2022-11-03 14:53:00

by Shigeru Yoshida

Subject: [PATCH] nilfs: Avoid use-after-free caused by nilfs->ns_writer

syzbot reported use-after-free in nilfs_segctor_sync() [1].

The use-after-free occurs with nilfs->ns_writer. The scenario for the
issue is as follows:

Task1 Task2
----------------------------------------------------------------------
nilfs_construct_segment
nilfs_segctor_sync
init_wait
init_waitqueue_entry
add_wait_queue
schedule
nilfs_detach_log_writer
nilfs_segctor_destroy
kfree
finish_wait
_raw_spin_lock_irqsave
__raw_spin_lock_irqsave
do_raw_spin_lock
debug_spin_lock_before <-- use-after-free

While Task1 is sleeping, nilfs->ns_writer is freed by Task2. After
Task1 waked up, Task1 accesses nilfs->ns_writer which is already
freed.

This patch fixes the issue by taking nilfs->ns_segctor_sem in
nilfs_construct_segment() so that nilfs->ns_segctor_sem cannot be
freed while nilfs_segctor_sync() is sleeping.

Link: https://syzkaller.appspot.com/bug?id=79a4c002e960419ca173d55e863bd09e8112df8b [1]
Reported-by: [email protected]
Signed-off-by: Shigeru Yoshida <[email protected]>
---
fs/nilfs2/segment.c | 14 +++++++++++---
1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/fs/nilfs2/segment.c b/fs/nilfs2/segment.c
index b4cebad21b48..d4f10d82664d 100644
--- a/fs/nilfs2/segment.c
+++ b/fs/nilfs2/segment.c
@@ -2239,16 +2239,24 @@ static void nilfs_segctor_wakeup(struct nilfs_sc_info *sci, int err)
int nilfs_construct_segment(struct super_block *sb)
{
struct the_nilfs *nilfs = sb->s_fs_info;
- struct nilfs_sc_info *sci = nilfs->ns_writer;
+ struct nilfs_sc_info *sci;
struct nilfs_transaction_info *ti;
+ int ret;

- if (!sci)
+ down_write(&nilfs->ns_segctor_sem);
+ sci = nilfs->ns_writer;
+ if (!sci) {
+ up_write(&nilfs->ns_segctor_sem);
return -EROFS;
+ }

/* A call inside transactions causes a deadlock. */
BUG_ON((ti = current->journal_info) && ti->ti_magic == NILFS_TI_MAGIC);

- return nilfs_segctor_sync(sci);
+ ret = nilfs_segctor_sync(sci);
+ up_write(&nilfs->ns_segctor_sem);
+
+ return ret;
}

/**
--
2.37.3

2022-11-03 19:29:24

by Ryusuke Konishi

Subject: Re: [PATCH] nilfs: Avoid use-after-free caused by nilfs->ns_writer

On Thu, Nov 3, 2022 at 11:18 PM Shigeru Yoshida wrote:
>
> syzbot reported use-after-free in nilfs_segctor_sync() [1].
>
> The use-after-free occurs with nilfs->ns_writer. The scenario for the
> issue is as follows:
>
> Task1 Task2
> ----------------------------------------------------------------------
> nilfs_construct_segment
> nilfs_segctor_sync
> init_wait
> init_waitqueue_entry
> add_wait_queue
> schedule
> nilfs_detach_log_writer
> nilfs_segctor_destroy
> kfree
> finish_wait
> _raw_spin_lock_irqsave
> __raw_spin_lock_irqsave
> do_raw_spin_lock
> debug_spin_lock_before <-- use-after-free
>
> While Task1 is sleeping, nilfs->ns_writer is freed by Task2. After
> Task1 waked up, Task1 accesses nilfs->ns_writer which is already
> freed.
>
> This patch fixes the issue by taking nilfs->ns_segctor_sem in
> nilfs_construct_segment() so that nilfs->ns_segctor_sem cannot be
> freed while nilfs_segctor_sync() is sleeping.
>
> Link: https://syzkaller.appspot.com/bug?id=79a4c002e960419ca173d55e863bd09e8112df8b [1]
> Reported-by: [email protected]
> Signed-off-by: Shigeru Yoshida <[email protected]>
> ---
> fs/nilfs2/segment.c | 14 +++++++++++---
> 1 file changed, 11 insertions(+), 3 deletions(-)
>
> diff --git a/fs/nilfs2/segment.c b/fs/nilfs2/segment.c
> index b4cebad21b48..d4f10d82664d 100644
> --- a/fs/nilfs2/segment.c
> +++ b/fs/nilfs2/segment.c
> @@ -2239,16 +2239,24 @@ static void nilfs_segctor_wakeup(struct nilfs_sc_info *sci, int err)
> int nilfs_construct_segment(struct super_block *sb)
> {
> struct the_nilfs *nilfs = sb->s_fs_info;
> - struct nilfs_sc_info *sci = nilfs->ns_writer;
> + struct nilfs_sc_info *sci;
> struct nilfs_transaction_info *ti;
> + int ret;
>
> - if (!sci)
> + down_write(&nilfs->ns_segctor_sem);
> + sci = nilfs->ns_writer;
> + if (!sci) {
> + up_write(&nilfs->ns_segctor_sem);
> return -EROFS;
> + }
>
> /* A call inside transactions causes a deadlock. */
> BUG_ON((ti = current->journal_info) && ti->ti_magic == NILFS_TI_MAGIC);
>
> - return nilfs_segctor_sync(sci);
> + ret = nilfs_segctor_sync(sci);
> + up_write(&nilfs->ns_segctor_sem);
> +
> + return ret;
> }
>
> /**
> --
> 2.37.3
>

Thank you for your help Yoshida-san.

Your analysis of the bug cause is quite correct.

However, this patch caused deadlock for tests with fsync() or mkcp
command (i.e. nilfs_ioctl_sync).
They both call nilfs_construct_segment().
The approach implemented in this patch may not avoid the deadlock regression.

I'd like to consider other approaches.
My current thoughts are either to stop detaching ns_writer on remount
which is the root cause of the UAF races, or to introduce a refcount
on struct nilfs_sc_info.

Regards,
Ryusuke Konishi