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Message-ID: <20210705111548.GD15373@quack2.suse.cz>
Date: Mon, 5 Jul 2021 13:15:48 +0200
From: Jan Kara <jack@...e.cz>
To: Ye Bin <yebin10@...wei.com>
Cc: tytso@....edu, adilger.kernel@...ger.ca,
linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org,
jack@...e.cz
Subject: Re: [PATCH 1/2] ext4: Fix use-after-free about sbi->s_mmp_tsk
On Tue 29-06-21 22:36:02, Ye Bin wrote:
> After merge 618f003199c6("ext4: fix memory leak in ext4_fill_super") commit,
> we add delay in ext4_remount after "sb->s_flags |= SB_RDONLY", then
> remount filesystem with read-only kasan report following warning:
> [ 888.695326] ==================================================================
> [ 888.696566] BUG: KASAN: use-after-free in kthread_stop+0x4c/0x2f0
> [ 888.697599] Write of size 4 at addr ffff8883849e0020 by task mount/2013
> [ 888.698707]
> [ 888.698982] CPU: 4 PID: 2013 Comm: mount Not tainted 4.19.95-00013-ga369a6189bbf-dirty #413
> [ 888.700376] Hardware name: QEMU Standard PC
> [ 888.702587] Call Trace:
> [ 888.703017] dump_stack+0x108/0x15f
> [ 888.703615] print_address_description+0xa5/0x372
> [ 888.704420] kasan_report.cold+0x236/0x2a8
> [ 888.705761] check_memory_region+0x240/0x270
> [ 888.706486] kasan_check_write+0x20/0x30
> [ 888.707156] kthread_stop+0x4c/0x2f0
> [ 888.707776] ext4_stop_mmpd+0x32/0x90
> [ 888.708262] ext4_remount.cold+0xf6/0x116
> [ 888.712671] do_remount_sb+0xff/0x460
> [ 888.714007] do_mount+0xce3/0x1be0
> [ 888.717749] ksys_mount+0xb2/0x150
> [ 888.718163] __x64_sys_mount+0x6a/0x80
> [ 888.718607] do_syscall_64+0xd9/0x1f0
> [ 888.719047] entry_SYSCALL_64_after_hwframe+0x44/0xa9
>
> As kmmpd will exit if filesystem is read-only. Then sbi->s_mmp_tsk become wild
> ptr, lead to use-after-free. As kmmpd will exit by others(call ktread_stop)
> or by itself. After 618f003199c6 commit we can trigger this issue very easy.
> Before this commit also exist this issue.
> If kmmpd exit by itself, after merge 618f003199c6 commit there will trigger UAF
> when umount filesystem.
> To fix this issue, introduce sbi->s_mmp_lock to protect sbi->s_mmp_tsk. If kmmpd
> exit by itself, we set sbi->s_mmp_tsk with NULL, and release mmp buffer_head.
>
> Fixes: 618f003199c6 ("ext4: fix memory leak in ext4_fill_super")
> Signed-off-by: Ye Bin <yebin10@...wei.com>
> ---
> fs/ext4/ext4.h | 1 +
> fs/ext4/mmp.c | 24 ++++++++++++++++++++++--
> fs/ext4/super.c | 1 +
> 3 files changed, 24 insertions(+), 2 deletions(-)
>
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index 8d3446746718..a479da37fed4 100644
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -1489,6 +1489,7 @@ struct ext4_sb_info {
> struct completion s_kobj_unregister;
> struct super_block *s_sb;
> struct buffer_head *s_mmp_bh;
> + struct mutex s_mmp_lock;
>
> /* Journaling */
> struct journal_s *s_journal;
> diff --git a/fs/ext4/mmp.c b/fs/ext4/mmp.c
> index 6cb598b549ca..fc18a8c205c7 100644
> --- a/fs/ext4/mmp.c
> +++ b/fs/ext4/mmp.c
> @@ -128,8 +128,9 @@ void __dump_mmp_msg(struct super_block *sb, struct mmp_struct *mmp,
> static int kmmpd(void *data)
> {
> struct super_block *sb = (struct super_block *) data;
> - struct ext4_super_block *es = EXT4_SB(sb)->s_es;
> - struct buffer_head *bh = EXT4_SB(sb)->s_mmp_bh;
> + struct ext4_sb_info *sbi = EXT4_SB(sb);
> + struct ext4_super_block *es = sbi->s_es;
> + struct buffer_head *bh = sbi->s_mmp_bh;
> struct mmp_struct *mmp;
> ext4_fsblk_t mmp_block;
> u32 seq = 0;
> @@ -245,16 +246,35 @@ static int kmmpd(void *data)
> retval = write_mmp_block(sb, bh);
>
> exit_thread:
> + /*
> + * Maybe s_mmp_tsk kthread is stoped by others or by itself. If exit
> + * by itself then sbi->s_mmp_tsk will be wild ptr, so there is need
> + * set sbi->s_mmp_tsk with NULL, and also release mmp buffer_head.
> + */
> + while (!kthread_should_stop()) {
> + if (!mutex_trylock(&sbi->s_mmp_lock))
> + continue;
> +
> + if (sbi->s_mmp_tsk) {
> + sbi->s_mmp_tsk = NULL;
> + brelse(bh);
> + }
> + mutex_unlock(&sbi->s_mmp_lock);
> + break;
> + }
> +
> return retval;
> }
Honestly, this is just ugly. I think it would be saner if ext4_stop_mmpd()
did:
void ext4_stop_mmpd(struct ext4_sb_info *sbi)
{
struct task_struct *tsk;
mutex_lock(&sbi->s_mmp_lock);
tsk = sbi->s_mmp_tsk;
if (tsk) {
sbi->s_mmp_tsk = NULL;
get_task_struct(tsk);
}
mutex_unlock(&sbi->s_mmp_lock);
if (tsk) {
kthread_stop(tsk);
put_task_struct(k);
}
}
And you leave freeing of 'bh' to exit path from kmmpd() which can also use
normal locking scheme for zeroing s_mmp_tsk now.
That being said for this scheme spinlock is enough, you don't need a mutex
for s_mmp_lock.
Honza
> void ext4_stop_mmpd(struct ext4_sb_info *sbi)
> {
> + mutex_lock(&sbi->s_mmp_lock);
> if (sbi->s_mmp_tsk) {
> kthread_stop(sbi->s_mmp_tsk);
> brelse(sbi->s_mmp_bh);
> sbi->s_mmp_tsk = NULL;
> }
> + mutex_unlock(&sbi->s_mmp_lock);
> }
>
> /*
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index c3942804b57f..5bc3230553fb 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -4786,6 +4786,7 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
> needs_recovery = (es->s_last_orphan != 0 ||
> ext4_has_feature_journal_needs_recovery(sb));
>
> + mutex_init(&sbi->s_mmp_lock);
> if (ext4_has_feature_mmp(sb) && !sb_rdonly(sb))
> if (ext4_multi_mount_protect(sb, le64_to_cpu(es->s_mmp_block)))
> goto failed_mount3a;
> --
> 2.31.1
>
--
Jan Kara <jack@...e.com>
SUSE Labs, CR
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