Fix undefined behaviour in e_aes_cbc_hmac_sha256.c and e_aes_cbc_hmac…

…_sha1.c

In TLS mode of operation the padding value "pad" is obtained along with the
maximum possible padding value "maxpad". If pad > maxpad then the data is
invalid. However we must continue anyway because this is constant time code.

We calculate the payload length like this:

    inp_len = len - (SHA_DIGEST_LENGTH + pad + 1);

However if pad is invalid then inp_len ends up -ve (actually large +ve
because it is a size_t).

Later we do this:

    /* verify HMAC */
    out += inp_len;
    len -= inp_len;

This ends up with "out" pointing before the buffer which is undefined
behaviour. Next we calculate "p" like this:

    unsigned char *p =
        out + len - 1 - maxpad - SHA256_DIGEST_LENGTH;

Because of the "out + len" term the -ve inp_len value is cancelled out
so "p" points to valid memory (although technically the pointer arithmetic
is undefined behaviour again).

We only ever then dereference "p" and never "out" directly so there is
never an invalid read based on the bad pointer - so there is no security
issue.

This commit fixes the undefined behaviour by ensuring we use maxpad in
place of pad, if the supplied pad is invalid.

With thanks to Brian Carpenter for reporting this issue.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from #3832)

(cherry picked from commit 335d0a4)

crypto/evp/e_aes_cbc_hmac_sha1.c

@@ -528,7 +528,15 @@ static int aesni_cbc_hmac_sha1_cipher(EVP_CIPHER_CTX *ctx, unsigned char *out,
             maxpad |= (255 - maxpad) >> (sizeof(maxpad) * 8 - 8);
             maxpad &= 255;
 
-            ret &= constant_time_ge(maxpad, pad);
+            mask = constant_time_ge(maxpad, pad);
+            ret &= mask;
+            /*
+             * If pad is invalid then we will fail the above test but we must
+             * continue anyway because we are in constant time code. However,
+             * we'll use the maxpad value instead of the supplied pad to make
+             * sure we perform well defined pointer arithmetic.
+             */
+            pad = constant_time_select(mask, pad, maxpad);
 
             inp_len = len - (SHA_DIGEST_LENGTH + pad + 1);
             mask = (0 - ((inp_len - len) >> (sizeof(inp_len) * 8 - 1)));

crypto/evp/e_aes_cbc_hmac_sha256.c

@@ -538,7 +538,15 @@ static int aesni_cbc_hmac_sha256_cipher(EVP_CIPHER_CTX *ctx,
             maxpad |= (255 - maxpad) >> (sizeof(maxpad) * 8 - 8);
             maxpad &= 255;
 
-            ret &= constant_time_ge(maxpad, pad);
+            mask = constant_time_ge(maxpad, pad);
+            ret &= mask;
+            /*
+             * If pad is invalid then we will fail the above test but we must
+             * continue anyway because we are in constant time code. However,
+             * we'll use the maxpad value instead of the supplied pad to make
+             * sure we perform well defined pointer arithmetic.
+             */
+            pad = constant_time_select(mask, pad, maxpad);
 
             inp_len = len - (SHA256_DIGEST_LENGTH + pad + 1);
             mask = (0 - ((inp_len - len) >> (sizeof(inp_len) * 8 - 1)));