[PATCH 1/2] sysctl: read() must consume poll events, not poll()

["Jason A. Donenfeld" <Jason@zx2c4.com>]

Events that poll() responds to are supposed to be consumed when the file
is read(), not by the poll() itself. By putting it on the poll() itself,
it makes it impossible to poll() on a epoll file descriptor, since the
event gets consumed too early. Jann wrote a PoC, available in the link
below.

Reported-by: Jann Horn <jannh@google.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Cc: linux-fsdevel@vger.kernel.org
Link: https://lore.kernel.org/lkml/CAG48ez1F0P7Wnp=PGhiUej=u=8CSF6gpD9J=Oxxg0buFRqV1tA@mail.gmail.com/
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
 fs/proc/proc_sysctl.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
index 7d9cfc730bd4..1aa145794207 100644
--- a/fs/proc/proc_sysctl.c
+++ b/fs/proc/proc_sysctl.c
@@ -622,6 +622,14 @@ static ssize_t proc_sys_call_handler(struct kiocb *iocb, struct iov_iter *iter,
 
 static ssize_t proc_sys_read(struct kiocb *iocb, struct iov_iter *iter)
 {
+	struct inode *inode = file_inode(iocb->ki_filp);
+	struct ctl_table_header *head = grab_header(inode);
+	struct ctl_table *table = PROC_I(inode)->sysctl_entry;
+
+	if (!IS_ERR(head) && table->poll)
+		iocb->ki_filp->private_data = proc_sys_poll_event(table->poll);
+	sysctl_head_finish(head);
+
 	return proc_sys_call_handler(iocb, iter, 0);
 }
 
@@ -668,10 +676,8 @@ static __poll_t proc_sys_poll(struct file *filp, poll_table *wait)
 	event = (unsigned long)filp->private_data;
 	poll_wait(filp, &table->poll->wait, wait);
 
-	if (event != atomic_read(&table->poll->event)) {
-		filp->private_data = proc_sys_poll_event(table->poll);
+	if (event != atomic_read(&table->poll->event))
 		ret = EPOLLIN | EPOLLRDNORM | EPOLLERR | EPOLLPRI;
-	}
 
 out:
 	sysctl_head_finish(head);
-- 
2.35.1