random: fix error in entropy extraction (CVE-2007-2453 1 of 2)

Fix cast error in entropy extraction.
Add comments explaining the magic 16.
Remove extra confusing loop variable.

Signed-off-by: Matt Mackall <mpm@selenic.com>
Acked-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Adrian Bunk <bunk@kernel.org>

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 86be04b241e1b5c9fb5dd477f7766b47629af604..e237237b3c4a907d3da2bc749f50c16bff4ba6a3 100644 (file)
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -758,7 +758,7 @@ static size_t account(struct entropy_store *r, size_t nbytes, int min,
 
 static void extract_buf(struct entropy_store *r, __u8 *out)
 {
-       int i, x;
+       int i;
        __u32 data[16], buf[5 + SHA_WORKSPACE_WORDS];
 
        sha_init(buf);
@@ -770,9 +770,11 @@ static void extract_buf(struct entropy_store *r, __u8 *out)
         * attempts to find previous ouputs), unless the hash
         * function can be inverted.
         */
-       for (i = 0, x = 0; i < r->poolinfo->poolwords; i += 16, x+=2) {
-               sha_transform(buf, (__u8 *)r->pool+i, buf + 5);
-               add_entropy_words(r, &buf[x % 5], 1);
+       for (i = 0; i < r->poolinfo->poolwords; i += 16) {
+               /* hash blocks of 16 words = 512 bits */
+               sha_transform(buf, (__u8 *)(r->pool + i), buf + 5);
+               /* feed back portion of the resulting hash */
+               add_entropy_words(r, &buf[i % 5], 1);
        }
 
        /*
@@ -780,7 +782,7 @@ static void extract_buf(struct entropy_store *r, __u8 *out)
         * portion of the pool while mixing, and hash one
         * final time.
         */
-       __add_entropy_words(r, &buf[x % 5], 1, data);
+       __add_entropy_words(r, &buf[i % 5], 1, data);
        sha_transform(buf, (__u8 *)data, buf + 5);
 
        /*