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Date: Mon, 22 Oct 2012 11:36:47 +1030
From: Rusty Russell <rusty@...tcorp.com.au>
To: David Howells <dhowells@...hat.com>, torvalds@...ux-foundation.org
Cc: dhowells@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] MODSIGN: Move the magic string to the end of a module and eliminate the search
David Howells <dhowells@...hat.com> writes:
> Emit the magic string that indicates a module has a signature after the
> signature data instead of before it. This allows module_sig_check() to be
> made simpler and faster by the elimination of the search for the magic string.
> Instead we just need to do a single memcmp().
>
> This works because at the end of the signature data there is the fixed-length
> signature information block. This block then falls immediately prior to the
> magic number.
>
>>>From the contents of the information block, it is trivial to calculate the size
> of the signature data and thus the size of the actual module data.
Meh, I really wanted to separate the module signature locating (my
problem) from the decoding and checking (your problem).
If we're going to require such a header, we can simplify further (untested!).
BTW, I'm not convinced your use of bitfields here is portable; it may be
portable enough for Linux though.
Cheers,
Rusty.
diff --git a/include/linux/module.h b/include/linux/module.h
index 7760c6d..bfd1b2d 100644
--- a/include/linux/module.h
+++ b/include/linux/module.h
@@ -21,9 +21,6 @@
#include <linux/percpu.h>
#include <asm/module.h>
-/* In stripped ARM and x86-64 modules, ~ is surprisingly rare. */
-#define MODULE_SIG_STRING "~Module signature appended~\n"
-
/* Not Yet Implemented */
#define MODULE_SUPPORTED_DEVICE(name)
@@ -656,4 +653,29 @@ static inline void module_bug_finalize(const Elf_Ehdr *hdr,
static inline void module_bug_cleanup(struct module *mod) {}
#endif /* CONFIG_GENERIC_BUG */
+#ifdef CONFIG_MODULE_SIG
+#define MODULE_SIG_STRING "~Module signature appended~\n"
+
+/*
+ * Appended module signature information block.
+ *
+ * The constituents of the signature section are, in order:
+ *
+ * - Signer's name
+ * - Key identifier
+ * - Signature data
+ * - Information block
+ */
+struct module_signature {
+ enum pkey_algo algo : 8; /* Public-key crypto algorithm */
+ enum pkey_hash_algo hash : 8; /* Digest algorithm */
+ enum pkey_id_type id_type : 8; /* Key identifier type */
+ u8 signer_len; /* Length of signer's name */
+ u8 key_id_len; /* Length of key identifier */
+ u8 __pad[3];
+ __be32 sig_len; /* Length of signature data */
+ char marker[sizeof(MODULE_SIG_STRING)-1];
+};
+#endif /* CONFIG_MODULE_SIG */
+
#endif /* _LINUX_MODULE_H */
diff --git a/kernel/module.c b/kernel/module.c
index 6085f5e..254d6a3 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -2424,14 +2424,17 @@ static int module_sig_check(struct load_info *info,
const void *mod, unsigned long *_len)
{
int err = -ENOKEY;
- unsigned long markerlen = sizeof(MODULE_SIG_STRING) - 1;
+ struct module_signature sig;
unsigned long len = *_len;
- if (len > markerlen &&
- memcmp(mod + len - markerlen, MODULE_SIG_STRING, markerlen) == 0) {
- /* We truncate the module to discard the signature */
- *_len -= markerlen;
- err = mod_verify_sig(mod, _len);
+ if (len > sizeof(sig)) {
+ memcpy(&sig, mod + len - sizeof(sig), sizeof(sig));
+
+ if (!memcmp(sig.marker, MODULE_SIG_STRING, sizeof(sig.marker))) {
+ /* We truncate the module to discard the signature */
+ *_len -= sizeof(sig);
+ err = mod_verify_sig(mod, &sig, _len);
+ }
}
if (!err) {
diff --git a/kernel/module_signing.c b/kernel/module_signing.c
index d492a23..db8e836 100644
--- a/kernel/module_signing.c
+++ b/kernel/module_signing.c
@@ -17,26 +17,6 @@
#include "module-internal.h"
/*
- * Module signature information block.
- *
- * The constituents of the signature section are, in order:
- *
- * - Signer's name
- * - Key identifier
- * - Signature data
- * - Information block
- */
-struct module_signature {
- enum pkey_algo algo : 8; /* Public-key crypto algorithm */
- enum pkey_hash_algo hash : 8; /* Digest algorithm */
- enum pkey_id_type id_type : 8; /* Key identifier type */
- u8 signer_len; /* Length of signer's name */
- u8 key_id_len; /* Length of key identifier */
- u8 __pad[3];
- __be32 sig_len; /* Length of signature data */
-};
-
-/*
* Digest the module contents.
*/
static struct public_key_signature *mod_make_digest(enum pkey_hash_algo hash,
@@ -183,10 +163,10 @@ static struct key *request_asymmetric_key(const char *signer, size_t signer_len,
/*
* Verify the signature on a module.
*/
-int mod_verify_sig(const void *mod, unsigned long *_modlen)
+int mod_verify_sig(const void *mod,
+ const struct module_signature *ms, unsigned long *_modlen)
{
struct public_key_signature *pks;
- struct module_signature ms;
struct key *key;
const void *sig;
size_t modlen = *_modlen, sig_len;
@@ -194,44 +174,38 @@ int mod_verify_sig(const void *mod, unsigned long *_modlen)
pr_devel("==>%s(,%lu)\n", __func__, modlen);
- if (modlen <= sizeof(ms))
- return -EBADMSG;
-
- memcpy(&ms, mod + (modlen - sizeof(ms)), sizeof(ms));
- modlen -= sizeof(ms);
-
- sig_len = be32_to_cpu(ms.sig_len);
+ sig_len = be32_to_cpu(ms->sig_len);
if (sig_len >= modlen)
return -EBADMSG;
modlen -= sig_len;
if ((size_t)ms.signer_len + ms.key_id_len >= modlen)
return -EBADMSG;
- modlen -= (size_t)ms.signer_len + ms.key_id_len;
+ modlen -= (size_t)ms->signer_len + ms.key_id_len;
*_modlen = modlen;
sig = mod + modlen;
/* For the moment, only support RSA and X.509 identifiers */
- if (ms.algo != PKEY_ALGO_RSA ||
- ms.id_type != PKEY_ID_X509)
+ if (ms->algo != PKEY_ALGO_RSA ||
+ ms->id_type != PKEY_ID_X509)
return -ENOPKG;
- if (ms.hash >= PKEY_HASH__LAST ||
+ if (ms->hash >= PKEY_HASH__LAST ||
!pkey_hash_algo[ms.hash])
return -ENOPKG;
- key = request_asymmetric_key(sig, ms.signer_len,
- sig + ms.signer_len, ms.key_id_len);
+ key = request_asymmetric_key(sig, ms->signer_len,
+ sig + ms->signer_len, ms->key_id_len);
if (IS_ERR(key))
return PTR_ERR(key);
- pks = mod_make_digest(ms.hash, mod, modlen);
+ pks = mod_make_digest(ms->hash, mod, modlen);
if (IS_ERR(pks)) {
ret = PTR_ERR(pks);
goto error_put_key;
}
- ret = mod_extract_mpi_array(pks, sig + ms.signer_len + ms.key_id_len,
+ ret = mod_extract_mpi_array(pks, sig + ms->signer_len + ms->key_id_len,
sig_len);
if (ret < 0)
goto error_free_pks;
--
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