linux-kernel - Re: kernel crash in mknod

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Date: Mon, 25 Mar 2024 17:06:22 +0100
From: Christian Brauner <brauner@...nel.org>
To: Roberto Sassu <roberto.sassu@...wei.com>
Cc: Al Viro <viro@...iv.linux.org.uk>, Steve French <smfrench@...il.com>, 
	LKML <linux-kernel@...r.kernel.org>, linux-fsdevel <linux-fsdevel@...r.kernel.org>, 
	CIFS <linux-cifs@...r.kernel.org>, Paulo Alcantara <pc@...guebit.com>, 
	Christian Brauner <christian@...uner.io>, Mimi Zohar <zohar@...ux.ibm.com>, 
	Paul Moore <paul@...l-moore.com>, 
	"linux-integrity@...r.kernel.org" <linux-integrity@...r.kernel.org>, 
	"linux-security-module@...r.kernel.org" <linux-security-module@...r.kernel.org>
Subject: Re: kernel crash in mknod

On Sun, Mar 24, 2024 at 04:50:24PM +0000, Roberto Sassu wrote:
> > From: Al Viro [mailto:viro@....linux.org.uk] On Behalf Of Al Viro
> > Sent: Sunday, March 24, 2024 6:47 AM
> > On Sun, Mar 24, 2024 at 12:00:15AM -0500, Steve French wrote:
> > > Anyone else seeing this kernel crash in do_mknodat (I see it with a
> > > simple "mkfifo" on smb3 mount).  I started seeing this in 6.9-rc (did
> > > not see it in 6.8).   I did not see it with the 3/12/23 mainline
> > > (early in the 6.9-rc merge Window) but I do see it in the 3/22 build
> > > so it looks like the regression was introduced by:
> > 
> > 	FWIW, successful ->mknod() is allowed to return 0 and unhash
> > dentry, rather than bothering with lookups.  So commit in question
> > is bogus - lack of error does *NOT* mean that you have struct inode
> > existing, let alone attached to dentry.  That kind of behaviour
> > used to be common for network filesystems more than just for ->mknod(),
> > the theory being "if somebody wants to look at it, they can bloody
> > well pay the cost of lookup after dcache miss".
> > 
> > Said that, the language in D/f/vfs.rst is vague as hell and is very easy
> > to misread in direction of "you must instantiate".
> > 
> > Thankfully, there's no counterpart with mkdir - *there* it's not just
> > possible, it's inevitable in some cases for e.g. nfs.
> > 
> > What the hell is that hook doing in non-S_IFREG cases, anyway?  Move it
> > up and be done with it...
> 
> Hi Al
> 
> thanks for the patch. Indeed, it was like that before, when instead of
> an LSM hook there was an IMA call.

Could you please start adding lore links into your commit messages for
all messages that are sent to a mailing list? It really makes tracking
down the original thread a lot easier.

> However, I thought, since we were promoting it as an LSM hook,
> we should be as generic possible, and support more usages than
> what was needed for IMA.

I'm a bit confused now why this is taking a dentry. Nothing in IMA or
EVM cares about the dentry for these hooks so it really should have take
an inode in the first place?

And one minor other question I just realized. Why are some of the new
hooks called security_path_post_mknod() when they aren't actually taking
a path in contrast to say
security_path_{chown,chmod,mknod,chroot,truncate}() that do.