linux-kernel - Re: [PATCH 2/2] vfs: Fix anon_inode triggering VFS_BUG_ON_INODE in may

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Message-ID: <iufbqsvdp52sgpsjkyulfqgfpvksev3guds5hd556q7zxestgq@ixog46pumnry>
Date: Thu, 3 Apr 2025 04:26:37 +0200
From: Mateusz Guzik <mjguzik@...il.com>
To: Penglei Jiang <superman.xpt@...il.com>
Cc: viro@...iv.linux.org.uk, brauner@...nel.org, jack@...e.cz, 
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, 
	syzbot+5d8e79d323a13aa0b248@...kaller.appspotmail.com
Subject: Re: [PATCH 2/2] vfs: Fix anon_inode triggering VFS_BUG_ON_INODE in
 may_open()

On Wed, Apr 02, 2025 at 06:59:46PM -0700, Penglei Jiang wrote:
> may_open()
> {
>         ...
>         switch (inode->i_mode & S_IFMT) {
>         case S_IFLNK:
>         case S_IFDIR:
>         case S_IFBLK:
>         case S_IFCHR:
>         case S_IFIFO:
>         case S_IFSOCK:
>         case S_IFREG:
>         default:
>                 VFS_BUG_ON_INODE(1, inode);
>         }
>         ...
> }
> 
> Since some anonymous inodes do not have S_IFLNK, S_IFDIR, S_IFBLK,
> S_IFCHR, S_IFIFO, S_IFSOCK, or S_IFREG flags set when created, they
> end up in the default case branch.
> 
> When creating some anon_inode instances, the i_mode in the switch
> statement is not properly set, which causes the open operation to
> follow the default branch when opening anon_inode.
> 
> We could check whether the inode is an anon_inode before VFS_BUG_ON_INODE
> and trigger the assertion only if it's not. However, a more reasonable
> approach might be to set a flag during creation in alloc_anon_inode(),
> such as inode->i_flags |= S_ANON, to explicitly mark anonymous inodes.
> 

I think this is the right step, but there is a missing bit.

The assert was added because of a prior bug report involving ntfs, where
a misconstructed inode did not have an i_mode and was passed to execve.
That eventually landed in may_open() which did not check for MAY_EXEC as
the inode did not fit any of the modes. It was only caught by a
hardening check later.

Note all other cases handle MAY_EXEC.

So I think the safest way forward has 2 steps in the default case:
- detect inodes which purposefully don't have a valid mode and elide the
  assert if so, for example like in your patch
- add the permission check:
	if (acc_mode & MAY_EXEC)
		return -EACCES;

The hardening check comes with a WARN_ON if the inode is not S_ISREG.

No need to hope none of the anon inodes reach it, this should just be
short-circuited.