KEYS: encrypted: avoid encrypting/decrypting stack buffers

Since v4.9, the crypto API cannot (normally) be used to encrypt/decrypt
stack buffers because the stack may be virtually mapped.  Fix this for
the padding buffers in encrypted-keys by using ZERO_PAGE for the
encryption padding and by allocating a temporary heap buffer for the
decryption padding.

Tested with CONFIG_DEBUG_SG=y:
	keyctl new_session
	keyctl add user master "abcdefghijklmnop" @s
	keyid=$(keyctl add encrypted desc "new user:master 25" @s)
	datablob="$(keyctl pipe $keyid)"
	keyctl unlink $keyid
	keyid=$(keyctl add encrypted desc "load $datablob" @s)
	datablob2="$(keyctl pipe $keyid)"
	[ "$datablob" = "$datablob2" ] && echo "Success!"

Cc: Andy Lutomirski <luto@kernel.org>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Mimi Zohar <zohar@linux.vnet.ibm.com>
Cc: stable@vger.kernel.org # 4.9+
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: James Morris <james.l.morris@oracle.com>

1 files changed, 9 insertions, 8 deletions

diff --git a/security/keys/encrypted-keys/encrypted.c b/security/keys/encrypted-keys/encrypted.c
index 2ab48eab29a1..d14f1a47a130 100644
--- a/security/keys/encrypted-keys/encrypted.c
+++ b/security/keys/encrypted-keys/encrypted.c
@@ -479,12 +479,9 @@ static int derived_key_encrypt(struct encrypted_key_payload *epayload,
         struct skcipher_request *req;
         unsigned int encrypted_datalen;
         u8 iv[AES_BLOCK_SIZE];
-        unsigned int padlen;
-        char pad[16];
         int ret;
 
         encrypted_datalen = roundup(epayload->decrypted_datalen, blksize);
-        padlen = encrypted_datalen - epayload->decrypted_datalen;
 
         req = init_skcipher_req(derived_key, derived_keylen);
         ret = PTR_ERR(req);
@@ -492,11 +489,10 @@ static int derived_key_encrypt(struct encrypted_key_payload *epayload,
                 goto out;
         dump_decrypted_data(epayload);
 
-        memset(pad, 0, sizeof pad);
         sg_init_table(sg_in, 2);
         sg_set_buf(&sg_in[0], epayload->decrypted_data,
                    epayload->decrypted_datalen);
-        sg_set_buf(&sg_in[1], pad, padlen);
+        sg_set_page(&sg_in[1], ZERO_PAGE(0), AES_BLOCK_SIZE, 0);
 
         sg_init_table(sg_out, 1);
         sg_set_buf(sg_out, epayload->encrypted_data, encrypted_datalen);
@@ -583,9 +579,14 @@ static int derived_key_decrypt(struct encrypted_key_payload *epayload,
         struct skcipher_request *req;
         unsigned int encrypted_datalen;
         u8 iv[AES_BLOCK_SIZE];
-        char pad[16];
+        u8 *pad;
         int ret;
 
+        /* Throwaway buffer to hold the unused zero padding at the end */
+        pad = kmalloc(AES_BLOCK_SIZE, GFP_KERNEL);
+        if (!pad)
+                return -ENOMEM;
+
         encrypted_datalen = roundup(epayload->decrypted_datalen, blksize);
         req = init_skcipher_req(derived_key, derived_keylen);
         ret = PTR_ERR(req);
@@ -593,13 +594,12 @@ static int derived_key_decrypt(struct encrypted_key_payload *epayload,
                 goto out;
         dump_encrypted_data(epayload, encrypted_datalen);
 
-        memset(pad, 0, sizeof pad);
         sg_init_table(sg_in, 1);
         sg_init_table(sg_out, 2);
         sg_set_buf(sg_in, epayload->encrypted_data, encrypted_datalen);
         sg_set_buf(&sg_out[0], epayload->decrypted_data,
                    epayload->decrypted_datalen);
-        sg_set_buf(&sg_out[1], pad, sizeof pad);
+        sg_set_buf(&sg_out[1], pad, AES_BLOCK_SIZE);
 
         memcpy(iv, epayload->iv, sizeof(iv));
         skcipher_request_set_crypt(req, sg_in, sg_out, encrypted_datalen, iv);
@@ -611,6 +611,7 @@ static int derived_key_decrypt(struct encrypted_key_payload *epayload,
                 goto out;
         dump_decrypted_data(epayload);
 out:
+        kfree(pad);
         return ret;
 }