[PATCH] tracefs: Check for dentry->d_inode exists in set_gid()

From: "Steven Rostedt (Google)" <rostedt@goodmis.org>

If a getdents() is called on the tracefs directory but does not get all
the files, it can leave a "cursor" dentry in the d_subdirs list of tracefs
dentry. This cursor dentry does not have a d_inode for it. Before
referencing tracefs_inode from the dentry, the d_inode must first be
checked if it has content. If not, then it's not a tracefs_inode and can
be ignored.

The following caused a crash:

 #define getdents64(fd, dirp, count) syscall(SYS_getdents64, fd, dirp, count)
 #define BUF_SIZE 256
 #define TDIR "/tmp/file0"

 int main(void)
 {
	char buf[BUF_SIZE];
	int fd;
       	int n;

       	mkdir(TDIR, 0777);
	mount(NULL, TDIR, "tracefs", 0, NULL);
       	fd = openat(AT_FDCWD, TDIR, O_RDONLY);
       	n = getdents64(fd, buf, BUF_SIZE);
       	ret = mount(NULL, TDIR, NULL, MS_NOSUID|MS_REMOUNT|MS_RELATIME|MS_LAZYTIME,
		    "gid=1000");
	return 0;
 }

That's because the 256 BUF_SIZE was not big enough to read all the
dentries of the tracefs file system and it left a "cursor" dentry in the
subdirs of the tracefs root inode. Then on remounting with "gid=1000",
it would cause an iteration of all dentries which hit:

	ti = get_tracefs(dentry->d_inode);
	if (ti && (ti->flags & TRACEFS_EVENT_INODE))
		eventfs_update_gid(dentry, gid);

Which crashed because of the dereference of the cursor dentry which had a NULL
d_inode.

Link: https://lore.kernel.org/all/20240102135637.3a21fb10@gandalf.local.home/ (local)

Cc: stable@vger.kernel.org
Fixes: 7e8358edf503e ("eventfs: Fix file and directory uid and gid ownership")
Reported-by: "Ubisectech Sirius" <redacted>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
 fs/tracefs/inode.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/fs/tracefs/inode.c b/fs/tracefs/inode.c
index 62524b20964e..c29387a36bc8 100644
--- a/fs/tracefs/inode.c
+++ b/fs/tracefs/inode.c

@@ -219,10 +219,13 @@ static void set_gid(struct dentry *parent, kgid_t gid)
 
 		change_gid(dentry, gid);
 
-		/* If this is the events directory, update that too */
-		ti = get_tracefs(dentry->d_inode);
-		if (ti && (ti->flags & TRACEFS_EVENT_INODE))
-			eventfs_update_gid(dentry, gid);
+		/* Note, getdents() can add a cursor dentry with no inode */
+		if (dentry->d_inode) {
+			/* If this is the events directory, update that too */
+			ti = get_tracefs(dentry->d_inode);
+			if (ti && (ti->flags & TRACEFS_EVENT_INODE))
+				eventfs_update_gid(dentry, gid);
+		}
 
 		if (!list_empty(&dentry->d_subdirs)) {
 			spin_unlock(&this_parent->d_lock);

-- 
2.42.0