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SubjectRe: [PATCH 1/2] ext4: Fix use-after-free about sbi->s_mmp_tsk
On Mon, Jul 05, 2021 at 01:15:48PM +0200, Jan Kara wrote:
>
> That being said for this scheme spinlock is enough, you don't need a mutex
> for s_mmp_lock.

I think we can solve this without using using either a spinlock or a
mutex, and it's a smaller and simpler patch as a result. (This is the
-v2 version of this patch, which removes an unused label compared to
the earlier version.)

From 22ebc97aac75e27a5fd11acdb2bc3030d1da58d1 Mon Sep 17 00:00:00 2001
From: Theodore Ts'o <tytso@mit.edu>
Date: Fri, 2 Jul 2021 12:45:02 -0400
Subject: [PATCH] ext4: fix possible UAF when remounting r/o a mmp-protected file system

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/e525c0bf7b18da426bb3d3dd63830a3f85218a9e.1625244710.git.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>
---
fs/ext4/mmp.c | 33 +++++++++++++++++----------------
fs/ext4/super.c | 6 +++++-
2 files changed, 22 insertions(+), 17 deletions(-)

diff --git a/fs/ext4/mmp.c b/fs/ext4/mmp.c
index 6cb598b549ca..1e95cee3d8b7 100644
--- a/fs/ext4/mmp.c
+++ b/fs/ext4/mmp.c
@@ -157,6 +157,17 @@ static int kmmpd(void *data)
sizeof(mmp->mmp_nodename));

while (!kthread_should_stop()) {
+ 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 wait_to_exit;
+ }
+ if (sb_rdonly(sb)) {
+ if (!kthread_should_stop())
+ schedule_timeout_interruptible(HZ);
+ continue;
+ }
if (++seq > EXT4_MMP_SEQ_MAX)
seq = 1;

@@ -177,16 +188,6 @@ 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 +208,7 @@ 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 +222,7 @@ 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);
}
@@ -242,9 +243,11 @@ static int kmmpd(void *data)
mmp->mmp_seq = cpu_to_le32(EXT4_MMP_SEQ_CLEAN);
mmp->mmp_time = cpu_to_le64(ktime_get_real_seconds());

- retval = write_mmp_block(sb, bh);
+ return write_mmp_block(sb, bh);

-exit_thread:
+wait_to_exit:
+ while (!kthread_should_stop())
+ schedule();
return retval;
}

@@ -391,5 +394,3 @@ int ext4_multi_mount_protect(struct super_block *sb,
brelse(bh);
return 1;
}
-
-
diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index cdbe71d935e8..b8ff0399e171 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -5993,7 +5993,6 @@ static int ext4_remount(struct super_block *sb, int *flags, char *data)
*/
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) ||
@@ -6107,6 +6106,9 @@ static int ext4_remount(struct super_block *sb, int *flags, char *data)
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
@@ -6140,6 +6142,8 @@ static int ext4_remount(struct super_block *sb, int *flags, char *data)
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;
}
--
2.31.0
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