linux-kernel - [PATCH v2 3/5] signal: use fdget() since we don't allow O

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Date:   Thu, 18 Apr 2019 12:18:39 +0200
From:   Christian Brauner <christian@...uner.io>
To:     torvalds@...ux-foundation.org, viro@...iv.linux.org.uk,
        jannh@...gle.com, dhowells@...hat.com, oleg@...hat.com,
        linux-api@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     serge@...lyn.com, luto@...nel.org, arnd@...db.de,
        ebiederm@...ssion.com, keescook@...omium.org, tglx@...utronix.de,
        mtk.manpages@...il.com, akpm@...ux-foundation.org,
        cyphar@...har.com, joel@...lfernandes.org, dancol@...gle.com,
        Christian Brauner <christian@...uner.io>,
        Jann Horn <jann@...jh.net>
Subject: [PATCH v2 3/5] signal: use fdget() since we don't allow O_PATH

As stated in the original commit for pidfd_send_signal() we don't allow to
signal processes through O_PATH file descriptors since it is semantically
equivalent to a write on the pidfd. We already correctly error out right
now and return EBADF if an O_PATH fd is passed. This is because we use
file->f_op to detect whether a pidfd is passed and O_PATH fds have their
file->f_op set to empty_fops in do_dentry_open() and thus fail the test.
Thus, there is no regression. It's just semantically correct to use fdget()
and return an error right from there instead of taking a reference and
returning an error later.

Signed-off-by: Christian Brauner <christian@...uner.io>
Cc: Arnd Bergmann <arnd@...db.de>
Cc: "Eric W. Biederman" <ebiederm@...ssion.com>
Cc: Kees Cook <keescook@...omium.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Jann Horn <jann@...jh.net>
Cc: David Howells <dhowells@...hat.com>
Cc: "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
Cc: Andy Lutomirsky <luto@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Oleg Nesterov <oleg@...hat.com>
Cc: Aleksa Sarai <cyphar@...har.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Al Viro <viro@...iv.linux.org.uk>
---
/* changelog */
v1: patch not present
v2:
- Oleg Nesterov <oleg@...hat.com>:
  - split out from following patch since the s/fdget_raw()/fdget()/
    replacement is a fix unrelated to the theme of the following patch
---
 kernel/signal.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index f98448cf2def..227ba170298e 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -3581,7 +3581,7 @@ SYSCALL_DEFINE4(pidfd_send_signal, int, pidfd, int, sig,
 	if (flags)
 		return -EINVAL;
 
-	f = fdget_raw(pidfd);
+	f = fdget(pidfd);
 	if (!f.file)
 		return -EBADF;
 
-- 
2.21.0