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Message-ID: <CAL3q7H7D=uNsvgKwBcK+jtNXayDYPLO=ZowWyjjaeYK27ZgirQ@mail.gmail.com>
Date: Tue, 15 Oct 2024 13:49:04 +0100
From: Filipe Manana <fdmanana@...nel.org>
To: Edward Adam Davis <eadavis@...com>
Cc: syzbot+c3a3a153f0190dca5be9@...kaller.appspotmail.com, clm@...com,
dsterba@...e.com, josef@...icpanda.com, linux-btrfs@...r.kernel.org,
linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com
Subject: Re: [PATCH next] btrfs: Accessing head_ref within delayed_refs lock
On Tue, Oct 15, 2024 at 1:40 PM Edward Adam Davis <eadavis@...com> wrote:
>
> This is because the thread routine btrfs_work_helper released head_def after
> exiting delayed_refs->lock in add_delayed_ref.
This should be explained a lot better. Starting the changelog with
"This is because..." is odd. It should explain why the head reference
was freed (because delayed references were run).
> Causing add_delayed_ref to encounter uaf when accessing head_def->bytenr
> outside the delayed_refs->lock.
>
> Move head_ref->bytenr into the protection range of delayed_refs->lock
> to avoid uaf in add_delayed_ref.
This was already fixed yesterday, in a simpler way and with an
explanation of what's going on to trigger the use-after-free:
https://lore.kernel.org/linux-btrfs/02fc507b62b19be2348fc08de8b13bd7af1a440e.1728922973.git.fdmanana@suse.com/
Thanks.
>
> Fixes: a3aad8f4f5d9 ("btrfs: qgroups: remove bytenr field from struct btrfs_qgroup_extent_record")
> Reported-and-tested-by: syzbot+c3a3a153f0190dca5be9@...kaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=c3a3a153f0190dca5be9
> Signed-off-by: Edward Adam Davis <eadavis@...com>
> ---
> fs/btrfs/delayed-ref.c | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/fs/btrfs/delayed-ref.c b/fs/btrfs/delayed-ref.c
> index 13c2e00d1270..f50fc05847a1 100644
> --- a/fs/btrfs/delayed-ref.c
> +++ b/fs/btrfs/delayed-ref.c
> @@ -1012,6 +1012,7 @@ static int add_delayed_ref(struct btrfs_trans_handle *trans,
> int action = generic_ref->action;
> bool merged;
> int ret;
> + u64 bytenr;
>
> node = kmem_cache_alloc(btrfs_delayed_ref_node_cachep, GFP_NOFS);
> if (!node)
> @@ -1056,6 +1057,7 @@ static int add_delayed_ref(struct btrfs_trans_handle *trans,
> goto free_record;
> }
> head_ref = new_head_ref;
> + bytenr = head_ref->bytenr;
>
> merged = insert_delayed_ref(trans, head_ref, node);
> spin_unlock(&delayed_refs->lock);
> @@ -1074,7 +1076,7 @@ static int add_delayed_ref(struct btrfs_trans_handle *trans,
> kmem_cache_free(btrfs_delayed_ref_node_cachep, node);
>
> if (qrecord_inserted)
> - return btrfs_qgroup_trace_extent_post(trans, record, head_ref->bytenr);
> + return btrfs_qgroup_trace_extent_post(trans, record, bytenr);
> return 0;
>
> free_record:
> --
> 2.43.0
>
>
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