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/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>

diff --git a/fs/ext4/mmp.c b/fs/ext4/mmp.c
index 6cb598b549ca169af7bc533c87298ff369b0d539..bc364c119af6ac00bcfedcec131af0178ecfa428 100644 (file)
--- 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 @@ 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 cdbe71d935e8f5c3b4f2d03324404434920b0c5d..b8ff0399e171d19ae3b07c743099d7a88dc16446 100644 (file)
--- 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;
 }