[PATCH v3 1/2] fs: Fix file_set_fowner LSM hook inconsistencies

[Mickaël Salaün]

The fcntl's F_SETOWN command sets the process that handle SIGIO/SIGURG
for the related file descriptor.  Before this change, the
file_set_fowner LSM hook was always called, ignoring the VFS logic which
may not actually change the process that handles SIGIO (e.g. TUN, TTY,
dnotify), nor update the related UID/EUID.

Moreover, because security_file_set_fowner() was called without lock
(e.g. f_owner.lock), concurrent F_SETOWN commands could result to a race
condition and inconsistent LSM states (e.g. SELinux's fown_sid) compared
to struct fown_struct's UID/EUID.

This change makes sure the LSM states are always in sync with the VFS
state by moving the security_file_set_fowner() call close to the
UID/EUID updates and using the same f_owner.lock .

Rename f_modown() to __f_setown() to simplify code.

Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Casey Schaufler <casey@xxxxxxxxxxxxxxxx>
Cc: Christian Brauner <brauner@xxxxxxxxxx>
Cc: James Morris <jmorris@xxxxxxxxx>
Cc: Jann Horn <jannh@xxxxxxxxxx>
Cc: Ondrej Mosnacek <omosnace@xxxxxxxxxx>
Cc: Paul Moore <paul@xxxxxxxxxxxxxx>
Cc: Serge E. Hallyn <serge@xxxxxxxxxx>
Cc: Stephen Smalley <stephen.smalley.work@xxxxxxxxx>
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Mickaël Salaün <mic@xxxxxxxxxxx>
---

Changes since v2:
https://lore.kernel.org/r/20240812174421.1636724-1-mic@xxxxxxxxxxx
- Only keep the LSM hook move.

Changes since v1:
https://lore.kernel.org/r/20240812144936.1616628-1-mic@xxxxxxxxxxx
- Add back the file_set_fowner hook (but without user) as
  requested by Paul, but move it for consistency.
---
 fs/fcntl.c | 14 ++++----------
 1 file changed, 4 insertions(+), 10 deletions(-)

diff --git a/fs/fcntl.c b/fs/fcntl.c
index 300e5d9ad913..c28dc6c005f1 100644
--- a/fs/fcntl.c
+++ b/fs/fcntl.c
@@ -87,8 +87,8 @@ static int setfl(int fd, struct file * filp, unsigned int arg)
 	return error;
 }
 
-static void f_modown(struct file *filp, struct pid *pid, enum pid_type type,
-                     int force)
+void __f_setown(struct file *filp, struct pid *pid, enum pid_type type,
+		int force)
 {
 	write_lock_irq(&filp->f_owner.lock);
 	if (force || !filp->f_owner.pid) {
@@ -98,19 +98,13 @@ static void f_modown(struct file *filp, struct pid *pid, enum pid_type type,
 
 		if (pid) {
 			const struct cred *cred = current_cred();
+			security_file_set_fowner(filp);
 			filp->f_owner.uid = cred->uid;
 			filp->f_owner.euid = cred->euid;
 		}
 	}
 	write_unlock_irq(&filp->f_owner.lock);
 }
-
-void __f_setown(struct file *filp, struct pid *pid, enum pid_type type,
-		int force)
-{
-	security_file_set_fowner(filp);
-	f_modown(filp, pid, type, force);
-}
 EXPORT_SYMBOL(__f_setown);
 
 int f_setown(struct file *filp, int who, int force)
@@ -146,7 +140,7 @@ EXPORT_SYMBOL(f_setown);
 
 void f_delown(struct file *filp)
 {
-	f_modown(filp, NULL, PIDTYPE_TGID, 1);
+	__f_setown(filp, NULL, PIDTYPE_TGID, 1);
 }
 
 pid_t f_getown(struct file *filp)
-- 
2.46.0