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Date: Sun, 31 Jul 2016 10:55:04 +0000
From: "Reshetova, Elena" <elena.reshetova@...el.com>
To: Jann Horn <jann@...jh.net>, "kernel-hardening@...ts.openwall.com"
<kernel-hardening@...ts.openwall.com>
CC: "linux-security-module@...r.kernel.org"
<linux-security-module@...r.kernel.org>, "keescook@...omium.org"
<keescook@...omium.org>, "spender@...ecurity.net" <spender@...ecurity.net>,
"jmorris@...ei.org" <jmorris@...ei.org>, "Schaufler, Casey"
<casey.schaufler@...el.com>, "Leibowitz, Michael"
<michael.leibowitz@...el.com>, "Roberts, William C"
<william.c.roberts@...el.com>
Subject: RE: [RFC] [PATCH 1/5] path_fchdir and
path_fhandle LSM hooks
On Fri, Jul 29, 2016 at 10:34:36AM +0300, Elena Reshetova wrote:
> This introduces two new LSM hooks operating on paths.
>
> - security_path_fchdir() checks for permission on
> changing working directory. It can be used by
> LSMs concerned on fchdir system call
>I don't think security_path_fchdir() is a good abstraction level. It
neither covers the whole case of "cwd is changed" nor does it cover the
whole case of "someone uses a file descriptor to a directory to look up
stuff outside that directory".
Do you have a suggestion on what can be a good place?
>For example, security_path_fchdir() seems to be intended to prevent the use
of a leaked file descriptor to the outside world for accessing other files
in the outside world.
Yes, this was exactly the use case.
>But this is trivially bypassed by first using openat() directly instead of
fchdir()+open() (something that used to work against grsecurity, but was
fixed quite a while ago).
The way it has been addressed in grsecurity is having a check inside
filename_lookup() , but it doesn't look a very great place for putting a
hook. I was thinking about it , but so far didn't find any other good
alternatives.
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