From 3ba733f879c2a88910744647e41edeefbc0d92b2 Mon Sep 17 00:00:00 2001
From: Jan Kara <jack@suse.cz>
Date: Wed, 18 May 2022 11:33:29 +0200
Subject: [PATCH] ext4: avoid cycles in directory h-tree
Git-commit: 3ba733f879c2a88910744647e41edeefbc0d92b2
Patch-mainline: v5.19-rc1
References: bsc#1198577 CVE-2022-1184

A maliciously corrupted filesystem can contain cycles in the h-tree
stored inside a directory. That can easily lead to the kernel corrupting
tree nodes that were already verified under its hands while doing a node
split and consequently accessing unallocated memory. Fix the problem by
verifying traversed block numbers are unique.

Cc: stable@vger.kernel.org
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20220518093332.13986-2-jack@suse.cz
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Acked-by: Jan Kara <jack@suse.cz>

---
 fs/ext4/namei.c |   22 +++++++++++++++++++---
 1 file changed, 19 insertions(+), 3 deletions(-)

--- a/fs/ext4/namei.c
+++ b/fs/ext4/namei.c
@@ -734,12 +734,14 @@ static struct dx_frame *
 dx_probe(struct ext4_filename *fname, struct inode *dir,
 	 struct dx_hash_info *hinfo, struct dx_frame *frame_in)
 {
-	unsigned count, indirect;
+	unsigned count, indirect, level, i;
 	struct dx_entry *at, *entries, *p, *q, *m;
 	struct dx_root *root;
 	struct dx_frame *frame = frame_in;
 	struct dx_frame *ret_err = ERR_PTR(ERR_BAD_DX_DIR);
 	u32 hash;
+	ext4_lblk_t block;
+	ext4_lblk_t blocks[2];
 
 	frame->bh = ext4_read_dirblock(dir, 0, INDEX);
 	if (IS_ERR(frame->bh))
@@ -788,6 +790,8 @@ dx_probe(struct ext4_filename *fname, st
 	}
 
 	dxtrace(printk("Look up %x", hash));
+	level = 0;
+	blocks[0] = 0;
 	while (1) {
 		count = dx_get_count(entries);
 		if (!count || count > dx_get_limit(entries)) {
@@ -829,15 +833,27 @@ dx_probe(struct ext4_filename *fname, st
 			       dx_get_block(at)));
 		frame->entries = entries;
 		frame->at = at;
-		if (!indirect--)
+
+		block = dx_get_block(at);
+		for (i = 0; i <= level; i++) {
+			if (blocks[i] == block) {
+				ext4_warning_inode(dir,
+					"dx entry: tree cycle block %u points back to block %u",
+					blocks[level], block);
+				goto fail;
+			}
+		}
+		if (++level > indirect)
 			return frame;
+		blocks[level] = block;
 		frame++;
-		frame->bh = ext4_read_dirblock(dir, dx_get_block(at), INDEX);
+		frame->bh = ext4_read_dirblock(dir, block, INDEX);
 		if (IS_ERR(frame->bh)) {
 			ret_err = (struct dx_frame *) frame->bh;
 			frame->bh = NULL;
 			goto fail;
 		}
+
 		entries = ((struct dx_node *) frame->bh->b_data)->entries;
 
 		if (dx_get_limit(entries) != dx_node_limit(dir)) {