On Mon, 12 Dec 2011 01:21:40 +0000, David Howells <dhowells@xxxxxxxxxx> wrote:
> Rusty Russell <rusty@xxxxxxxxxx> wrote:
>
> > I think you misunderstand, I'm talking about the modinfo command, not
> > the .modinfo section.
>
> Sorry, yes. But why do you need to enhance modinfo?
I was suggesting that you want it to print the signatures, or at least
indicate their existence. Maybe check them too, but that might be a bit
too heavy for modinfo.
> > But I need to know exactly what these version-dependent mangling of
> > modules is. Is it real? Is it more than strip? Is it so hard to fix
> > that it makes sense to add 450 lines of dense kernel code to allow
> > alteration of a module after signing?
>
> The strip program (as far as I know that's the only binutil that we need worry
> about) rearranges and reorders the section, symbol and relocation tables when
> it discards stuff, and different versions of strip have done it
> differently.
OK, then you need to generate stripped modules as part of the build,
too. It's a bit of a pain, sure, but hardly a showstopper.
> However, you said it should be fairly easy to jump over the ELF parcel to get
> to the signature. How do you plan on doing that?
> I presume you would just parse sufficient of the ELF to find the
> theoretical ELF EOF and then look there for a whole string of
> signatures
You could do that. But there's an easier way. Took me longer to figure
out the damn crypto API than actually write the module part :(
Subject: module: simple signature support.
A signature contains a magic marker: it signs everything up to the
magic marker (ie. just append them):
SUM=`md5sum drivers/block/loop.ko | cut -d\ -f1`; echo "@Module signature:$SUM" >> drivers/block/loop.ko
We can have false positives, but at worst that make us report EINVAL
(bad signature) instead of ENOENT (no signature).
diff --git a/kernel/module.c b/kernel/module.c
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -2374,6 +2374,98 @@ free_hdr:
return err;
}
+/* CONFIG_MODULE_SIGN implies we don't trust modules: verify signature
+ * before we interpret (almost) anything. */
+#define MOD_SIGNATURE "@Module signature:"
+
+#include <linux/ctype.h>
+#include <crypto/hash.h>
+#include <crypto/md5.h>
+
+static int from_hex(char c)
+{
+ if (isdigit(c))
+ return c - '0';
+ if (isupper(c))
+ return c - 'A' + 10;
+ return c - 'a' + 10;
+}
+
+/* A signature signs everything before it. */
+static int try_signature(void *data, void *sig, unsigned long max_sig)
+{
+ unsigned long data_len = sig - data;
+
+ sig += strlen(MOD_SIGNATURE);
+ max_sig -= strlen(MOD_SIGNATURE);
+
+ /* Dummy: accept md5 as signature. */
+ {
+ struct crypto_api_blows {
+ struct shash_desc md5;
+ char morestuff[100];
+ } m;
+ u8 digest[MD5_DIGEST_SIZE], expected[MD5_DIGEST_SIZE];
+ char *s = sig;
+ int i;
+
+ /* Not a signature? */
+ if (max_sig < MD5_DIGEST_SIZE * 2) {
+ printk("Too close to end (%lu)\n", max_sig);
+ return -ENOENT;
+ }
+
+ for (i = 0; i < MD5_DIGEST_SIZE * 2; i += 2) {
+ /* Not a signature? */
+ if (!isxdigit(s[i]) || !isxdigit(s[i+1])) {
+ printk("Not hex digit (%c)\n", s[i]);
+ return -ENOENT;
+ }
+ digest[i/2] = (from_hex(s[i])<<4) | from_hex(s[i+1]);
+ }
+
+ m.md5.tfm = crypto_alloc_shash("md5", 0, 0);
+ if (IS_ERR(m.md5.tfm))
+ return PTR_ERR(m.md5.tfm);
+ m.md5.flags = 0;
+
+ crypto_shash_digest(&m.md5, data, data_len, expected);
+ crypto_free_shash(m.md5.tfm);
+
+ if (memcmp(digest, expected, sizeof(digest)) != 0) {
+ printk("Mismatch: given %02x%02x%02x...,"
+ " expect %02x%02x%02x...\n",
+ digest[0], digest[1], digest[2],
+ expected[0], expected[1], expected[2]);
+ return -EINVAL;
+ }
+ printk("Found valid signature!\n");
+ return 0;
+ }
+}
+
+/* -ENOENT if no signature found, -EINVAL if invalid, 0 if good. */
+static int find_and_check_signatures(struct load_info *info)
+{
+ void *p = info->hdr, *end = p + info->len;
+ const size_t sigsize = strlen(MOD_SIGNATURE);
+ int err = -ENOENT;
+
+ /* Poor man's memmem. len > sigsize */
+ while ((p = memchr(p, MOD_SIGNATURE[0], end - p))) {
+ if (p + sigsize > end)
+ break;
+
+ if (memcmp(p, MOD_SIGNATURE, sigsize) == 0) {
+ err = try_signature(info->hdr, p, end - p);
+ if (!err)
+ break;
+ }
+ p++;
+ }
+ return err;
+}
+
static void free_copy(struct load_info *info)
{
vfree(info->hdr);
@@ -2819,6 +2911,11 @@ static struct module *load_module(void _
if (err)
return ERR_PTR(err);
+ /* Before we trust it, carefully check signatures. */
+ err = find_and_check_signatures(&info);
+ if (err)
+ goto free_copy;
+
/* Figure out module layout, and allocate all the memory. */
mod = layout_and_allocate(&info);
if (IS_ERR(mod)) {
--
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