[GIT PULL] proc fix for 5.7-rc1

Eric W. Biederman
2020-04-10T13:06:04Z

Linus,

Please pull the for-linus branch from the git tree:

git://git.kernel.org/pub/scm/linux/kernel/git/ebiederm/user-namespace.git for-linus

HEAD: 63f818f46af9f8b3f17b9695501e8d08959feb60 proc: Use a dedicated lock in struct pid

A brown paper bag slipped through my proc changes, and syzcaller caught
it when the code ended up in your tree. I have opted to fix it the
simplest cleanest way I know how. So there is no reasonable chance
for the bug to repeat.

Eric

From 63f818f46af9f8b3f17b9695501e8d08959feb60 Mon Sep 17 00:00:00 2001
From: "Eric W. Biederman" <ebiederm@xmission.com>
Date: Tue, 7 Apr 2020 09:43:04 -0500
Subject: [PATCH] proc: Use a dedicated lock in struct pid

syzbot wrote:
> ========================================================
> WARNING: possible irq lock inversion dependency detected
> 5.6.0-syzkaller #0 Not tainted
> --------------------------------------------------------
> swapper/1/0 just changed the state of lock:
> ffffffff898090d8 (tasklist_lock){.+.?}-{2:2}, at: send_sigurg+0x9f/0x320 fs/fcntl.c:840
> but this lock took another, SOFTIRQ-unsafe lock in the past:
> (&pid->wait_pidfd){+.+.}-{2:2}
>
>
> and interrupts could create inverse lock ordering between them.
>
>
> other info that might help us debug this:
> Possible interrupt unsafe locking scenario:
>
> CPU0 CPU1
> ---- ----
> lock(&pid->wait_pidfd);
> local_irq_disable();
> lock(tasklist_lock);
> lock(&pid->wait_pidfd);
> <Interrupt>
> lock(tasklist_lock);
>
> *** DEADLOCK ***
>
> 4 locks held by swapper/1/0:

The problem is that because wait_pidfd.lock is taken under the tasklist
lock. It must always be taken with irqs disabled as tasklist_lock can be
taken from interrupt context and if wait_pidfd.lock was already taken this
would create a lock order inversion.

Oleg suggested just disabling irqs where I have added extra calls to
wait_pidfd.lock. That should be safe and I think the code will eventually
do that. It was rightly pointed out by Christian that sharing the
wait_pidfd.lock was a premature optimization.

It is also true that my pre-merge window testing was insufficient. So
remove the premature optimization and give struct pid a dedicated lock of
it's own for struct pid things. I have verified that lockdep sees all 3
paths where we take the new pid->lock and lockdep does not complain.

It is my current day dream that one day pid->lock can be used to guard the
task lists as well and then the tasklist_lock won't need to be held to
deliver signals. That will require taking pid->lock with irqs disabled.

Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Link: https://lore.kernel.org/lkml/00000000000011d66805a25cd73f@google.com/
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Christian Brauner <christian.brauner@ubuntu.com>
Reported-by: syzbot+343f75cdeea091340956@syzkaller.appspotmail.com
Reported-by: syzbot+832aabf700bc3ec920b9@syzkaller.appspotmail.com
Reported-by: syzbot+f675f964019f884dbd0f@syzkaller.appspotmail.com
Reported-by: syzbot+a9fb1457d720a55d6dc5@syzkaller.appspotmail.com
Fixes: 7bc3e6e55acf ("proc: Use a list of inodes to flush from proc")
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
---
fs/proc/base.c | 10 +++++-----
include/linux/pid.h | 1 +
kernel/pid.c | 1 +
3 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 74f948a6b621..6042b646ab27 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1839,9 +1839,9 @@ void proc_pid_evict_inode(struct proc_inode *ei)
struct pid *pid = ei->pid;

if (S_ISDIR(ei->vfs_inode.i_mode)) {
- spin_lock(&pid->wait_pidfd.lock);
+ spin_lock(&pid->lock);
hlist_del_init_rcu(&ei->sibling_inodes);
- spin_unlock(&pid->wait_pidfd.lock);
+ spin_unlock(&pid->lock);
}

put_pid(pid);
@@ -1877,9 +1877,9 @@ struct inode *proc_pid_make_inode(struct super_block * sb,
/* Let the pid remember us for quick removal */
ei->pid = pid;
if (S_ISDIR(mode)) {
- spin_lock(&pid->wait_pidfd.lock);
+ spin_lock(&pid->lock);
hlist_add_head_rcu(&ei->sibling_inodes, &pid->inodes);
- spin_unlock(&pid->wait_pidfd.lock);
+ spin_unlock(&pid->lock);
}

task_dump_owner(task, 0, &inode->i_uid, &inode->i_gid);
@@ -3273,7 +3273,7 @@ static const struct inode_operations proc_tgid_base_inode_operations = {

void proc_flush_pid(struct pid *pid)
{
- proc_invalidate_siblings_dcache(&pid->inodes, &pid->wait_pidfd.lock);
+ proc_invalidate_siblings_dcache(&pid->inodes, &pid->lock);
put_pid(pid);
}

diff --git a/include/linux/pid.h b/include/linux/pid.h
index 01a0d4e28506..cc896f0fc4e3 100644
--- a/include/linux/pid.h
+++ b/include/linux/pid.h
@@ -60,6 +60,7 @@ struct pid
{
refcount_t count;
unsigned int level;
+ spinlock_t lock;
/* lists of tasks that use this pid */
struct hlist_head tasks[PIDTYPE_MAX];
struct hlist_head inodes;
diff --git a/kernel/pid.c b/kernel/pid.c
index efd34874b3d1..517d0855d4cf 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -246,6 +246,7 @@ struct pid *alloc_pid(struct pid_namespace *ns, pid_t *set_tid,

get_pid_ns(ns);
refcount_set(&pid->count, 1);
+ spin_lock_init(&pid->lock);
for (type = 0; type < PIDTYPE_MAX; ++type)
INIT_HLIST_HEAD(&pid->tasks[type]);

--
2.20.1