linux-kernel - Re: [PATCH v8 2/3] mm: add a field to store names for private anonymous memory

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Message-ID: <202108272228.7D36F0373@keescook>
Date:   Fri, 27 Aug 2021 22:52:33 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Suren Baghdasaryan <surenb@...gle.com>, akpm@...ux-foundation.org,
        ccross@...gle.com, sumit.semwal@...aro.org, mhocko@...e.com,
        dave.hansen@...el.com, kirill.shutemov@...ux.intel.com,
        vbabka@...e.cz, hannes@...xchg.org, corbet@....net,
        viro@...iv.linux.org.uk, rdunlap@...radead.org,
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Subject: Re: [PATCH v8 2/3] mm: add a field to store names for private
 anonymous memory

On Sat, Aug 28, 2021 at 02:47:03AM +0100, Matthew Wilcox wrote:
> On Fri, Aug 27, 2021 at 12:18:57PM -0700, Suren Baghdasaryan wrote:
> > +		anon_name = vma_anon_name(vma);
> > +		if (anon_name) {
> > +			seq_pad(m, ' ');
> > +			seq_puts(m, "[anon:");
> > +			seq_write(m, anon_name, strlen(anon_name));
> > +			seq_putc(m, ']');
> > +		}

Maybe after seq_pad, use: seq_printf(m, "[anon:%s]", anon_name);

> 
> ...
> 
> > +	case PR_SET_VMA_ANON_NAME:
> > +		name = strndup_user((const char __user *)arg,
> > +				    ANON_VMA_NAME_MAX_LEN);
> > +
> > +		if (IS_ERR(name))
> > +			return PTR_ERR(name);
> > +
> > +		for (pch = name; *pch != '\0'; pch++) {
> > +			if (!isprint(*pch)) {
> > +				kfree(name);
> > +				return -EINVAL;
> 
> I think isprint() is too weak a check.  For example, I would suggest
> forbidding the following characters: ':', ']', '[', ' '.  Perhaps
> isalnum() would be better?  (permit a-zA-Z0-9)  I wouldn't necessarily
> be opposed to some punctuation characters, but let's avoid creating
> confusion.  Do you happen to know which characters are actually in use
> today?

There's some sense in refusing [, ], and :, but removing " " seems
unhelpful for reasonable descriptors. As long as weird stuff is escaped,
I think it's fine. Any parser can just extract with m|\[anon:(.*)\]$|

For example, just escape it here instead of refusing to take it. Something
like:

	name = strndup_user((const char __user *)arg,
			    ANON_VMA_NAME_MAX_LEN);
	escaped = kasprintf(GFP_KERNEL, "%pE", name);
	if (escaped) {
		kfree(name);
		return -ENOMEM;
	}
	kfree(name);
	name = escaped;

-- 
Kees Cook