random: restore O_NONBLOCK support

commit cd4f24ae9404fd31fc461066e57889be3b68641b upstream.

Prior to 5.6, when /dev/random was opened with O_NONBLOCK, it would
return -EAGAIN if there was no entropy. When the pools were unified in
5.6, this was lost. The post 5.6 behavior of blocking until the pool is
initialized, and ignoring O_NONBLOCK in the process, went unnoticed,
with no reports about the regression received for two and a half years.
However, eventually this indeed did break somebody's userspace.

So we restore the old behavior, by returning -EAGAIN if the pool is not
initialized. Unlike the old /dev/random, this can only occur during
early boot, after which it never blocks again.

In order to make this O_NONBLOCK behavior consistent with other
expectations, also respect users reading with preadv2(RWF_NOWAIT) and
similar.

Fixes: 30c08efec888 ("random: make /dev/random be almost like /dev/urandom")
Reported-by: Guozihua <guozihua@huawei.com>
Reported-by: Zhongguohua <zhongguohua1@huawei.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Theodore Ts'o <tytso@mit.edu>
Cc: Andrew Lutomirski <luto@kernel.org>
Cc: stable@vger.kernel.org
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index 1c596b5..d8e3b54 100644 (file)
--- a/drivers/char/mem.c
+++ b/drivers/char/mem.c
@@ -702,8 +702,8 @@ static const struct memdev {
 #endif
         [5] = { "zero", 0666, &zero_fops, 0 },
         [7] = { "full", 0666, &full_fops, 0 },
-        [8] = { "random", 0666, &random_fops, 0 },
-        [9] = { "urandom", 0666, &urandom_fops, 0 },
+        [8] = { "random", 0666, &random_fops, FMODE_NOWAIT },
+        [9] = { "urandom", 0666, &urandom_fops, FMODE_NOWAIT },
 #ifdef CONFIG_PRINTK
        [11] = { "kmsg", 0644, &kmsg_fops, 0 },
 #endif

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 7bd6eb1..50dc861 100644 (file)
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1298,6 +1298,11 @@ static ssize_t random_read_iter(struct kiocb *kiocb, struct iov_iter *iter)
 {
        int ret;
 
+       if (!crng_ready() &&
+           ((kiocb->ki_flags & (IOCB_NOWAIT | IOCB_NOIO)) ||
+            (kiocb->ki_filp->f_flags & O_NONBLOCK)))
+               return -EAGAIN;
+
        ret = wait_for_random_bytes();
        if (ret != 0)
                return ret;