Re: [PATCH 1/3] fs: add O_BENEATH flag to openat(2)

[Andy Lutomirski]

On Mon, Nov 3, 2014 at 7:20 AM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> On Mon, Nov 03, 2014 at 11:48:23AM +0000, David Drysdale wrote:
>> Add a new O_BENEATH flag for openat(2) which restricts the
>> provided path, rejecting (with -EACCES) paths that are not beneath
>> the provided dfd.  In particular, reject:
>>  - paths that contain .. components
>>  - paths that begin with /
>>  - symlinks that have paths as above.
>
> Yecch...  The degree of usefulness aside (and I'm not convinced that it
> is non-zero),

This is extremely useful in conjunction with seccomp.

> WTF pass one bit out of nameidata->flags in a separate argument?
> Through the mutual recursion, no less...  And then you are not even attempting
> to detect symlinks that are not followed by interpretation of _any_ pathname.

How many symlinks like that are there?  Is there anything except
nd_jump_link users?  All of those are in /proc.  Arguably O_BENEATH
should prevent traversal of all of those links.

--Andy
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