linux-kernel - Re: [PATCH] bcachefs: Fix invalid shift in validate_sb

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Message-ID: <jaxnjw5ergn7zrnjuzzosgclpmv5st77gtfaj75my3hblufrn2@b4si2orrodw5>
Date: Thu, 24 Oct 2024 07:24:13 -0400
From: Kent Overstreet <kent.overstreet@...ux.dev>
To: Gianfranco Trad <gianf.trad@...il.com>
Cc: linux-bcachefs@...r.kernel.org, linux-kernel@...r.kernel.org, 
	skhan@...uxfoundation.org, syzbot+089fad5a3a5e77825426@...kaller.appspotmail.com
Subject: Re: [PATCH] bcachefs: Fix invalid shift in validate_sb_layout()

On Thu, Oct 24, 2024 at 12:48:30PM +0200, Gianfranco Trad wrote:
> 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@...kaller.appspotmail.com
> > Closes: https://syzkaller.appspot.com/bug?extid=089fad5a3a5e77825426
> > Fixes: 03ef80b469d5 ("bcachefs: Ignore unknown mount options")
> > Tested-by: syzbot+089fad5a3a5e77825426@...kaller.appspotmail.com
> > Signed-off-by: Gianfranco Trad <gianf.trad@...il.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]:

Your first patch looks better, we want to know if we're feeding it
garbage

> 
> 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
> 
> 
> 
> 
>