linux-kernel - Re: [PATCH] CRED: Fix check_unsafe

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Date:	Thu, 12 Mar 2009 13:23:41 +0000
From:	David Howells <dhowells@...hat.com>
To:	Hugh Dickins <hugh@...itas.com>
Cc:	dhowells@...hat.com, jmalicki@...acarta.com, chrisw@...s-sol.org,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	linux-security-module@...r.kernel.org
Subject: Re: [PATCH] CRED: Fix check_unsafe_exec()

Hugh Dickins <hugh@...itas.com> wrote:

> We do.  See the original thread.  It's here at
> http://lkml.org/lkml/2009/2/26/233
> and appended below for convenience.  We do know that patch did not
> fix Joe's problem, and we don't yet know whether addressing the
> files->count issue will actually fix it, but I'm hopeful.

Looks reasonable.  One thing that should be added, though, is a comment in
struct fs_struct to give a warning about the consequences of incrementing the
usage count for anything other than CLONE_FS.

David
---
From: David Howells <dhowells@...hat.com>
Subject: [PATCH] Annotate struct fs_struct's usage count to indicate the restrictions upon it

Annotate struct fs_struct's usage count to indicate the restrictions upon it.
It may not be incremented, except by clone(CLONE_FS), as this affects the
check in check_unsafe_exec() in fs/exec.c.

Signed-off-by: David Howells <dhowells@...hat.com>
---

 include/linux/fs_struct.h |    6 +++++-
 1 files changed, 5 insertions(+), 1 deletions(-)


diff --git a/include/linux/fs_struct.h b/include/linux/fs_struct.h
index a97c053..b12ede4 100644
--- a/include/linux/fs_struct.h
+++ b/include/linux/fs_struct.h
@@ -4,7 +4,11 @@
 #include <linux/path.h>
 
 struct fs_struct {
-	atomic_t count;
+	atomic_t count;	/* This usage count is used by check_unsafe_exec() for
+			 * security checking purposes - therefore it may not be
+			 * incremented, except by clone(CLONE_FS).
+			 */
+
 	rwlock_t lock;
 	int umask;
 	struct path root, pwd;
--
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