[PATCH 5.10 67/79] fcntl: make F_GETOWN(EX) return 0 on dead owner task

[Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>]

From: Pavel Tikhomirov <ptikhomirov@xxxxxxxxxxxxx>

[ Upstream commit cc4a3f885e8f2bc3c86a265972e94fef32d68f67 ]

Currently there is no way to differentiate the file with alive owner
from the file with dead owner but pid of the owner reused. That's why
CRIU can't actually know if it needs to restore file owner or not,
because if it restores owner but actual owner was dead, this can
introduce unexpected signals to the "false"-owner (which reused the
pid).

Let's change the api, so that F_GETOWN(EX) returns 0 in case actual
owner is dead already. This comports with the POSIX spec, which
states that a PID of 0 indicates that no signal will be sent.

Cc: Jeff Layton <jlayton@xxxxxxxxxx>
Cc: "J. Bruce Fields" <bfields@xxxxxxxxxxxx>
Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: linux-fsdevel@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: Cyrill Gorcunov <gorcunov@xxxxxxxxx>
Cc: Andrei Vagin <avagin@xxxxxxxxx>
Signed-off-by: Pavel Tikhomirov <ptikhomirov@xxxxxxxxxxxxx>
Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
Stable-dep-of: f671a691e299 ("fcntl: fix potential deadlocks for &fown_struct.lock")
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 fs/fcntl.c | 19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/fs/fcntl.c b/fs/fcntl.c
index 71b43538fa44..5a56351f1fc3 100644
--- a/fs/fcntl.c
+++ b/fs/fcntl.c
@@ -148,11 +148,15 @@ void f_delown(struct file *filp)
 
 pid_t f_getown(struct file *filp)
 {
-	pid_t pid;
+	pid_t pid = 0;
 	read_lock(&filp->f_owner.lock);
-	pid = pid_vnr(filp->f_owner.pid);
-	if (filp->f_owner.pid_type == PIDTYPE_PGID)
-		pid = -pid;
+	rcu_read_lock();
+	if (pid_task(filp->f_owner.pid, filp->f_owner.pid_type)) {
+		pid = pid_vnr(filp->f_owner.pid);
+		if (filp->f_owner.pid_type == PIDTYPE_PGID)
+			pid = -pid;
+	}
+	rcu_read_unlock();
 	read_unlock(&filp->f_owner.lock);
 	return pid;
 }
@@ -200,11 +204,14 @@ static int f_setown_ex(struct file *filp, unsigned long arg)
 static int f_getown_ex(struct file *filp, unsigned long arg)
 {
 	struct f_owner_ex __user *owner_p = (void __user *)arg;
-	struct f_owner_ex owner;
+	struct f_owner_ex owner = {};
 	int ret = 0;
 
 	read_lock(&filp->f_owner.lock);
-	owner.pid = pid_vnr(filp->f_owner.pid);
+	rcu_read_lock();
+	if (pid_task(filp->f_owner.pid, filp->f_owner.pid_type))
+		owner.pid = pid_vnr(filp->f_owner.pid);
+	rcu_read_unlock();
 	switch (filp->f_owner.pid_type) {
 	case PIDTYPE_PID:
 		owner.type = F_OWNER_TID;
-- 
2.35.1