[PATCH] fs/namespace.c: fix use-after-free of mount in mnt_warn_timestamp_expiry()

[Eric Biggers <ebiggers@kernel.org>]

From: Eric Biggers <ebiggers@google.com>

After do_add_mount() returns success, the caller doesn't hold a
reference to the 'struct mount' anymore.  So it's invalid to access it
in mnt_warn_timestamp_expiry().

Fix it by instead passing the super_block and the mnt_flags.  It's safe
to access the super_block because it's pinned by fs_context::root.

Reported-by: syzbot+da4f525235510683d855@syzkaller.appspotmail.com
Fixes: f8b92ba67c5d ("mount: Add mount warning for impending timestamp expiry")
Signed-off-by: Eric Biggers <ebiggers@google.com>
---
 fs/namespace.c | 15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index fe0e9e1410fe..7ef8edaaed69 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2466,12 +2466,11 @@ static void set_mount_attributes(struct mount *mnt, unsigned int mnt_flags)
 	unlock_mount_hash();
 }
 
-static void mnt_warn_timestamp_expiry(struct path *mountpoint, struct vfsmount *mnt)
+static void mnt_warn_timestamp_expiry(struct path *mountpoint,
+				      struct super_block *sb, int mnt_flags)
 {
-	struct super_block *sb = mnt->mnt_sb;
-
-	if (!__mnt_is_readonly(mnt) &&
-	   (ktime_get_real_seconds() + TIME_UPTIME_SEC_MAX > sb->s_time_max)) {
+	if (!(mnt_flags & MNT_READONLY) && !sb_rdonly(sb) &&
+	    (ktime_get_real_seconds() + TIME_UPTIME_SEC_MAX > sb->s_time_max)) {
 		char *buf = (char *)__get_free_page(GFP_KERNEL);
 		char *mntpath = buf ? d_path(mountpoint, buf, PAGE_SIZE) : ERR_PTR(-ENOMEM);
 		struct tm tm;
@@ -2512,7 +2511,7 @@ static int do_reconfigure_mnt(struct path *path, unsigned int mnt_flags)
 		set_mount_attributes(mnt, mnt_flags);
 	up_write(&sb->s_umount);
 
-	mnt_warn_timestamp_expiry(path, &mnt->mnt);
+	mnt_warn_timestamp_expiry(path, sb, mnt_flags);
 
 	return ret;
 }
@@ -2555,7 +2554,7 @@ static int do_remount(struct path *path, int ms_flags, int sb_flags,
 		up_write(&sb->s_umount);
 	}
 
-	mnt_warn_timestamp_expiry(path, &mnt->mnt);
+	mnt_warn_timestamp_expiry(path, sb, mnt_flags);
 
 	put_fs_context(fc);
 	return err;
@@ -2770,7 +2769,7 @@ static int do_new_mount_fc(struct fs_context *fc, struct path *mountpoint,
 		return error;
 	}
 
-	mnt_warn_timestamp_expiry(mountpoint, mnt);
+	mnt_warn_timestamp_expiry(mountpoint, sb, mnt_flags);
 
 	return error;
 }
-- 
2.23.0