[PATCH v3 2/2] fs/fcntl: f_setown, avoid undefined behaviour

[Jiri Slaby <jslaby@suse.cz>]

fcntl(0, F_SETOWN, 0x80000000) triggers:
UBSAN: Undefined behaviour in fs/fcntl.c:118:7
negation of -2147483648 cannot be represented in type 'int':
CPU: 1 PID: 18261 Comm: syz-executor Not tainted 4.8.1-0-syzkaller #1
...
Call Trace:
...
 [<ffffffffad8f0868>] ? f_setown+0x1d8/0x200
 [<ffffffffad8f19a9>] ? SyS_fcntl+0x999/0xf30
 [<ffffffffaed1fb00>] ? entry_SYSCALL_64_fastpath+0x23/0xc1

Fix that by checking the arg parameter properly (against INT_MAX) before
"who = -who". And return immediatelly with -EINVAL in case it is wrong.
Note that according to POSIX we can return EINVAL:
    http://pubs.opengroup.org/onlinepubs/9699919799/functions/fcntl.html

    [EINVAL]
        The cmd argument is F_SETOWN and the value of the argument
        is not valid as a process or process group identifier.

[v2] returns an error, v1 used to fail silently
[v3] implement proper check for the bad value INT_MIN

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Jeff Layton <jlayton@poochiereds.net>
Cc: "J. Bruce Fields" <bfields@fieldses.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
---
 fs/fcntl.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/fs/fcntl.c b/fs/fcntl.c
index 313eba860346..693322e28751 100644
--- a/fs/fcntl.c
+++ b/fs/fcntl.c
@@ -116,6 +116,10 @@ int f_setown(struct file *filp, unsigned long arg, int force)
 	int who = arg;
 	type = PIDTYPE_PID;
 	if (who < 0) {
+		/* avoid overflow below */
+		if (who == INT_MIN)
+			return -EINVAL;
+
 		type = PIDTYPE_PGID;
 		who = -who;
 	}
-- 
2.13.1