Re: [PATCH 1/2] quota: fix null-ptr-deref in ext4_acquire_dquot()

[yebin (H)]

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.
> > 			                              ext4_acquire_dquot
> > 						       -->dquot_to_inode(dquot) is NULL
> >
> > To solve above issue, if quota has been loaded, there's unneed to cleaup quota
> > inode if dquot_load_quota_sb() return failed when do dquot_resume();
> >
> > Reported-by: syzbot+e633c79ceaecbf479854@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Signed-off-by: Ye Bin <yebin10@xxxxxxxxxx>
> > ---
> >   fs/quota/dquot.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/fs/quota/dquot.c b/fs/quota/dquot.c
> > index ffd40dc3e4e9..4b913faa48ec 100644
> > --- a/fs/quota/dquot.c
> > +++ b/fs/quota/dquot.c
> > @@ -2476,7 +2476,7 @@ int dquot_resume(struct super_block *sb, int type)
> >   		flags = dquot_generic_flag(flags, cnt);
> >   		ret = dquot_load_quota_sb(sb, cnt, dqopt->info[cnt].dqi_fmt_id,
> >   					  flags);
> > -		if (ret < 0)
> > +		if (ret < 0 && !sb_has_quota_loaded(sb, cnt))
> >   			vfs_cleanup_quota_inode(sb, cnt);
> >   	}
> >   
> > --
> > 2.31.1