ima: Add __counted_by for struct modsig and use struct_size()

Prepare for the coming implementation by GCC and Clang of the __counted_by
attribute. Flexible array members annotated with __counted_by can have
their accesses bounds-checked at run-time via CONFIG_UBSAN_BOUNDS (for
array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
functions).

Also, relocate `hdr->raw_pkcs7_len = sig_len;` so that the __counted_by
annotation has effect, and flex-array member `raw_pkcs7` can be properly
bounds-checked at run-time.

While there, use struct_size() helper, instead of the open-coded
version, to calculate the size for the allocation of the whole
flexible structure, including of course, the flexible-array member.

This code was found with the help of Coccinelle, and audited and
fixed manually.

Signed-off-by: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Reviewed-by: Mimi Zohar <zohar@linux.ibm.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/ZSRaDcJNARUUWUwS@work
Signed-off-by: Kees Cook <keescook@chromium.org>

Author: GustavoARSilva

security/integrity/ima/ima_modsig.c

@@ -29,7 +29,7 @@ struct modsig {
 	 * storing the signature.
 	 */
 	int raw_pkcs7_len;
-	u8 raw_pkcs7[];
+	u8 raw_pkcs7[] __counted_by(raw_pkcs7_len);
 };
 
 /*
@@ -65,10 +65,11 @@ int ima_read_modsig(enum ima_hooks func, const void *buf, loff_t buf_len,
 	buf_len -= sig_len + sizeof(*sig);
 
 	/* Allocate sig_len additional bytes to hold the raw PKCS#7 data. */
-	hdr = kzalloc(sizeof(*hdr) + sig_len, GFP_KERNEL);
+	hdr = kzalloc(struct_size(hdr, raw_pkcs7, sig_len), GFP_KERNEL);
 	if (!hdr)
 		return -ENOMEM;
 
+	hdr->raw_pkcs7_len = sig_len;
 	hdr->pkcs7_msg = pkcs7_parse_message(buf + buf_len, sig_len);
 	if (IS_ERR(hdr->pkcs7_msg)) {
 		rc = PTR_ERR(hdr->pkcs7_msg);
@@ -77,7 +78,6 @@ int ima_read_modsig(enum ima_hooks func, const void *buf, loff_t buf_len,
 	}
 
 	memcpy(hdr->raw_pkcs7, buf + buf_len, sig_len);
-	hdr->raw_pkcs7_len = sig_len;
 
 	/* We don't know the hash algorithm yet. */
 	hdr->hash_algo = HASH_ALGO__LAST;