linux-kernel - Re: [PATCH 1/2] quota: fix null-ptr-deref in ext4_acquire

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Date:   Mon, 5 Jun 2023 13:51:43 +0200
From:   Jan Kara <jack@...e.cz>
To:     "yebin (H)" <yebin10@...wei.com>
Cc:     Jan Kara <jack@...e.cz>, jack@...e.com,
        linux-kernel@...r.kernel.org,
        syzbot+e633c79ceaecbf479854@...kaller.appspotmail.com
Subject: Re: [PATCH 1/2] quota: fix null-ptr-deref in ext4_acquire_dquot()

On Sat 03-06-23 14:14:11, yebin (H) wrote:
> 
> 
> On 2023/5/30 17:57, Jan Kara wrote:
> > On Sat 27-05-23 09:40:17, Ye Bin wrote:
> > > Syzbot found the following issue:
> > > Unable to handle kernel paging request at virtual address dfff800000000005
> > > KASAN: null-ptr-deref in range [0x0000000000000028-0x000000000000002f]
> > ...
> > > CPU: 0 PID: 6080 Comm: syz-executor747 Not tainted 6.3.0-rc7-syzkaller-g14f8db1c0f9a #0
> > > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/14/2023
> > > pstate: 80400005 (Nzcv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
> > > pc : ext4_acquire_dquot+0x1d4/0x398 fs/ext4/super.c:6766
> > > lr : dquot_to_inode fs/ext4/super.c:6740 [inline]
> > > lr : ext4_acquire_dquot+0x1ac/0x398 fs/ext4/super.c:6766
> > OK, this is bad...
> > 
> > > Above issue may happens as follows:
> > > ProcessA              ProcessB                    ProcessC
> > > sys_fsconfig
> > >    vfs_fsconfig_locked
> > >     reconfigure_super
> > >       ext4_remount
> > >        dquot_suspend -> suspend all type quota
> > > 
> > >                   sys_fsconfig
> > > 		  vfs_fsconfig_locked
> > > 		    reconfigure_super
> > > 		     ext4_remount
> > > 		      dquot_resume
> > > 		       ret = dquot_load_quota_sb
> > >                          add_dquot_ref
> > > 		                           do_open  -> open file O_RDWR
> > > 					    vfs_open
> > > 					     do_dentry_open
> > > 					      get_write_access
> > > 					       atomic_inc_unless_negative(&inode->i_writecount)
> > >                                                ext4_file_open
> > > 					       dquot_file_open
> > > 					        dquot_initialize
> > > 						  __dquot_initialize
> > > 						   dqget
> > > 						    if (!test_bit(DQ_ACTIVE_B, &dquot->dq_flags))
> > > 
> > > 			  __dquot_initialize
> > > 			   __dquot_initialize
> > > 			    dqget
> > > 			     if (!test_bit(DQ_ACTIVE_B, &dquot->dq_flags))
> > > 	                       ext4_acquire_dquot -> Return error
> > > 		       if (ret < 0)
> > > 	                 vfs_cleanup_quota_inode
> > > 			  dqopt->files[type] = NULL;
> > But I don't see how this can happen. The code in dquot_load_quota_sb()
> > looks like:
> > 
> >          error = add_dquot_ref(sb, type);
> >          if (error)
> >                  dquot_disable(sb, type, flags);
> > 
> > So if an error happens in add_dquot_ref(), we'll call dquot_disable().
> > dquot_disable() then does:
> > 
> >                  drop_dquot_ref(sb, cnt);
> >                  invalidate_dquots(sb, cnt);
> > 
> > and invalidate_dquots() waits for reference count of all dquots to drop to
> > 0. Hence if dqget() returned a dquot pointer to ProcessC, then ProcessB
> > should wait until ProcessC drops the dquot reference (hence
> > ext4_acquire_dquot() is done).
> > 
> > What am I missing?
> > 
> > 								Honza
> My reproduction condition is:
> mkfs.ext4 -F /dev/sda
> tune2fs  -Q usrquota /dev/sda
> 
> dquot_disable
> ...
>          if ((flags & DQUOT_USAGE_ENABLED && !(flags &
> DQUOT_LIMITS_ENABLED))
>              || (flags & DQUOT_SUSPENDED && flags & (DQUOT_LIMITS_ENABLED |
>              DQUOT_USAGE_ENABLED)))
>                  return -EINVAL;
> ...
> If without enable DQUOT_LIMITS_ENABLED dquot_disable() will just return
> -EINVAL.

Aha, that is the bug! Does attached patch fix your problem?

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR

View attachment "0001-quota-Properly-disable-quotas-when-add_dquot_ref-fai.patch" of type "text/x-patch" (1302 bytes)