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

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Message-ID: <43c3r56uapoeyc33lt6tunn5x772ngflxq6lpntodoj3g32s6t@ohktx3g4pkis>
Date: Fri, 25 Oct 2024 21:17:09 -0400
From: Kent Overstreet <kent.overstreet@...ux.dev>
To: Lizhi Xu <lizhi.xu@...driver.com>
Cc: bfoster@...hat.com, linux-bcachefs@...r.kernel.org, 
	linux-kernel@...r.kernel.org, syzbot+9fc4dac4775d07bcfe34@...kaller.appspotmail.com, 
	syzkaller-bugs@...glegroups.com
Subject: Re: [PATCH] bcachefs: Retrieve ext again after sb is reallocated

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@...kaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=9fc4dac4775d07bcfe34
> Signed-off-by: Lizhi Xu <lizhi.xu@...driver.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@...ux.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@...kaller.appspotmail.com
Signed-off-by: Kent Overstreet <kent.overstreet@...ux.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);