linux-kernel - [PATCH 2/2] CRED: Guard the setprocattr security hook against ptrace

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Message-ID: <20090508125527.8488.82217.stgit@warthog.procyon.org.uk>
Date:	Fri, 08 May 2009 13:55:27 +0100
From:	David Howells <dhowells@...hat.com>
To:	roland@...hat.com, jmorris@...ei.org, chrisw@...s-sol.org,
	oleg@...hat.com
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	linux-security-module@...r.kernel.org, eparis@...hat.com,
	sds@...ho.nsa.gov, David Howells <dhowells@...hat.com>
Subject: [PATCH 2/2] CRED: Guard the setprocattr security hook against ptrace

Guard the setprocattr security hook against ptrace by taking the target task's
cred_guard_mutex around it.  The problem is that setprocattr() may otherwise
note the lack of a debugger, and then perform an action on that basis whilst
letting a debugger attach between the two points.  Holding cred_guard_mutex
across the test and the action prevents ptrace_attach() from doing that.

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

 fs/proc/base.c |    6 ++++++
 1 files changed, 6 insertions(+), 0 deletions(-)


diff --git a/fs/proc/base.c b/fs/proc/base.c
index fb45615..23342e1 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -2128,9 +2128,15 @@ static ssize_t proc_pid_attr_write(struct file * file, const char __user * buf,
 	if (copy_from_user(page, buf, count))
 		goto out_free;
 
+	/* Guard against adverse ptrace interaction */
+	length = mutex_lock_interruptible(&task->cred_guard_mutex);
+	if (length < 0)
+		goto out_free;
+
 	length = security_setprocattr(task,
 				      (char*)file->f_path.dentry->d_name.name,
 				      (void*)page, count);
+	mutex_unlock(&task->cred_guard_mutex);
 out_free:
 	free_page((unsigned long) page);
 out:

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