Re: [PATCH V3] bcachefs: Add journal v2 entry nr value check

From: Kent Overstreet
Date: Tue Aug 20 2024 - 23:17:07 EST


On Wed, Aug 21, 2024 at 11:10:00AM GMT, Lizhi Xu wrote:
> On Tue, 20 Aug 2024 23:00:05 -0400, Kent Overstreet wrote:
> > > When the nr value of a signle entry or their sum overflows, it will
> > > cause the value of ja->nr to be incorrect, this will result in the
> > > allocated memory to ja->buckets being too small, leading to out of
> > > bounds access in bch2_dev_journal_init.
> > >
> > > Reported-by: syzbot+47ecc948aadfb2ab3efc@xxxxxxxxxxxxxxxxxxxxxxxxx
> > > Signed-off-by: Lizhi Xu <lizhi.xu@xxxxxxxxxxxxx>
> > > ---
> > > fs/bcachefs/journal_sb.c | 16 +++++++++++++++-
> > > 1 file changed, 15 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/fs/bcachefs/journal_sb.c b/fs/bcachefs/journal_sb.c
> > > index db80e506e3ab..230ed99130e4 100644
> > > --- a/fs/bcachefs/journal_sb.c
> > > +++ b/fs/bcachefs/journal_sb.c
> > > @@ -107,6 +107,7 @@ static int bch2_sb_journal_v2_validate(struct bch_sb *sb, struct bch_sb_field *f
> > > unsigned nr;
> > > unsigned i;
> > > struct u64_range *b;
> > > + u64 total_nr = 0, entry_nr;
> > >
> > > nr = bch2_sb_field_journal_v2_nr_entries(journal);
> > > if (!nr)
> > > @@ -117,8 +118,21 @@ static int bch2_sb_journal_v2_validate(struct bch_sb *sb, struct bch_sb_field *f
> > > return -BCH_ERR_ENOMEM_sb_journal_v2_validate;
> > >
> > > for (i = 0; i < nr; i++) {
> > > + entry_nr = le64_to_cpu(journal->d[i].nr);
> > > + if (entry_nr > UINT_MAX) {
> > > + prt_printf(err, "Journal v2 entry d[%u] nr %llu overflow\n",
> > > + i, entry_nr);
> > > + goto err;
> > > + }
> >
> > This check is unnecessary; we know the sum can't overflow a u64 because
> > we're also checking that the entries are nonoverlapping.
> You didn't read my previous email carefully.
> In this issue, journal->d[0] is 7, journal->d[1] is 18446744073709551615,
> so the sum of their u64 type values will definitely overflow.

It doesn't matter. We're already checking that the entries are
nonoverlapping, and within the range of [1, nbuckets), so total_nr can't
overflow nbuckets, much less an s64 (not that that matters).