0xff))
+                        return 0;
+                if (pkcs7[1] == 0x80)
+                        return 0;
+                if (pkcs7[1] > 0x82)
+                        return -EMSGSIZE;
+        }
+
+        pr_debug("Signature data not PKCS#7\n");
+        return -ELIBBAD;
+}
+
 /**
  * verify_pefile_signature - Verify the signature on a PE binary image
  * @pebuf: Buffer containing the PE binary image
@@ -159,5 +226,9 @@ int verify_pefile_signature(const void *pebuf, unsigned pelen,
         if (ret < 0)
                 return ret;
 
+        ret = pefile_strip_sig_wrapper(pebuf, &ctx);
+        if (ret < 0)
+                return ret;
+
         return -ENOANO; // Not yet complete
 }