Fix slab-out-of-bounds Write in dbgfs_rm_context_write

This patchset is for fixing (patch 1) the syzbot-reported slab-out-of-bounds write in dbgfs_rm_context_write[1], and adding a selftest for the bug (patch 2).

[1] https://lore.kernel.org/damon/[email protected]/

SeongJae Park (2):
mm/damon/dbgfs: check if rm_contexts input is for a real context selftests/damon: test non-context inputs to rm_contexts file

 mm/damon/dbgfs.c                              |  7 +++++++
 tools/testing/selftests/damon/Makefile        |  1 +
 .../damon/debugfs_rm_non_contexts.sh          | 19 +++++++++++++++++++
 3 files changed, 27 insertions(+)
 create mode 100755 tools/testing/selftests/damon/debugfs_rm_non_contexts.sh

2.25.1

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/[email protected]/

---
 mm/damon/dbgfs.c | 7 +++++++
 1 file changed, 7 insertions(+)

index 6f0ae7d3ae39..b3f454a5c682 100644
@@ -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

There was a bug[1] that triggered by writing non-context DAMON debugfs file names to the 'rm_contexts' DAMON debugfs file. Add a selftest for the bug to avoid it happen again.

[1] https://lore.kernel.org/damon/[email protected]/

---
 tools/testing/selftests/damon/Makefile        |  1 +
 .../damon/debugfs_rm_non_contexts.sh          | 19 +++++++++++++++++++
 2 files changed, 20 insertions(+)
 create mode 100755 tools/testing/selftests/damon/debugfs_rm_non_contexts.sh

index af490acc5348..838a8e49f77b 100644
@@ -7,6 +7,7 @@ TEST_FILES = _chk_dependency.sh _debugfs_common.sh
 TEST_PROGS = debugfs_attrs.sh debugfs_schemes.sh debugfs_target_ids.sh
 TEST_PROGS += debugfs_empty_targets.sh debugfs_huge_count_read_write.sh
 TEST_PROGS += debugfs_duplicate_context_creation.sh
+TEST_PROGS += debugfs_rm_non_contexts.sh
 TEST_PROGS += sysfs.sh
 TEST_PROGS += reclaim.sh lru_sort.sh
 

new file mode 100755
index 000000000000..48b7af6b022c
@@ -0,0 +1,19 @@
+#!/bin/bash
+# SPDX-License-Identifier: GPL-2.0
+
+source _debugfs_common.sh
+
+# Test putting non-ctx files/dirs to rm_contexts file
+# ===================================================
+
+dmesg -C
+
+for file in "$DBGFS/"*
+do
+	echo "$(basename "$f")" > "$DBGFS/rm_contexts"
+	if dmesg | grep -q BUG
+	then
+		dmesg
+		exit 1
+	fi
+done

2.25.1

Hi Andrew,

May I ask you to merge this fix in your tree if you have no concern for this? I think it deserves stable@.

Thanks,
SJ