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Date:   Thu, 15 Oct 2020 00:08:12 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Andrii Nakryiko <andrii@kernel.org>
Cc:     linux-fsdevel@vger.kernel.org, torvalds@linux-foundation.org,
        kernel-team@fb.com, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] fs: fix NULL dereference due to data race in
 prepend_path()
Message-ID: <20201014230812.GK3576660@ZenIV.linux.org.uk>
References: <20201014204529.934574-1-andrii@kernel.org>
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On Wed, Oct 14, 2020 at 01:45:28PM -0700, Andrii Nakryiko wrote:
> Fix data race in prepend_path() with re-reading mnt->mnt_ns twice without
> holding the lock. is_mounted() does check for NULL, but is_anon_ns(mnt->mnt_ns)
> might re-read the pointer again which could be NULL already, if in between
> reads one of kern_unmount()/kern_unmount_array()/umount_tree() sets mnt->mnt_ns
> to NULL.

Cute...  What config/compiler has resulted in that?  I agree with the analysis, but
I really hate the open-coded (and completely unexplained) use of IS_ERR_OR_NULL()
there.

> -			if (is_mounted(vfsmnt) && !is_anon_ns(mnt->mnt_ns))
> +			mnt_ns = READ_ONCE(mnt->mnt_ns);
> +			/* open-coded is_mounted() to use local mnt_ns */
> +			if (!IS_ERR_OR_NULL(mnt_ns) && !is_anon_ns(mnt_ns))
>  				error = 1;	// absolute root
>  			else
>  				error = 2;	// detached or not attached yet

Better turn that into an inlined helper in fs/mount.h, next to is_mounted(), IMO,
and kill that IS_ERR_OR_NULL garbage there.  What that thing does is
	if (ns == NULL || ns == MNT_NS_INTERNAL)
and it's *not* on any kind of hot path to warrant that kind of microoptimizations.

So let's make that

static inline bool is_real_ns(struct mnt_namespace *mnt_ns)
{
	return mnt_ns && mnt_ns != MNT_NS_INTERNAL;
}

turn is_mounted(m) into is_real_ns(m->mnt_ns) and replace that line in your fix
with
			if (is_real_ns(mnt_ns) && !is_anon_ns(mnt_ns))

Objections?