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Subject: Re: Null-ptr-deref due to "sanitized pathwalk machinery (v4)"
From:   Qian Cai <cai@lca.pw>
In-Reply-To: <20200325055830.GL23230@ZenIV.linux.org.uk>
Date:   Wed, 25 Mar 2020 15:43:06 -0400
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        linux-fsdevel@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>
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To:     Al Viro <viro@zeniv.linux.org.uk>
Sender: linux-kernel-owner@vger.kernel.org
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> On Mar 25, 2020, at 1:58 AM, Al Viro <viro@zeniv.linux.org.uk> wrote:
>=20
> On Wed, Mar 25, 2020 at 04:03:59AM +0000, Al Viro wrote:
>=20
>> Lovely.  So
>> 	* we really do get NULL nd->path.dentry there; I've not misread =
the
>> trace.
>> 	* on the entry into link_path_walk() nd->path.dentry is =
non-NULL.
>> 	* *ALL* components should've been LAST_NORM ones
>> 	* not a single symlink in sight, unless the setup is rather =
unusual
>> 	* possibly not even a single mountpoint along the way (depending
>> upon the userland used)
>=20
> OK, I see one place where that could occur, but I really don't see how =
that
> could be triggered on this pathname, short of very odd symlink layout =
in
> the filesystem on the testbox.  Does the following fix your =
reproducer?
>=20
> diff --git a/fs/namei.c b/fs/namei.c
> index 311e33dbac63..4082b70f32ff 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -1805,6 +1805,8 @@ static const char *handle_dots(struct nameidata =
*nd, int type)
> 			error =3D step_into(nd, WALK_NOFOLLOW,
> 					 parent, inode, seq);
> 		}
> +		if (unlikely(error))
> +			return ERR_PTR(error);
>=20
> 		if (unlikely(nd->flags & LOOKUP_IS_SCOPED)) {
> 			/*

Since that one has a compilation warning, I have tested this patch and =
seen no crash so far.

diff --git a/fs/namei.c b/fs/namei.c
index 311e33dbac63..73851acdbf3a 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1806,6 +1806,9 @@ static const char *handle_dots(struct nameidata =
*nd, int type)
                                         parent, inode, seq);
                }
=20
+               if (unlikely(error))
+                       return error;
+
                if (unlikely(nd->flags & LOOKUP_IS_SCOPED)) {
                        /*
                         * If there was a racing rename or mount along =
our=