[PATCH v2 1/2] mm/damon/dbgfs: check if rm_contexts input is for a real context

[SeongJae Park <sj@kernel.org>]

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/

Reported-by: syzbot+6087eafb76a94c4ac9eb@syzkaller.appspotmail.com
Fixes: 75c1c2b53c78 ("mm/damon/dbgfs: support multiple contexts")
Cc: <stable@vger.kernel.org> # 5.15.x
Signed-off-by: SeongJae Park <sj@kernel.org>
---
 mm/damon/dbgfs.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/mm/damon/dbgfs.c b/mm/damon/dbgfs.c
index 6f0ae7d3ae39..b3f454a5c682 100644
--- a/mm/damon/dbgfs.c
+++ b/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) {
-- 
2.25.1