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@redhat.com>
Acked-by: Roland McGrath <roland@redhat.com>
[ Fixed lock/unlock typo  - Hugh ]
Acked-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/fs/exec.c b/fs/exec.c
index a2e6989..a3a8ce8 100644 (file)
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -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_binprm *bprm)
 
        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;
@@ -1083,8 +1083,6 @@ int check_unsafe_exec(struct linux_binprm *bprm)
                        res = 1;
                }
        }
-
-       unlock_task_sighand(p, &flags);
        write_unlock(&p->fs->lock);
 
        return res;