[PATCH] btrfs: Fix NULL pointer access in btrfs_check_leaked_roots()

[Dewei Meng <mengdewei@cqsoftware.com.cn>]

If fs_info->super_copy or fs_info->super_for_commit is NULL in
btrfs_get_tree_subvol(), the btrfs_check_leaked_roots() will get the
btrfs_root list entry using the fs_info->allocated_roots->next
which is NULL.

syzkaller reported the following information:
  ------------[ cut here ]------------
  BUG: unable to handle page fault for address: fffffffffffffbb0
  #PF: supervisor read access in kernel mode
  #PF: error_code(0x0000) - not-present page
  PGD 64c9067 P4D 64c9067 PUD 64cb067 PMD 0
  Oops: Oops: 0000 [#1] SMP KASAN PTI
  CPU: 0 UID: 0 PID: 1402 Comm: syz.1.35 Not tainted 6.15.8 #4 PREEMPT(lazy)
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), (...)
  RIP: 0010:arch_atomic_read arch/x86/include/asm/atomic.h:23 [inline]
  RIP: 0010:raw_atomic_read include/linux/atomic/atomic-arch-fallback.h:457 [inline]
  RIP: 0010:atomic_read include/linux/atomic/atomic-instrumented.h:33 [inline]
  RIP: 0010:refcount_read include/linux/refcount.h:170 [inline]
  RIP: 0010:btrfs_check_leaked_roots+0x18f/0x2c0 fs/btrfs/disk-io.c:1230
  [...]
  Call Trace:
   <TASK>
   btrfs_free_fs_info+0x310/0x410 fs/btrfs/disk-io.c:1280
   btrfs_get_tree_subvol+0x592/0x6b0 fs/btrfs/super.c:2029
   btrfs_get_tree+0x63/0x80 fs/btrfs/super.c:2097
   vfs_get_tree+0x98/0x320 fs/super.c:1759
   do_new_mount+0x357/0x660 fs/namespace.c:3899
   path_mount+0x716/0x19c0 fs/namespace.c:4226
   do_mount fs/namespace.c:4239 [inline]
   __do_sys_mount fs/namespace.c:4450 [inline]
   __se_sys_mount fs/namespace.c:4427 [inline]
   __x64_sys_mount+0x28c/0x310 fs/namespace.c:4427
   do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
   do_syscall_64+0x92/0x180 arch/x86/entry/syscall_64.c:94
   entry_SYSCALL_64_after_hwframe+0x76/0x7e
  RIP: 0033:0x7f032eaffa8d
  [...]

This should check if the fs_info->allocated_roots->next is NULL before
accessing it.

Fixes: 3bb17a25bcb0 ("btrfs: add get_tree callback for new mount API")
Signed-off-by: Dewei Meng <mengdewei@cqsoftware.com.cn>
---
 fs/btrfs/disk-io.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 0aa7e5d1b05f..76db7f98187a 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -1213,6 +1213,9 @@ void btrfs_check_leaked_roots(const struct btrfs_fs_info *fs_info)
 #ifdef CONFIG_BTRFS_DEBUG
 	struct btrfs_root *root;
 
+	if (!fs_info->allocated_roots.next)
+		return;
+
 	while (!list_empty(&fs_info->allocated_roots)) {
 		char buf[BTRFS_ROOT_NAME_BUF_LEN];
 
-- 
2.43.5