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()
Date: Thu, 15 Oct 2020 00:08:12 +0100 [thread overview]
Message-ID: <20201014230812.GK3576660@ZenIV.linux.org.uk> (raw)
In-Reply-To: <20201014204529.934574-1-andrii@kernel.org>
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?
next prev parent reply other threads:[~2020-10-15 1:35 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-14 20:45 [PATCH] fs: fix NULL dereference due to data race in prepend_path() Andrii Nakryiko
2020-10-14 21:12 ` Josef Bacik
2020-10-14 21:49 ` Linus Torvalds
2020-10-14 23:08 ` Al Viro
2020-10-14 23:12 ` Linus Torvalds
2020-10-14 23:08 ` Al Viro [this message]
2020-10-15 1:07 ` Andrii Nakryiko
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