KEYS: fix writing past end of user-supplied buffer in keyring_read()

Userspace can call keyctl_read() on a keyring to get the list of IDs of
keys in the keyring.  But if the user-supplied buffer is too small, the
kernel would write the full list anyway --- which will corrupt whatever
userspace memory happened to be past the end of the buffer.  Fix it by
only filling the space that is available.

Fixes: b2a4df200d57 ("KEYS: Expand the capacity of a keyring")
Cc: <stable@vger.kernel.org>	[v3.13+]
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: David Howells <dhowells@redhat.com>

diff --git a/security/keys/keyring.c b/security/keys/keyring.c
index de81793..94f0389 100644 (file)
--- a/security/keys/keyring.c
+++ b/security/keys/keyring.c
@@ -423,7 +423,7 @@ static void keyring_describe(const struct key *keyring, struct seq_file *m)
 }
 
 struct keyring_read_iterator_context {
-       size_t                  qty;
+       size_t                  buflen;
        size_t                  count;
        key_serial_t __user     *buffer;
 };
@@ -435,9 +435,9 @@ static int keyring_read_iterator(const void *object, void *data)
        int ret;
 
        kenter("{%s,%d},,{%zu/%zu}",
-              key->type->name, key->serial, ctx->count, ctx->qty);
+              key->type->name, key->serial, ctx->count, ctx->buflen);
 
-       if (ctx->count >= ctx->qty)
+       if (ctx->count >= ctx->buflen)
                return 1;
 
        ret = put_user(key->serial, ctx->buffer);
@@ -472,16 +472,12 @@ static long keyring_read(const struct key *keyring,
                return 0;
 
        /* Calculate how much data we could return */
-       ctx.qty = nr_keys * sizeof(key_serial_t);
-
        if (!buffer || !buflen)
-               return ctx.qty;
-
-       if (buflen > ctx.qty)
-               ctx.qty = buflen;
+               return nr_keys * sizeof(key_serial_t);
 
        /* Copy the IDs of the subscribed keys into the buffer */
        ctx.buffer = (key_serial_t __user *)buffer;
+       ctx.buflen = buflen;
        ctx.count = 0;
        ret = assoc_array_iterate(&keyring->keys, keyring_read_iterator, &ctx);
        if (ret < 0) {