linux-kernel - Re: [PATCH] autofs: fix null deref in autofs_fill

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Message-ID: <20231114044110.GR1957730@ZenIV>
Date:   Tue, 14 Nov 2023 04:41:10 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Ian Kent <raven@...maw.net>
Cc:     Edward Adam Davis <eadavis@...com>,
        syzbot+662f87a8ef490f45fa64@...kaller.appspotmail.com,
        autofs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com
Subject: Re: [PATCH] autofs: fix null deref in autofs_fill_super

On Tue, Nov 14, 2023 at 12:25:35PM +0800, Ian Kent wrote:
> >         root_inode = autofs_get_inode(s, S_IFDIR | 0755);
> > +       if (!root_inode)
> > +               goto fail;
> 
> Yes, I think this is the only thing it could be.
> 
> There's one small problem though, it leaks the dentry info. ino,
> allocated just above. I think this should goto label fail_ino instead.
> 
> Note that once the root dentry is allocated then the ino struct will
> be freed when the dentry is freed so ino doesn't need to be freed.

There's a simpler solution:

        root_inode = autofs_get_inode(s, S_IFDIR | 0755);
	if (root_inode) {
		root_inode->i_uid = ctx->uid;
		root_inode->i_gid = ctx->gid;
	}
        root = d_make_root(root_inode);
        if (!root)
                goto fail_ino;

d_make_root(NULL) will quietly return NULL, which is all you
need.  FWIW, I would probably bring the rest of initialization
        root_inode->i_fop = &autofs_root_operations;
        root_inode->i_op = &autofs_dir_inode_operations;
in there as well.

Incidentally, why bother with separate fail_dput thing?  Shove it
into ->s_root and return ret - autofs_kill_sb() will take care
of dropping it...

How about the following:
static int autofs_fill_super(struct super_block *s, struct fs_context *fc)
{
	struct autofs_fs_context *ctx = fc->fs_private;
	struct autofs_sb_info *sbi = s->s_fs_info;
	struct inode *root_inode;
	struct autofs_info *ino;

	pr_debug("starting up, sbi = %p\n", sbi);

	sbi->sb = s;
	s->s_blocksize = 1024;
	s->s_blocksize_bits = 10;
	s->s_magic = AUTOFS_SUPER_MAGIC;
	s->s_op = &autofs_sops;
	s->s_d_op = &autofs_dentry_operations;
	s->s_time_gran = 1;

	/*
	 * Get the root inode and dentry, but defer checking for errors.
	 */
	ino = autofs_new_ino(sbi);
	if (!ino)
		return -ENOMEM;

	root_inode = autofs_get_inode(s, S_IFDIR | 0755);
	if (root_inode) {
		root_inode->i_uid = ctx->uid;
		root_inode->i_gid = ctx->gid;
		root_inode->i_fop = &autofs_root_operations;
		root_inode->i_op = &autofs_dir_inode_operations;
	}
	s->s_root = d_make_root(root_inode);
	if (unlikely(!s->s_root)) {
		autofs_free_ino(ino);
		return -ENOMEM;
	}
	s->s_root->d_fsdata = ino;

	if (ctx->pgrp_set) {
		sbi->oz_pgrp = find_get_pid(ctx->pgrp);
		if (!sbi->oz_pgrp) {
			int ret = invalf(fc, "Could not find process group %d",
				     ctx->pgrp);
			return ret;
		}
	} else {
		sbi->oz_pgrp = get_task_pid(current, PIDTYPE_PGID);
	}

	if (autofs_type_trigger(sbi->type))
		managed_dentry_set_managed(s->s_root);

	pr_debug("pipe fd = %d, pgrp = %u\n",
		 sbi->pipefd, pid_nr(sbi->oz_pgrp));

	sbi->flags &= ~AUTOFS_SBI_CATATONIC;
	return 0;
}

No gotos, no labels to keep track of...