linux-kernel - Re: [syzbot] [bcachefs?] KMSAN: uninit-value in bch2_btree_node_iter

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Message-ID: <bazxq46aftvo7bv5st4tkpebntekbi6th42vcsgvguoafl3qvb@d3zygvbnkb6m>
Date: Sun, 17 Nov 2024 19:39:51 -0500
From: Kent Overstreet <kent.overstreet@...ux.dev>
To: Piotr Zalewski <pZ010001011111@...ton.me>
Cc: syzbot <syzbot+62f5ae3a10a9e97accd4@...kaller.appspotmail.com>, 
	linux-bcachefs@...r.kernel.org, linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [bcachefs?] KMSAN: uninit-value in
 bch2_btree_node_iter_init (2)

On Sun, Nov 17, 2024 at 11:51:39PM +0000, Piotr Zalewski wrote:
> Hi, all
> 
> This seems to be the proper way to fix this?:
> 
> diff --git a/fs/bcachefs/btree_io.c b/fs/bcachefs/btree_io.c
> index 89a42ee81e5c..f92a3fb946de 100644
> --- a/fs/bcachefs/btree_io.c
> +++ b/fs/bcachefs/btree_io.c
> @@ -324,6 +324,7 @@ static void btree_node_sort(struct bch_fs *c, struct btree *b,
>  	start_time = local_clock();
>  
>  	u64s = bch2_sort_keys(out->keys.start, &sort_iter.iter);
> +	memset((out->keys.start + u64s), 0, bytes - sizeof(struct btree_node) - u64s * sizeof(u64));
>  
>  	out->keys.u64s = cpu_to_le16(u64s);

Did you do any analysis you can share?

It seems like zeroing out the rest of the buffer shouldn't be necessary:
we do that at write time, where it really is necessary, otherwise we
shouldn't be accessing parts of the buffer that don't have valid keys in
them.

varint_decode_fast, used for inodes, does do reads (that aren't used) up
to 7 bytes past the valid data, so it could be needed if we were seeing
kmsan splats there - but this is bkey comparison, so something fishy
seems to be going on.