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