[patch 43/58] check_unsafe_exec: s/lock_task_sighand/rcu_read_lock/

[Greg KH]

2.6.29-stable review patch.  If anyone has any objections, please let us know.

------------------

From: Oleg Nesterov <oleg@xxxxxxxxxx>

commit 437f7fdb607f32b737e4da9f14bebcfdac2c90c3 upstream.

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@xxxxxxxxxx>
Acked-by: Roland McGrath <roland@xxxxxxxxxx>
[ Fixed lock/unlock typo  - Hugh ]
Acked-by: Hugh Dickins <hugh@xxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxx>

---
 fs/exec.c |    6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1052,7 +1052,6 @@ 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;
 
@@ -1060,11 +1059,12 @@ 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_unlock();
 
 	if (p->fs->users > n_fs) {
 		bprm->unsafe |= LSM_UNSAFE_SHARE;
@@ -1075,8 +1075,6 @@ 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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