Re: Null-ptr-deref due to "sanitized pathwalk machinery (v4)"

[Qian Cai <cai@lca.pw>]

> On Mar 25, 2020, at 1:58 AM, Al Viro <viro@zeniv.linux.org.uk> wrote:
> 
> 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)) {
> 			/*

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);
                }
 
+               if (unlikely(error))
+                       return error;
+
                if (unlikely(nd->flags & LOOKUP_IS_SCOPED)) {
                        /*
                         * If there was a racing rename or mount along our