linux-kernel - [PATCH 2/2] check_unsafe_exec: s/lock_task_sighand/rcu_read

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Date:	Fri, 24 Apr 2009 01:02:45 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Hugh Dickins <hugh@...itas.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Al Viro <viro@...IV.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Joe Malicki <jmalicki@...acarta.com>,
	Michael Itz <mitz@...acarta.com>,
	Kenneth Baker <bakerk@...acarta.com>,
	Chris Wright <chrisw@...s-sol.org>,
	David Howells <dhowells@...hat.com>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Roland McGrath <roland@...hat.com>
Subject: [PATCH 2/2] check_unsafe_exec: s/lock_task_sighand/rcu_read_lock/

write_lock(&current->fs->lock) guarantees we can't wrongly miss
LSM_UNSAFE_SHARE, this is what we care about. Use rcu_read_lock()
instead of ->siglock to iterate over the sub-threads. We must see
all CLONE_THREAD|CLONE_FS threads which didn't pass exit_fs(), it
takes fs->lock too.

With or without this patch we can miss the freshly cloned thread
and set LSM_UNSAFE_SHARE, we don't care.

Signed-off-by: Oleg Nesterov <oleg@...hat.com>

--- PTRACE/fs/exec.c~2_NO_SIGLOCK	2009-04-24 00:01:53.000000000 +0200
+++ PTRACE/fs/exec.c	2009-04-24 00:36:22.000000000 +0200
@@ -1060,7 +1060,6 @@ EXPORT_SYMBOL(install_exec_creds);
 int check_unsafe_exec(struct linux_binprm *bprm)
 {
 	struct task_struct *p = current, *t;
-	unsigned long flags;
 	unsigned n_fs;
 	int res = 0;
 
@@ -1068,11 +1067,12 @@ int check_unsafe_exec(struct linux_binpr
 
 	n_fs = 1;
 	write_lock(&p->fs->lock);
-	lock_task_sighand(p, &flags);
+	rcu_read_lock();
 	for (t = next_thread(p); t != p; t = next_thread(t)) {
 		if (t->fs == p->fs)
 			n_fs++;
 	}
+	rcu_read_lock();
 
 	if (p->fs->users > n_fs) {
 		bprm->unsafe |= LSM_UNSAFE_SHARE;
@@ -1083,8 +1083,6 @@ int check_unsafe_exec(struct linux_binpr
 			res = 1;
 		}
 	}
-
-	unlock_task_sighand(p, &flags);
 	write_unlock(&p->fs->lock);
 
 	return res;

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