seccomp: fix memory leak on filter attach

This sets the correct error code when final filter memory is unavailable,
and frees the raw filter no matter what.

unreferenced object 0xffff8800d6ea4000 (size 512):
  comm "sshd", pid 278, jiffies 4294898315 (age 46.653s)
  hex dump (first 32 bytes):
    21 00 00 00 04 00 00 00 15 00 01 00 3e 00 00 c0  !...........>...
    06 00 00 00 00 00 00 00 21 00 00 00 00 00 00 00  ........!.......
  backtrace:
    [<ffffffff8151414e>] kmemleak_alloc+0x4e/0xb0
    [<ffffffff811a3a40>] __kmalloc+0x280/0x320
    [<ffffffff8110842e>] prctl_set_seccomp+0x11e/0x3b0
    [<ffffffff8107bb6b>] SyS_prctl+0x3bb/0x4a0
    [<ffffffff8152ef2d>] system_call_fastpath+0x1a/0x1f
    [<ffffffffffffffff>] 0xffffffffffffffff
Reported-by: Masami Ichikawa <masami256@gmail.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Tested-by: Masami Ichikawa <masami256@gmail.com>
Acked-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

kernel/seccomp.c

@@ -255,6 +255,7 @@ static long seccomp_attach_filter(struct sock_fprog *fprog)
 		goto free_prog;
 
 	/* Allocate a new seccomp_filter */
+	ret = -ENOMEM;
 	filter = kzalloc(sizeof(struct seccomp_filter) +
 			 sizeof(struct sock_filter_int) * new_len,
 			 GFP_KERNEL|__GFP_NOWARN);
@@ -264,6 +265,7 @@ static long seccomp_attach_filter(struct sock_fprog *fprog)
 	ret = sk_convert_filter(fp, fprog->len, filter->insnsi, &new_len);
 	if (ret)
 		goto free_filter;
+	kfree(fp);
 
 	atomic_set(&filter->usage, 1);
 	filter->len = new_len;