linux-kernel - [PATCH] proc: Update inode upon changing task security attribute

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Message-ID: <20231130003704.31928-1-kamatam@amazon.com>
Date:   Thu, 30 Nov 2023 00:37:04 +0000
From:   Munehisa Kamata <kamatam@...zon.com>
To:     <linux-fsdevel@...r.kernel.org>,
        <linux-security-module@...r.kernel.org>
CC:     <linux-kernel@...r.kernel.org>, <akpm@...ux-foundation.org>,
        "Munehisa Kamata" <kamatam@...zon.com>
Subject: [PATCH] proc: Update inode upon changing task security attribute

I'm not clear whether VFS is a better (or worse) place[1] to fix the
problem described below and would like to hear opinion.

If the /proc/[pid] directory is bind-mounted on a system with Smack
enabled, and if the task updates its current security attribute, the task
may lose access to files in its own /proc/[pid] through the mountpoint.

 $ sudo capsh --drop=cap_mac_override --
 # mkdir -p dir
 # mount --bind /proc/$$ dir
 # echo AAA > /proc/$$/task/current		# assuming built-in echo
 # cat /proc/$$/task/current			# revalidate
 AAA
 # echo BBB > dir/attr/current
 # cat dir/attr/current
 cat: dir/attr/current: Permission denied
 # ls dir/
 ls: cannot access dir/: Permission denied
 # cat /proc/$$/attr/current			# revalidate
 BBB
 # cat dir/attr/current
 BBB
 # echo CCC > /proc/$$/attr/current
 # cat dir/attr/current
 cat: dir/attr/current: Permission denied

This happens because path lookup doesn't revalidate the dentry of the
/proc/[pid] when traversing the filesystem boundary, so the inode security
blob of the /proc/[pid] doesn't get updated with the new task security
attribute. Then, this may lead security modules to deny an access to the
directory. Looking at the code[2] and the /proc/pid/attr/current entry in
proc man page, seems like the same could happen with SELinux. Though, I
didn't find relevant reports.

The steps above are quite artificial. I actually encountered such an
unexpected denial of access with an in-house application sandbox
framework; each app has its own dedicated filesystem tree where the
process's /proc/[pid] is bind-mounted to and the app enters into via
chroot.

With this patch, writing to /proc/[pid]/attr/current (and its per-security
module variant) updates the inode security blob of /proc/[pid] or
/proc/[pid]/task/[tid] (when pid != tid) with the new attribute.

[1] https://lkml.kernel.org/linux-fsdevel/4A2D15AF.8090000@sun.com/
[2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/security/selinux/hooks.c#n4220

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Munehisa Kamata <kamatam@...zon.com>
---
 fs/proc/base.c | 23 ++++++++++++++++++++---
 1 file changed, 20 insertions(+), 3 deletions(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index dd31e3b6bf77..bdb7bea53475 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -2741,6 +2741,7 @@ static ssize_t proc_pid_attr_write(struct file * file, const char __user * buf,
 {
 	struct inode * inode = file_inode(file);
 	struct task_struct *task;
+	const char *name = file->f_path.dentry->d_name.name;
 	void *page;
 	int rv;

@@ -2784,10 +2785,26 @@ static ssize_t proc_pid_attr_write(struct file * file, const char __user * buf,
 	if (rv < 0)
 		goto out_free;

-	rv = security_setprocattr(PROC_I(inode)->op.lsm,
-				  file->f_path.dentry->d_name.name, page,
-				  count);
+	rv = security_setprocattr(PROC_I(inode)->op.lsm, name, page, count);
 	mutex_unlock(&current->signal->cred_guard_mutex);
+
+	/*
+	 *  Update the inode security blob in advance if the task's security
+	 *  attribute was updated
+	 */
+	if (rv > 0 && !strcmp(name, "current")) {
+		struct pid *pid;
+		struct proc_inode *cur, *ei;
+
+		rcu_read_lock();
+		pid = get_task_pid(current, PIDTYPE_PID);
+		hlist_for_each_entry(cur, &pid->inodes, sibling_inodes)
+			ei = cur;
+		put_pid(pid);
+		pid_update_inode(current, &ei->vfs_inode);
+		rcu_read_unlock();
+	}
+
 out_free:
 	kfree(page);
 out:
-- 
2.40.1