linux-ext4 - [PATCH] ext4: fix kernel BUG in ext4_free

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Date:   Wed, 13 Jul 2022 11:59:04 -0700
From:   Tadeusz Struk <tadeusz.struk@...aro.org>
To:     Theodore Ts'o <tytso@....edu>
Cc:     Andreas Dilger <adilger.kernel@...ger.ca>,
        linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org,
        Tadeusz Struk <tadeusz.struk@...aro.org>,
        syzbot+15cd994e273307bf5cfa@...kaller.appspotmail.com
Subject: [PATCH] ext4: fix kernel BUG in ext4_free_blocks

Syzbot reported a BUG in ext4_free_blocks.
The issue is triggered from ext4_mb_clear_bb(). What happens is the
block number passed to ext4_get_group_no_and_offset() is 0 and the
es->s_first_data_block is 1. This makes block group number returned
from ext4_get_group_no_and_offset equal to -1. This is then passed to
ext4_get_group_info() and hits a BUG:
BUG_ON(group >= EXT4_SB(sb)->s_groups_count),
what can be seen in the trace below.
This patch adds an assertion to ext4_get_group_no_and_offset() that
checks if block number is not smaller than es->s_first_data_block.

kernel BUG at fs/ext4/ext4.h:3319!
invalid opcode: 0000 [#1] PREEMPT SMP KASAN
CPU: 0 PID: 337 Comm: repro Not tainted 5.19.0-rc6-00105-g4e8e898e4107-dirty #14
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.0-1.fc36 04/01/2014
RIP: 0010:ext4_mb_clear_bb+0x1bd6/0x1be0
Call Trace:
 <TASK>
 ext4_free_blocks+0x9b3/0xc90
 ext4_clear_blocks+0x344/0x3b0
 ext4_ind_truncate+0x967/0x1050
 ext4_truncate+0xb1b/0x1210
 ext4_evict_inode+0xf06/0x16f0
 evict+0x2a3/0x630
 iput+0x618/0x850
 ext4_enable_quotas+0x578/0x920
 ext4_orphan_cleanup+0x539/0x1200
 ext4_fill_super+0x94d8/0x9bc0
 get_tree_bdev+0x40c/0x630
 ext4_get_tree+0x1c/0x20
 vfs_get_tree+0x88/0x290
 do_new_mount+0x289/0xac0
 path_mount+0x607/0xfd0
 __se_sys_mount+0x2c4/0x3b0
 __x64_sys_mount+0xbf/0xd0
 do_syscall_64+0x3d/0x90
 entry_SYSCALL_64_after_hwframe+0x63/0xcd
 </TASK>

Link: https://syzkaller.appspot.com/bug?id=5266d464285a03cee9dbfda7d2452a72c3c2ae7c
Reported-by: syzbot+15cd994e273307bf5cfa@...kaller.appspotmail.com
Signed-off-by: Tadeusz Struk <tadeusz.struk@...aro.org>
---
 fs/ext4/balloc.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/ext4/balloc.c b/fs/ext4/balloc.c
index 78ee3ef795ae..1175750ad05f 100644
--- a/fs/ext4/balloc.c
+++ b/fs/ext4/balloc.c
@@ -56,6 +56,9 @@ void ext4_get_group_no_and_offset(struct super_block *sb, ext4_fsblk_t blocknr,
 	struct ext4_super_block *es = EXT4_SB(sb)->s_es;
 	ext4_grpblk_t offset;

+	if (blocknr < le32_to_cpu(es->s_first_data_block))
+		blocknr = le32_to_cpu(es->s_first_data_block);
+
 	blocknr = blocknr - le32_to_cpu(es->s_first_data_block);
 	offset = do_div(blocknr, EXT4_BLOCKS_PER_GROUP(sb)) >>
 		EXT4_SB(sb)->s_cluster_bits;
-- 
2.36.1