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Date:   Wed, 25 Mar 2020 05:58:30 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Qian Cai <cai@lca.pw>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        linux-fsdevel@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>
Subject: Re: Null-ptr-deref due to "sanitized pathwalk machinery (v4)"
Message-ID: <20200325055830.GL23230@ZenIV.linux.org.uk>
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On Wed, Mar 25, 2020 at 04:03:59AM +0000, Al Viro wrote:

> 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)

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?

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 = step_into(nd, WALK_NOFOLLOW,
 					 parent, inode, seq);
 		}
+		if (unlikely(error))
+			return ERR_PTR(error);
 
 		if (unlikely(nd->flags & LOOKUP_IS_SCOPED)) {
 			/*