FAILED: patch "[PATCH] btrfs: fix a NULL pointer dereference when failed to start a" failed to apply to 4.19-stable tree

[<gregkh@linuxfoundation.org>]

The patch below does not apply to the 4.19-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-4.19.y
git checkout FETCH_HEAD
git cherry-pick -x c3b47f49e83197e8dffd023ec568403bcdbb774b
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '2024100756-parasail-agreement-0453@gregkh' --subject-prefix 'PATCH 4.19.y' HEAD^..

Possible dependencies:

c3b47f49e831 ("btrfs: fix a NULL pointer dereference when failed to start a new trasacntion")
d2311e698578 ("btrfs: relocation: Delay reloc tree deletion after merge_reloc_roots")
83354f0772cd ("btrfs: catch cow on deleting snapshots")
61fa90c16b0b ("btrfs: switch BTRFS_ROOT_* to enums")

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From c3b47f49e83197e8dffd023ec568403bcdbb774b Mon Sep 17 00:00:00 2001
From: Qu Wenruo <wqu@suse.com>
Date: Sat, 28 Sep 2024 08:05:58 +0930
Subject: [PATCH] btrfs: fix a NULL pointer dereference when failed to start a
 new trasacntion

[BUG]
Syzbot reported a NULL pointer dereference with the following crash:

  FAULT_INJECTION: forcing a failure.
   start_transaction+0x830/0x1670 fs/btrfs/transaction.c:676
   prepare_to_relocate+0x31f/0x4c0 fs/btrfs/relocation.c:3642
   relocate_block_group+0x169/0xd20 fs/btrfs/relocation.c:3678
  ...
  BTRFS info (device loop0): balance: ended with status: -12
  Oops: general protection fault, probably for non-canonical address 0xdffffc00000000cc: 0000 [#1] PREEMPT SMP KASAN NOPTI
  KASAN: null-ptr-deref in range [0x0000000000000660-0x0000000000000667]
  RIP: 0010:btrfs_update_reloc_root+0x362/0xa80 fs/btrfs/relocation.c:926
  Call Trace:
   <TASK>
   commit_fs_roots+0x2ee/0x720 fs/btrfs/transaction.c:1496
   btrfs_commit_transaction+0xfaf/0x3740 fs/btrfs/transaction.c:2430
   del_balance_item fs/btrfs/volumes.c:3678 [inline]
   reset_balance_state+0x25e/0x3c0 fs/btrfs/volumes.c:3742
   btrfs_balance+0xead/0x10c0 fs/btrfs/volumes.c:4574
   btrfs_ioctl_balance+0x493/0x7c0 fs/btrfs/ioctl.c:3673
   vfs_ioctl fs/ioctl.c:51 [inline]
   __do_sys_ioctl fs/ioctl.c:907 [inline]
   __se_sys_ioctl+0xf9/0x170 fs/ioctl.c:893
   do_syscall_x64 arch/x86/entry/common.c:52 [inline]
   do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
   entry_SYSCALL_64_after_hwframe+0x77/0x7f

[CAUSE]
The allocation failure happens at the start_transaction() inside
prepare_to_relocate(), and during the error handling we call
unset_reloc_control(), which makes fs_info->balance_ctl to be NULL.

Then we continue the error path cleanup in btrfs_balance() by calling
reset_balance_state() which will call del_balance_item() to fully delete
the balance item in the root tree.

However during the small window between set_reloc_contrl() and
unset_reloc_control(), we can have a subvolume tree update and created a
reloc_root for that subvolume.

Then we go into the final btrfs_commit_transaction() of
del_balance_item(), and into btrfs_update_reloc_root() inside
commit_fs_roots().

That function checks if fs_info->reloc_ctl is in the merge_reloc_tree
stage, but since fs_info->reloc_ctl is NULL, it results a NULL pointer
dereference.

[FIX]
Just add extra check on fs_info->reloc_ctl inside
btrfs_update_reloc_root(), before checking
fs_info->reloc_ctl->merge_reloc_tree.

That DEAD_RELOC_TREE handling is to prevent further modification to the
reloc tree during merge stage, but since there is no reloc_ctl at all,
we do not need to bother that.

Reported-by: syzbot+283673dbc38527ef9f3d@syzkaller.appspotmail.com
Link: https://lore.kernel.org/linux-btrfs/66f6bfa7.050a0220.38ace9.0019.GAE@google.com/
CC: stable@vger.kernel.org # 4.19+
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c
index cf1dfeaaf2d8..f3834f8d26b4 100644
--- a/fs/btrfs/relocation.c
+++ b/fs/btrfs/relocation.c
@@ -856,7 +856,7 @@ int btrfs_update_reloc_root(struct btrfs_trans_handle *trans,
 	btrfs_grab_root(reloc_root);
 
 	/* root->reloc_root will stay until current relocation finished */
-	if (fs_info->reloc_ctl->merge_reloc_tree &&
+	if (fs_info->reloc_ctl && fs_info->reloc_ctl->merge_reloc_tree &&
 	    btrfs_root_refs(root_item) == 0) {
 		set_bit(BTRFS_ROOT_DEAD_RELOC_TREE, &root->state);
 		/*