From: "Steven Rostedt (Google)" <rostedt@xxxxxxxxxxx>
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.
In the subdir loop of the dentry lookup of set_gid(), if a child has a
NULL d_inode, simply skip it.
Link: https://lore.kernel.org/all/20240102135637.3a21fb10@xxxxxxxxxxxxxxxxxx/
Link: https://lore.kernel.org/linux-trace-kernel/20240102151249.05da244d@xxxxxxxxxxxxxxxxxx
Cc: stable@xxxxxxxxxxxxxxx
Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
Cc: Mark Rutland <mark.rutland@xxxxxxx>
Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Fixes: 7e8358edf503e ("eventfs: Fix file and directory uid and gid ownership")
Reported-by: "Ubisectech Sirius" <bugreport@xxxxxxxxxxxxxx>
Signed-off-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
---
fs/tracefs/inode.c | 4 ++++
1 file changed, 4 insertions(+)
diff --git a/fs/tracefs/inode.c b/fs/tracefs/inode.c
index 62524b20964e..bc86ffdb103b 100644
--- a/fs/tracefs/inode.c
+++ b/fs/tracefs/inode.c
@@ -215,6 +215,10 @@ static void set_gid(struct dentry *parent, kgid_t gid)
struct dentry *dentry = list_entry(tmp, struct dentry, d_child);
next = tmp->next;
+ /* Note, getdents() can add a cursor dentry with no inode */
+ if (!dentry->d_inode)
+ continue;
+
spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
change_gid(dentry, gid);
--
2.42.0
