nilfs2: fix use-after-free of timer for log writer thread

Patch series "nilfs2: fix log writer related issues".

This bug fix series covers three nilfs2 log writer-related issues,
including a timer use-after-free issue and potential deadlock issue on
unmount, and a potential freeze issue in event synchronization found
during their analysis.  Details are described in each commit log.

This patch (of 3):

A use-after-free issue has been reported regarding the timer sc_timer on
the nilfs_sc_info structure.

The problem is that even though it is used to wake up a sleeping log
writer thread, sc_timer is not shut down until the nilfs_sc_info structure
is about to be freed, and is used regardless of the thread's lifetime.

Fix this issue by limiting the use of sc_timer only while the log writer
thread is alive.

Link: https://lkml.kernel.org/r/20240520132621.4054-1-konishi.ryusuke@gmail.com
Link: https://lkml.kernel.org/r/20240520132621.4054-2-konishi.ryusuke@gmail.com
Fixes: fdce895ea5dd ("nilfs2: change sc_timer from a pointer to an embedded one in struct nilfs_sc_info")
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Reported-by: "Bai, Shuangpeng" <sjb7183@psu.edu>
Closes: https://groups.google.com/g/syzkaller/c/MK_LYqtt8ko/m/8rgdWeseAwAJ
Tested-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/fs/nilfs2/segment.c b/fs/nilfs2/segment.c
index 6be7dd4..7cb34e1 100644 (file)
--- a/fs/nilfs2/segment.c
+++ b/fs/nilfs2/segment.c
@@ -2118,8 +2118,10 @@ static void nilfs_segctor_start_timer(struct nilfs_sc_info *sci)
 {
        spin_lock(&sci->sc_state_lock);
        if (!(sci->sc_state & NILFS_SEGCTOR_COMMIT)) {
-               sci->sc_timer.expires = jiffies + sci->sc_interval;
-               add_timer(&sci->sc_timer);
+               if (sci->sc_task) {
+                       sci->sc_timer.expires = jiffies + sci->sc_interval;
+                       add_timer(&sci->sc_timer);
+               }
                sci->sc_state |= NILFS_SEGCTOR_COMMIT;
        }
        spin_unlock(&sci->sc_state_lock);
@@ -2320,10 +2322,21 @@ int nilfs_construct_dsync_segment(struct super_block *sb, struct inode *inode,
  */
 static void nilfs_segctor_accept(struct nilfs_sc_info *sci)
 {
+       bool thread_is_alive;
+
        spin_lock(&sci->sc_state_lock);
        sci->sc_seq_accepted = sci->sc_seq_request;
+       thread_is_alive = (bool)sci->sc_task;
        spin_unlock(&sci->sc_state_lock);
-       del_timer_sync(&sci->sc_timer);
+
+       /*
+        * This function does not race with the log writer thread's
+        * termination.  Therefore, deleting sc_timer, which should not be
+        * done after the log writer thread exits, can be done safely outside
+        * the area protected by sc_state_lock.
+        */
+       if (thread_is_alive)
+               del_timer_sync(&sci->sc_timer);
 }
 
 /**
@@ -2349,7 +2362,7 @@ static void nilfs_segctor_notify(struct nilfs_sc_info *sci, int mode, int err)
                        sci->sc_flush_request &= ~FLUSH_DAT_BIT;
 
                /* re-enable timer if checkpoint creation was not done */
-               if ((sci->sc_state & NILFS_SEGCTOR_COMMIT) &&
+               if ((sci->sc_state & NILFS_SEGCTOR_COMMIT) && sci->sc_task &&
                    time_before(jiffies, sci->sc_timer.expires))
                        add_timer(&sci->sc_timer);
        }
@@ -2539,6 +2552,7 @@ static int nilfs_segctor_thread(void *arg)
        int timeout = 0;
 
        sci->sc_timer_task = current;
+       timer_setup(&sci->sc_timer, nilfs_construction_timeout, 0);
 
        /* start sync. */
        sci->sc_task = current;
@@ -2606,6 +2620,7 @@ static int nilfs_segctor_thread(void *arg)
  end_thread:
        /* end sync. */
        sci->sc_task = NULL;
+       timer_shutdown_sync(&sci->sc_timer);
        wake_up(&sci->sc_wait_task); /* for nilfs_segctor_kill_thread() */
        spin_unlock(&sci->sc_state_lock);
        return 0;
@@ -2669,7 +2684,6 @@ static struct nilfs_sc_info *nilfs_segctor_new(struct super_block *sb,
        INIT_LIST_HEAD(&sci->sc_gc_inodes);
        INIT_LIST_HEAD(&sci->sc_iput_queue);
        INIT_WORK(&sci->sc_iput_work, nilfs_iput_work_func);
-       timer_setup(&sci->sc_timer, nilfs_construction_timeout, 0);
 
        sci->sc_interval = HZ * NILFS_SC_DEFAULT_TIMEOUT;
        sci->sc_mjcp_freq = HZ * NILFS_SC_DEFAULT_SR_FREQ;
@@ -2748,7 +2762,6 @@ static void nilfs_segctor_destroy(struct nilfs_sc_info *sci)
 
        down_write(&nilfs->ns_segctor_sem);
 
-       timer_shutdown_sync(&sci->sc_timer);
        kfree(sci);
 }