mm/damon/dbgfs: check if rm_contexts input is for a real context

A user could write a name of a file under 'damon/' debugfs directory,
which is not a user-created context, to 'rm_contexts' file.  In the case,
'dbgfs_rm_context()' just assumes it's the valid DAMON context directory
only if a file of the name exist.  As a result, invalid memory access
could happen as below.  Fix the bug by checking if the given input is for
a directory.  This check can filter out non-context inputs because
directories under 'damon/' debugfs directory can be created via only
'mk_contexts' file.

This bug has found by syzbot[1].

[1] https://lore.kernel.org/damon/000000000000ede3ac05ec4abf8e@google.com/

Link: https://lkml.kernel.org/r/20221107165001.5717-2-sj@kernel.org
Fixes: 75c1c2b5 ("mm/damon/dbgfs: support multiple contexts")
Signed-off-by: SeongJae Park <sj@kernel.org>
Reported-by: syzbot+6087eafb76a94c4ac9eb@syzkaller.appspotmail.com
Cc: <stable@vger.kernel.org>	[5.15.x]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/damon/dbgfs.c

@@ -890,6 +890,7 @@ static ssize_t dbgfs_mk_context_write(struct file *file,
 static int dbgfs_rm_context(char *name)
 {
 	struct dentry *root, *dir, **new_dirs;
+	struct inode *inode;
 	struct damon_ctx **new_ctxs;
 	int i, j;
 	int ret = 0;
@@ -905,6 +906,12 @@ static int dbgfs_rm_context(char *name)
 	if (!dir)
 		return -ENOENT;
 
+	inode = d_inode(dir);
+	if (!S_ISDIR(inode->i_mode)) {
+		ret = -EINVAL;
+		goto out_dput;
+	}
+
 	new_dirs = kmalloc_array(dbgfs_nr_ctxs - 1, sizeof(*dbgfs_dirs),
 			GFP_KERNEL);
 	if (!new_dirs) {