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

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Message-ID: <vr4n6n25sq5dtcr3njl44ev2dxrawpz6s6xwv6cpjfm33uqmay@uhieqhcxeuns>
Date: Tue, 20 Aug 2024 23:16:50 -0400
From: Kent Overstreet <kent.overstreet@...ux.dev>
To: Lizhi Xu <lizhi.xu@...driver.com>
Cc: linux-bcachefs@...r.kernel.org, linux-kernel@...r.kernel.org, 
	syzbot+47ecc948aadfb2ab3efc@...kaller.appspotmail.com, syzkaller-bugs@...glegroups.com
Subject: Re: [PATCH V3] bcachefs: Add journal v2 entry nr value check

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@...kaller.appspotmail.com
> > > Signed-off-by: Lizhi Xu <lizhi.xu@...driver.com>
> > > ---
> > >  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).