On Thu, Oct 13, 2022 at 09:36:52AM -0600, Jason A. Donenfeld wrote:
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
drivers/char/mem.c | 4 ++-- drivers/char/random.c | 5 +++++ 2 files changed, 7 insertions(+), 2 deletions(-)
Still breaks on older kernels:
drivers/char/random.c: In function ‘random_read_iter’: drivers/char/random.c:1299:33: error: ‘IOCB_NOWAIT’ undeclared (first use in this function); did you mean ‘IPC_NOWAIT’? 1299 | ((kiocb->ki_flags & IOCB_NOWAIT) || | ^~~~~~~~~~~ | IPC_NOWAIT drivers/char/random.c:1299:33: note: each undeclared identifier is reported only once for each function it appears in drivers/char/mem.c:872:48: error: ‘FMODE_NOWAIT’ undeclared here (not in a function); did you mean ‘FOLL_NOWAIT’? 872 | [8] = { "random", 0666, &random_fops, FMODE_NOWAIT }, | ^~~~~~~~~~~~ | FOLL_NOWAIT