f2fs: fix race in concurrent f2fs_stop_gc_thread

Long Li · smb49 · commit cf8b3de1e948 · 2025-03-14T14:31:02.000+01:00
BugLink: https://bugs.launchpad.net/bugs/2101915 [ Upstream commit 7b0033d ] In my test case, concurrent calls to f2fs shutdown report the following stack trace: Oops: general protection fault, probably for non-canonical address 0xc6cfff63bb5513fc: 0000 [#1] PREEMPT SMP PTI CPU: 0 UID: 0 PID: 678 Comm: f2fs_rep_shutdo Not tainted 6.12.0-rc5-next-20241029-g6fb2fa9805c5-dirty torvalds#85 Call Trace: <TASK> ? show_regs+0x8b/0xa0 ? __die_body+0x26/0xa0 ? die_addr+0x54/0x90 ? exc_general_protection+0x24b/0x5c0 ? asm_exc_general_protection+0x26/0x30 ? kthread_stop+0x46/0x390 f2fs_stop_gc_thread+0x6c/0x110 f2fs_do_shutdown+0x309/0x3a0 f2fs_ioc_shutdown+0x150/0x1c0 __f2fs_ioctl+0xffd/0x2ac0 f2fs_ioctl+0x76/0xe0 vfs_ioctl+0x23/0x60 __x64_sys_ioctl+0xce/0xf0 x64_sys_call+0x2b1b/0x4540 do_syscall_64+0xa7/0x240 entry_SYSCALL_64_after_hwframe+0x76/0x7e The root cause is a race condition in f2fs_stop_gc_thread() called from different f2fs shutdown paths: [CPU0] [CPU1] ---------------------- ----------------------- f2fs_stop_gc_thread f2fs_stop_gc_thread gc_th = sbi->gc_thread gc_th = sbi->gc_thread kfree(gc_th) sbi->gc_thread = NULL < gc_th != NULL > kthread_stop(gc_th->f2fs_gc_task) //UAF The commit c7f114d ("f2fs: fix to avoid use-after-free in f2fs_stop_gc_thread()") attempted to fix this issue by using a read semaphore to prevent races between shutdown and remount threads, but it fails to prevent all race conditions. Fix it by converting to write lock of s_umount in f2fs_do_shutdown(). Fixes: 7950e9a ("f2fs: stop gc/discard thread after fs shutdown") Signed-off-by: Long Li <leo.lilong@huawei.com> Reviewed-by: Chao Yu <chao@kernel.org> Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org> Signed-off-by: Sasha Levin <sashal@kernel.org> CVE-2024-53218 Signed-off-by: Koichiro Den <koichiro.den@canonical.com> Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
@@ -2327,9 +2327,12 @@ int f2fs_do_shutdown(struct f2fs_sb_info *sbi, unsigned int flag,
 	if (readonly)
 		goto out;
 
-	/* grab sb->s_umount to avoid racing w/ remount() */
+	/*
+	 * grab sb->s_umount to avoid racing w/ remount() and other shutdown
+	 * paths.
+	 */
 	if (need_lock)
-		down_read(&sbi->sb->s_umount);
+		down_write(&sbi->sb->s_umount);
 
 	f2fs_stop_gc_thread(sbi);
 	f2fs_stop_discard_thread(sbi);
@@ -2338,7 +2341,7 @@ int f2fs_do_shutdown(struct f2fs_sb_info *sbi, unsigned int flag,
 	clear_opt(sbi, DISCARD);
 
 	if (need_lock)
-		up_read(&sbi->sb->s_umount);
+		up_write(&sbi->sb->s_umount);
 
 	f2fs_update_time(sbi, REQ_TIME);
 out:
