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

[Gianfranco Trad]

On 24/10/24 13:24, Kent Overstreet wrote:
> 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@xxxxxxxxxxxxxxxxxxxxxxxxx
> > > Closes: https://syzkaller.appspot.com/bug?extid=089fad5a3a5e77825426
> > > Fixes: 03ef80b469d5 ("bcachefs: Ignore unknown mount options")
> > > Tested-by: syzbot+089fad5a3a5e77825426@xxxxxxxxxxxxxxxxxxxxxxxxx
> > > Signed-off-by: Gianfranco Trad <gianf.trad@xxxxxxxxx>
> > > ---
> > >    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

Understood. Thanks Kent.
Have a nice day,
--Gian
> > 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