[PATCH 3.18 34/52] fs/fcntl: f_setown, avoid undefined behaviour

[Greg Kroah-Hartman]

3.18-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Jiri Slaby <jslaby@xxxxxxx>

commit fc3dc67471461c0efcb1ed22fb7595121d65fad9 upstream.

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@xxxxxxx>
Cc: Jeff Layton <jlayton@xxxxxxxxxxxxxxx>
Cc: "J. Bruce Fields" <bfields@xxxxxxxxxxxx>
Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: linux-fsdevel@xxxxxxxxxxxxxxx
Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 fs/fcntl.c |    4 ++++
 1 file changed, 4 insertions(+)

--- a/fs/fcntl.c
+++ b/fs/fcntl.c
@@ -113,6 +113,10 @@ void f_setown(struct file *filp, unsigne
 	int who = arg;
 	type = PIDTYPE_PID;
 	if (who < 0) {
+		/* avoid overflow below */
+		if (who == INT_MIN)
+			return;
+
 		type = PIDTYPE_PGID;
 		who = -who;
 	}