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 --- drivers/char/mem.c | 4 ++-- drivers/char/random.c | 5 +++++ 2 files changed, 7 insertions(+), 2 deletions(-)
diff --git a/drivers/char/mem.c b/drivers/char/mem.c index 84ca98ed1dad..c2b37009b11e 100644 --- a/drivers/char/mem.c +++ b/drivers/char/mem.c @@ -706,8 +706,8 @@ static const struct memdev { #endif [5] = { "zero", 0666, &zero_fops, FMODE_NOWAIT }, [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 79d7d4e4e582..c8cc23515568 100644 --- a/drivers/char/random.c +++ b/drivers/char/random.c @@ -1347,6 +1347,11 @@ 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;