linux-kernel - Re: [PATCH] bfs: put a inode if link count is 0

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <20250109073950.2142484-1-lizhi.xu@windriver.com>
Date: Thu, 9 Jan 2025 15:39:50 +0800
From: Lizhi Xu <lizhi.xu@...driver.com>
To: <viro@...iv.linux.org.uk>
CC: <aivazian.tigran@...il.com>, <linux-kernel@...r.kernel.org>,
        <lizhi.xu@...driver.com>,
        <syzbot+80e60df48923e1b7691d@...kaller.appspotmail.com>,
        <syzkaller-bugs@...glegroups.com>
Subject: Re: [PATCH] bfs: put a inode if link count is 0

On Thu, 9 Jan 2025 07:32:28 +0000, Al Viro wrote:
> > On Thu, 9 Jan 2025 06:22:16 +0000, Al Viro wrote:
> > > > The reproducer performs the rename operation on the file twice in succession
> > > > and changes the file to the same file name.  After the first rename operation,
> > > > the number of links in the inode is set to 0. In the second execution, the
> > > > same inode is used, resulting in a 0 value warning for i_nlink.
> > > >
> > > > To avoid this issue, put the target inode before exiting the bfs_rename.
> > >
> > >         This is completely insane - you get an extra drop of in-core inode
> > > refcount, which *will* end up with dangling pointer and memory corruption.
> > > Besides, there is a perfectly legitimate case when you open a file and
> > > rename something on top of it.  It MUST remain open and alive until the
> > > last in-core reference to inode goes away, which must not happen before
> > > close().
> > In the reproducer, changes the file to the same file, the same file name is
> > "file0", file0 uses mknod to create its inode and sets the i_nlink value to 1.
> > There is no operation to open file0 in the reproducer. Is this situation also
> > as you said?
> 
> I'm not sure I understand your sentence, to be honest.
> 
> Your patch is 100% wrong - you must *not*, under any circumstances, have
> ->rename() drop references to in-core struct inode instances.  It's *always*
> wrong; the reference to new_inode in new_dentry->d_inode remains there (as
> it ought to) and its contribution to new_inode refcount remains unchanged.
> It has nothing to do with ->i_nlink; you are decrementing ->i_count, which
> controls the lifetime of in-core struct inode instance.  As soon as that
> reaches zero, struct inode instance will be freed.  And destructor of
> new_dentry *will* call iput() on its ->d_inode, so you'll end up with
> attempt to decrement refcount of already freed memory object.
> 
> Again, that has nothing to do with ->i_nlink.  I'm not familiar with
> bfs layout, so I can't tell what's going on with the corrupted image syzbot
> are messing with just by visual examination.  Is there, by any chance,
> a preexisting file0 in the root of that bfs image?  Does mknod() succeed
> there?  Because if it doesn't and what you have is a buggered image with
> 'file0' being there having zero link count, yes, renaming over it would
> trigger warnings about detected corrupted filesystem - we are trying to
> remove a link (on disk) to something that claims (on disk) to have 0 links
> at the time of that operation; something is clearly wrong and deserves
> a warning.
> 
> Again, that iput() in there is basically introducing random memory
> corruption; it might make the warning go away, but it's not a fix.
I also have deep doubts about using iput in rename.
Thanks for your analysis.

Lizhi