Re: [PATCH] bcachefs: Retrieve ext again after sb is reallocated

[Lizhi Xu <lizhi.xu@windriver.com>]

On Fri, 25 Oct 2024 21:17:09 -0400, Kent Overstreet wrote:
> On Sat, Oct 26, 2024 at 09:09:53AM +0800, Lizhi Xu wrote:
> > Syzbot reported a slab-use-after-free Read in bch2_reconstruct_alloc.
> > The sb counters resize will cause sb reallocation and release the old sb,
> > which leads to uaf in ext.
> > After disk_sb.sb is reallocated, ext should be retrieved again to avoid uaf.
> >
> > Reported-and-tested-by: syzbot+9fc4dac4775d07bcfe34@syzkaller.appspotmail.com
> > Closes: https://syzkaller.appspot.com/bug?extid=9fc4dac4775d07bcfe34
> > Signed-off-by: Lizhi Xu <lizhi.xu@windriver.com>
> > ---
> >  fs/bcachefs/recovery.c | 4 ++++
> >  1 file changed, 4 insertions(+)
> >
> > diff --git a/fs/bcachefs/recovery.c b/fs/bcachefs/recovery.c
> > index 55e1504a8130..9df0969c29ce 100644
> > --- a/fs/bcachefs/recovery.c
> > +++ b/fs/bcachefs/recovery.c
> > @@ -59,6 +59,7 @@ static void bch2_reconstruct_alloc(struct bch_fs *c)
> >
> >  	mutex_lock(&c->sb_lock);
> >  	struct bch_sb_field_ext *ext = bch2_sb_field_get(c->disk_sb.sb, ext);
> > +	void *sb;
> >
> >  	__set_bit_le64(BCH_RECOVERY_PASS_STABLE_check_allocations, ext->recovery_passes_required);
> >  	__set_bit_le64(BCH_RECOVERY_PASS_STABLE_check_alloc_info, ext->recovery_passes_required);
> > @@ -94,7 +95,10 @@ static void bch2_reconstruct_alloc(struct bch_fs *c)
> >  	__set_bit_le64(BCH_FSCK_ERR_accounting_mismatch, ext->errors_silent);
> >  	c->sb.compat &= ~(1ULL << BCH_COMPAT_alloc_info);
> >
> > +	sb = c->disk_sb.sb;
> >  	bch2_write_super(c);
> > +	if (sb != c->disk_sb.sb)
> > +		ext = bch2_sb_field_get(c->disk_sb.sb, ext);
> >  	mutex_unlock(&c->sb_lock);
> >
> >  	c->opts.recovery_passes |= bch2_recovery_passes_from_stable(le64_to_cpu(ext->recovery_passes_required[0]));
> 
> There's a simpler fix:
> 
> 
> From 8e910ca20e112d7f06ba3bf631a06ddb5ce14657 Mon Sep 17 00:00:00 2001
> From: Kent Overstreet <kent.overstreet@linux.dev>
> Date: Fri, 25 Oct 2024 13:13:05 -0400
> Subject: [PATCH] bcachefs: Fix UAF in bch2_reconstruct_alloc()
> 
> write_super() -> sb_counters_from_cpu() may reallocate the superblock
> 
> Reported-by: syzbot+9fc4dac4775d07bcfe34@syzkaller.appspotmail.com
> Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
> 
> diff --git a/fs/bcachefs/recovery.c b/fs/bcachefs/recovery.c
> index 454b5a32dd7f..0ebc76dd7eb5 100644
> --- a/fs/bcachefs/recovery.c
> +++ b/fs/bcachefs/recovery.c
> @@ -94,11 +94,10 @@ static void bch2_reconstruct_alloc(struct bch_fs *c)
>  	__set_bit_le64(BCH_FSCK_ERR_accounting_mismatch, ext->errors_silent);
>  	c->sb.compat &= ~(1ULL << BCH_COMPAT_alloc_info);
> 
> -	bch2_write_super(c);
> -	mutex_unlock(&c->sb_lock);
> -
>  	c->opts.recovery_passes |= bch2_recovery_passes_from_stable(le64_to_cpu(ext->recovery_passes_required[0]));
> 
> +	bch2_write_super(c);
> +	mutex_unlock(&c->sb_lock);
> 
>  	bch2_shoot_down_journal_keys(c, BTREE_ID_alloc,
>  				     0, BTREE_MAX_DEPTH, POS_MIN, SPOS_MAX);
Oh, Yes, it can fix, but my first fix is before your patch,

https://lore.kernel.org/all/20241025100205.635960-1-lizhi.xu@windriver.com/

Subject: Re: [syzbot] [bcachefs?] KASAN: slab-use-after-free Read in bch2_reconstruct_alloc
Date: Fri, 25 Oct 2024 18:02:05 +0800	[thread overview]
Message-ID: <20241025100205.635960-1-lizhi.xu@windriver.com> (raw)
In-Reply-To: <671907d4.050a0220.1e4b4d.008f.GAE@google.com>

The counters will cause sb to be realloc and release the old sb, which leads to uaf in ext

#syz test: upstream master

diff --git a/fs/bcachefs/recovery.c b/fs/bcachefs/recovery.c
index 55e1504a8130..9df0969c29ce 100644
--- a/fs/bcachefs/recovery.c
+++ b/fs/bcachefs/recovery.c
@@ -59,6 +59,7 @@ static void bch2_reconstruct_alloc(struct bch_fs *c)

 	mutex_lock(&c->sb_lock);
 	struct bch_sb_field_ext *ext = bch2_sb_field_get(c->disk_sb.sb, ext);
+	void *sb;

 	__set_bit_le64(BCH_RECOVERY_PASS_STABLE_check_allocations, ext->recovery_passes_required);
 	__set_bit_le64(BCH_RECOVERY_PASS_STABLE_check_alloc_info, ext->recovery_passes_required);
@@ -94,7 +95,10 @@ static void bch2_reconstruct_alloc(struct bch_fs *c)
 	__set_bit_le64(BCH_FSCK_ERR_accounting_mismatch, ext->errors_silent);
 	c->sb.compat &= ~(1ULL << BCH_COMPAT_alloc_info);

+	sb = c->disk_sb.sb;
 	bch2_write_super(c);
+	if (sb != c->disk_sb.sb)
+		ext = bch2_sb_field_get(c->disk_sb.sb, ext);
 	mutex_unlock(&c->sb_lock);

 	c->opts.recovery_passes |= bch2_recovery_passes_from_stable(le64_to_cpu(ext->recovery_passes_required[0]));