[PATCH 5.13 262/266] ext4: fix possible UAF when remounting r/o a mmp-protected file system

Greg Kroah-Hartman
2021-07-15T19:50:19Z

From: Theodore Ts'o <tytso@mit.edu>

commit 61bb4a1c417e5b95d9edb4f887f131de32e419cb upstream.

After commit 618f003199c6 ("ext4: fix memory leak in
ext4_fill_super"), after the file system is remounted read-only, there
is a race where the kmmpd thread can exit, causing sbi->s_mmp_tsk to
point at freed memory, which the call to ext4_stop_mmpd() can trip
over.

Fix this by only allowing kmmpd() to exit when it is stopped via
ext4_stop_mmpd().

Link: https://lore.kernel.org/r/20210707002433.3719773-1-tytso@mit.edu
Reported-by: Ye Bin <yebin10@huawei.com>
Bug-Report-Link: <20210629143603.2166962-1-yebin10@huawei.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
fs/ext4/mmp.c | 31 +++++++++++++++----------------
fs/ext4/super.c | 6 +++++-
2 files changed, 20 insertions(+), 17 deletions(-)

--- a/fs/ext4/mmp.c
+++ b/fs/ext4/mmp.c
@@ -156,7 +156,12 @@ static int kmmpd(void *data)
memcpy(mmp->mmp_nodename, init_utsname()->nodename,
sizeof(mmp->mmp_nodename));

- while (!kthread_should_stop()) {
+ while (!kthread_should_stop() && !sb_rdonly(sb)) {
+ if (!ext4_has_feature_mmp(sb)) {
+ ext4_warning(sb, "kmmpd being stopped since MMP feature"
+ " has been disabled.");
+ goto wait_to_exit;
+ }
if (++seq > EXT4_MMP_SEQ_MAX)
seq = 1;

@@ -177,16 +182,6 @@ static int kmmpd(void *data)
failed_writes++;
}

- if (!(le32_to_cpu(es->s_feature_incompat) &
- EXT4_FEATURE_INCOMPAT_MMP)) {
- ext4_warning(sb, "kmmpd being stopped since MMP feature"
- " has been disabled.");
- goto exit_thread;
- }
-
- if (sb_rdonly(sb))
- break;
-
diff = jiffies - last_update_time;
if (diff < mmp_update_interval * HZ)
schedule_timeout_interruptible(mmp_update_interval *
@@ -207,7 +202,7 @@ static int kmmpd(void *data)
ext4_error_err(sb, -retval,
"error reading MMP data: %d",
retval);
- goto exit_thread;
+ goto wait_to_exit;
}

mmp_check = (struct mmp_struct *)(bh_check->b_data);
@@ -221,7 +216,7 @@ static int kmmpd(void *data)
ext4_error_err(sb, EBUSY, "abort");
put_bh(bh_check);
retval = -EBUSY;
- goto exit_thread;
+ goto wait_to_exit;
}
put_bh(bh_check);
}
@@ -244,7 +239,13 @@ static int kmmpd(void *data)

retval = write_mmp_block(sb, bh);

-exit_thread:
+wait_to_exit:
+ while (!kthread_should_stop()) {
+ set_current_state(TASK_INTERRUPTIBLE);
+ if (!kthread_should_stop())
+ schedule();
+ }
+ set_current_state(TASK_RUNNING);
return retval;
}

@@ -391,5 +392,3 @@ failed:
brelse(bh);
return 1;
}
-
-
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -5996,7 +5996,6 @@ static int ext4_remount(struct super_blo
*/
ext4_mark_recovery_complete(sb, es);
}
- ext4_stop_mmpd(sbi);
} else {
/* Make sure we can mount this feature set readwrite */
if (ext4_has_feature_readonly(sb) ||
@@ -6110,6 +6109,9 @@ static int ext4_remount(struct super_blo
if (!test_opt(sb, BLOCK_VALIDITY) && sbi->s_system_blks)
ext4_release_system_zone(sb);

+ if (!ext4_has_feature_mmp(sb) || sb_rdonly(sb))
+ ext4_stop_mmpd(sbi);
+
/*
* Some options can be enabled by ext4 and/or by VFS mount flag
* either way we need to make sure it matches in both *flags and
@@ -6143,6 +6145,8 @@ restore_opts:
for (i = 0; i < EXT4_MAXQUOTAS; i++)
kfree(to_free[i]);
#endif
+ if (!ext4_has_feature_mmp(sb) || sb_rdonly(sb))
+ ext4_stop_mmpd(sbi);
kfree(orig_data);
return err;
}