Re: [PATCH] bcachefs: Fix invalid shift in validate_sb_layout()

[Gianfranco Trad <gianf.trad@gmail.com>]

On 23/10/24 23:30, Gianfranco Trad wrote:
> Add check on layout->sb_max_size_bits against BCH_SB_LAYOUT_SIZE_BITS_MAX
> to prevent UBSAN shift-out-of-bounds in validate_sb_layout().
> 
> Reported-by: syzbot+089fad5a3a5e77825426@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=089fad5a3a5e77825426
> Fixes: 03ef80b469d5 ("bcachefs: Ignore unknown mount options")
> Tested-by: syzbot+089fad5a3a5e77825426@syzkaller.appspotmail.com
> Signed-off-by: Gianfranco Trad <gianf.trad@gmail.com>
> ---
>   fs/bcachefs/errcode.h  | 1 +
>   fs/bcachefs/super-io.c | 5 +++++
>   2 files changed, 6 insertions(+)
> 
> diff --git a/fs/bcachefs/errcode.h b/fs/bcachefs/errcode.h
> index 649263516ab1..b6cbd716000b 100644
> --- a/fs/bcachefs/errcode.h
> +++ b/fs/bcachefs/errcode.h
> @@ -222,6 +222,7 @@
>   	x(BCH_ERR_invalid_sb_layout,	invalid_sb_layout_type)			\
>   	x(BCH_ERR_invalid_sb_layout,	invalid_sb_layout_nr_superblocks)	\
>   	x(BCH_ERR_invalid_sb_layout,	invalid_sb_layout_superblocks_overlap)	\
> +	x(BCH_ERR_invalid_sb_layout,    invalid_sb_layout_sb_max_size_bits)     \
>   	x(BCH_ERR_invalid_sb,		invalid_sb_members_missing)		\
>   	x(BCH_ERR_invalid_sb,		invalid_sb_members)			\
>   	x(BCH_ERR_invalid_sb,		invalid_sb_disk_groups)			\
> diff --git a/fs/bcachefs/super-io.c b/fs/bcachefs/super-io.c
> index ce7410d72089..44d0ac9b00dd 100644
> --- a/fs/bcachefs/super-io.c
> +++ b/fs/bcachefs/super-io.c
> @@ -287,6 +287,11 @@ static int validate_sb_layout(struct bch_sb_layout *layout, struct printbuf *out
>   		return -BCH_ERR_invalid_sb_layout_nr_superblocks;
>   	}
>   
> +	if (layout->sb_max_size_bits > BCH_SB_LAYOUT_SIZE_BITS_MAX) {
> +		prt_printf(out, "Invalid superblock layout: max_size_bits too high");
> +		return -BCH_ERR_invalid_sb_layout_sb_max_size_bits;
> +	}
> +
>   	max_sectors = 1 << layout->sb_max_size_bits;
>   
>   	prev_offset = le64_to_cpu(layout->sb_offset[0]);

Wondering if this other patch might be considered more correct to 
prevent shift oob, given also [0]:

diff --git a/fs/bcachefs/super-io.c b/fs/bcachefs/super-io.c
index ce7410d72089..428172897501 100644
--- a/fs/bcachefs/super-io.c
+++ b/fs/bcachefs/super-io.c
@@ -287,7 +287,7 @@ static int validate_sb_layout(struct bch_sb_layout 
*layout, struct printbuf *out
  		return -BCH_ERR_invalid_sb_layout_nr_superblocks;
  	}

-	max_sectors = 1 << layout->sb_max_size_bits;
+	max_sectors = 1 << min(BCH_SB_LAYOUT_SIZE_BITS_MAX, 
layout->sb_max_size_bits);

  	prev_offset = le64_to_cpu(layout->sb_offset[0]);

Also this patch was already tested by syzbot [1]

[0] 71dac2482ad3c8d4a8b8998a96751f009bad895f ("bcachefs: 
BCH_SB_LAYOUT_SIZE_BITS_MAX")
[1] https://syzkaller.appspot.com/x/log.txt?x=1640b640580000

Thanks for your time,

-- Gian