memory leak in ext4_multi_mount_protect

memory leak in ext4_multi_mount_protect

[Pavel Skripkin]

Hi!

I've done debugging on this issue
https://syzkaller.appspot.com/bug?id=420258a304e5d92cfef6b0097f87b42506e1db08
and I want to ask you about 
proper way of fixing it. The problem was in case sbi->s_mmp_tsk hasn’t
started at the time of kthread_stop() call. In that case allocated data
won't be freed.

I wrote fix patch, but I am confused about it, because I didn't find
any kernel code like this. I don't think, that adding new members to
struct super_block is good idea, that's why I came to that decision: 

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index b9693680463a..9c33e97bd5c5 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -5156,8 +5156,10 @@ static int ext4_fill_super(struct super_block
*sb, void *data, int silent)
 failed_mount3:
 	flush_work(&sbi->s_error_work);
 	del_timer_sync(&sbi->s_err_report);
-	if (sbi->s_mmp_tsk)
-		kthread_stop(sbi->s_mmp_tsk);
+	if (sbi->s_mmp_tsk) {
+		if (kthread_stop(sbi->s_mmp_tsk) == -EINTR)
+			kfree(kthread_data(sbi->s_mmp_tsk));
+	}
 failed_mount2:
 	rcu_read_lock();
 	group_desc = rcu_dereference(sbi->s_group_desc);


I look forward to hearing your perspective on this patch :)

With regards,
Pavel Skripkin

memory leak in ext4_multi_mount_protect

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    467f8165 Merge tag 'close-range-cloexec-unshare-v5.11' of ..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=12b7fccb500000
kernel config:  https://syzkaller.appspot.com/x/.config?x=37c889fb8b2761af
dashboard link: https://syzkaller.appspot.com/bug?extid=d9e482e303930fa4f6ff
compiler:       gcc (GCC) 10.1.0-syz 20200507
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1230f8a7500000

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+d9e482e303930fa4f6ff@syzkaller.appspotmail.com

BUG: memory leak
unreferenced object 0xffff88812560f120 (size 32):
  comm "syz-executor.3", pid 11391, jiffies 4294966956 (age 10.520s)
  hex dump (first 32 bytes):
    28 2a e4 20 81 88 ff ff 00 f8 32 24 81 88 ff ff  (*. ......2$....
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  backtrace:
    [<000000001fd6256c>] kmalloc include/linux/slab.h:552 [inline]
    [<000000001fd6256c>] ext4_multi_mount_protect+0x4a6/0x5d0 fs/ext4/mmp.c:367
    [<00000000ab3084f2>] ext4_fill_super+0x551e/0x5ac0 fs/ext4/super.c:4779
    [<00000000b7304a28>] mount_bdev+0x223/0x260 fs/super.c:1366
    [<00000000b580b323>] legacy_get_tree+0x2b/0x90 fs/fs_context.c:592
    [<000000005310f7d7>] vfs_get_tree+0x28/0x100 fs/super.c:1496
    [<000000006fc429ab>] do_new_mount fs/namespace.c:2875 [inline]
    [<000000006fc429ab>] path_mount+0xc5e/0x1170 fs/namespace.c:3205
    [<000000004f8c23d3>] do_mount fs/namespace.c:3218 [inline]
    [<000000004f8c23d3>] __do_sys_mount fs/namespace.c:3426 [inline]
    [<000000004f8c23d3>] __se_sys_mount fs/namespace.c:3403 [inline]
    [<000000004f8c23d3>] __x64_sys_mount+0x18e/0x1d0 fs/namespace.c:3403
    [<000000002cff8f95>] do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
    [<00000000779cd3d5>] entry_SYSCALL_64_after_hwframe+0x44/0xa9



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