linux-kernel - Re: [PATCH 2/3] minix: Add required sanity checking to minix_check

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Message-ID: <406257584.911266.1765202095235@kpc.webmail.kpnmail.nl>
Date: Mon, 8 Dec 2025 14:54:55 +0100 (CET)
From: Jori Koolstra <jkoolstra@...all.nl>
To: Jan Kara <jack@...e.cz>
Cc: Christian Brauner <brauner@...nel.org>,
	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
	Taotao Chen <chentaotao@...iglobal.com>,
	Shuah Khan <skhan@...uxfoundation.org>, linux-kernel@...r.kernel.org,
	syzbot+5ad0824204c7bf9b67f2@...kaller.appspotmail.com
Subject: Re: [PATCH 2/3] minix: Add required sanity checking to
 minix_check_superblock()


> Op 08-12-2025 10:51 CET schreef Jan Kara <jack@...e.cz>:
> 
>  
> On Sun 07-12-25 19:36:44, Jori Koolstra wrote:
> > > > 
> > > > diff --git a/fs/minix/inode.c b/fs/minix/inode.c
> > > > index 7897f5123b3d..8eb26ff91adf 100644
> > > > --- a/fs/minix/inode.c
> > > > +++ b/fs/minix/inode.c
> > > > @@ -171,7 +171,14 @@ static bool minix_check_superblock(struct super_block *sb)
> > > >  {
> > > >  	struct minix_sb_info *sbi = minix_sb(sb);
> > > >  
> > > > -	if (sbi->s_imap_blocks == 0 || sbi->s_zmap_blocks == 0)
> > > > +	if (sbi->s_log_zone_size != 0) {
> > > > +		printk("minix-fs error: zone size must equal block size. "
> > > > +		       "s_log_zone_size > 0 is not supported.\n");
> > > > +		return false;
> > > > +	}
> > > > +
> > > > +	if (sbi->s_ninodes < 1 || sbi->s_firstdatazone <= 4 ||
> > > > +	    sbi->s_firstdatazone >= sbi->s_nzones)
> > > >  		return false;
> > > 
> > > OK, but why did you remove the s_imap_blocks and s_zmap_blocks checks?
> > > 
> > 
> > I replied to this a while ago that this already happens later in
> > minix_fill_superblock:
> > https://elixir.bootlin.com/linux/v6.18-rc7/source/fs/minix/inode.c#L280
> > 
> > Maybe you missed it. Let me know if this answers your question when you
> > have time.
> 
> True, a stricter check happens later in minix_fill_super() but AFAICS
> minix_set_bit() calls before this stricter check will crash if
> s_imap_blocks or s_zmap_blocks are zero. So I think the original check is
> still needed.
> 

Ah, how stupid. I missed that. I think it is better to move the logic to the
minix_check_super_block anyhow. What do you think of this:

diff --git a/fs/minix/inode.c b/fs/minix/inode.c
index 51ea9bdc813f..c8c6b2135abe 100644
--- a/fs/minix/inode.c
+++ b/fs/minix/inode.c
@@ -170,10 +170,38 @@ static int minix_reconfigure(struct fs_context *fc)
 static bool minix_check_superblock(struct super_block *sb)
 {
 	struct minix_sb_info *sbi = minix_sb(sb);
+	unsigned long block;
 
-	if (sbi->s_imap_blocks == 0 || sbi->s_zmap_blocks == 0)
+	if (sbi->s_log_zone_size != 0) {
+		printk("minix-fs error: zone size must equal block size. "
+		       "s_log_zone_size > 0 is not supported.\n");
+		return false;
+	}
+
+	if (sbi->s_ninodes < 1 || sbi->s_firstdatazone <= 4 ||
+	    sbi->s_firstdatazone >= sbi->s_nzones)
 		return false;
 
+	/* Apparently minix can create filesystems that allocate more blocks for
+	 * the bitmaps than needed.  We simply ignore that, but verify it didn't
+	 * create one with not enough blocks and bail out if so.
+	 */
+	block = minix_blocks_needed(sbi->s_ninodes, sb->s_blocksize);
+	if (sbi->s_imap_blocks < block) {
+		printk("MINIX-fs: file system does not have enough "
+		       "imap blocks allocated. Refusing to mount.\n");
+		return false;
+	}
+
+	block = minix_blocks_needed(
+			(sbi->s_nzones - sbi->s_firstdatazone + 1),
+			sb->s_blocksize);
+	if (sbi->s_zmap_blocks < block) {
+		printk("MINIX-fs: file system does not have enough "
+		       "zmap blocks allocated. Refusing to mount.\n");
+		return false;
+	}
+
 	/*
 	 * s_max_size must not exceed the block mapping limitation.  This check
 	 * is only needed for V1 filesystems, since V2/V3 support an extra level
@@ -293,26 +321,6 @@ static int minix_fill_super(struct super_block *s, struct fs_context *fc)
 	minix_set_bit(0,sbi->s_imap[0]->b_data);
 	minix_set_bit(0,sbi->s_zmap[0]->b_data);
 
-	/* Apparently minix can create filesystems that allocate more blocks for
-	 * the bitmaps than needed.  We simply ignore that, but verify it didn't
-	 * create one with not enough blocks and bail out if so.
-	 */
-	block = minix_blocks_needed(sbi->s_ninodes, s->s_blocksize);
-	if (sbi->s_imap_blocks < block) {
-		printk("MINIX-fs: file system does not have enough "
-				"imap blocks allocated.  Refusing to mount.\n");
-		goto out_no_bitmap;
-	}
-
-	block = minix_blocks_needed(
-			(sbi->s_nzones - sbi->s_firstdatazone + 1),
-			s->s_blocksize);
-	if (sbi->s_zmap_blocks < block) {
-		printk("MINIX-fs: file system does not have enough "
-				"zmap blocks allocated.  Refusing to mount.\n");
-		goto out_no_bitmap;
-	}
-
 	/* set up enough so that it can read an inode */
 	s->s_op = &minix_sops;
 	s->s_time_min = 0;
-- 

Let me know and if it's OK I will submit it again (that is the right procedure,
correct?)

Many Thanks!

Jori.