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Date:   Tue, 08 Dec 2020 08:36:01 +0000
From:   David Howells <dhowells@...hat.com>
To:     Randy Dunlap <rdunlap@...radead.org>
Cc:     dhowells@...hat.com,
        syzbot <syzbot+86dc6632faaca40133ab@...kaller.appspotmail.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        syzkaller-bugs@...glegroups.com, viro@...iv.linux.org.uk
Subject: Re: memory leak in generic_parse_monolithic [+PATCH]

Randy Dunlap <rdunlap@...radead.org> wrote:

> Otherwise please look at the patch below.

The patch won't help, since it's not going through sys_fsconfig() - worse, it
introduces two new errors.

>  		fc->source = param->string;
> -		param->string = NULL;

This will cause the string now attached to fc->source to be freed by the
caller.  No, the original is doing the correct thing here.  The point is to
steal the string.

> @@ -262,7 +262,9 @@ static int vfs_fsconfig_locked(struct fs
>
> -		return vfs_parse_fs_param(fc, param);
> +		ret = vfs_parse_fs_param(fc, param);
> +		kfree(param->string);
> +		return ret;

But your stack trace shows you aren't going through sys_fsconfig(), so this
function isn't involved.  Further, this introduces a double free, since
sys_fsconfig() frees param.string after it drops uapi_mutex.

Looking at the backtrace:

>      kmemdup_nul+0x2d/0x70 mm/util.c:151
>      vfs_parse_fs_string+0x6e/0xd0 fs/fs_context.c:155
>      generic_parse_monolithic+0xe0/0x130 fs/fs_context.c:201
>      do_new_mount fs/namespace.c:2871 [inline]
>      path_mount+0xbbb/0x1170 fs/namespace.c:3205
>      do_mount fs/namespace.c:3218 [inline]
>      __do_sys_mount fs/namespace.c:3426 [inline]
>      __se_sys_mount fs/namespace.c:3403 [inline]
>      __x64_sys_mount+0x18e/0x1d0 fs/namespace.c:3403

A couple of possibilities spring to mind from that: maybe
vfs_parse_fs_string() is not releasing the param.string - but that's not the
problem since we stole the string and the free is definitely there at the
bottom of the function:

	int vfs_parse_fs_string(struct fs_context *fc, const char *key,
				const char *value, size_t v_size)
	{
	...
		kfree(param.string);
		return ret;
	}

or fc->source is not being cleaned up in vfs_clean_context() - but that's
there as well:

	void vfs_clean_context(struct fs_context *fc)
	{
	...
		kfree(fc->source);
		fc->source = NULL;

In either of these cases, I would expect this to have already become evident
from other filesystem mounts as there would be a lot of leaking going on,
particularly with the first.

Now the backtrace only shows what the state was when the string was allocated;
it doesn't show what happened to it after that, so another possibility is that
the filesystem being mounted nicked what vfs_parse_fs_param() had rightfully
stolen, transferring fc->source somewhere else and then failed to release it -
most likely on mount failure (ie. it's an error handling bug in the
filesystem).

Do we know what filesystem it was?

David

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