linux-hardening - Re: [PATCH] mm/vmalloc: randomize vmalloc() allocations

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Message-ID: <20201202185334.GG11935@casper.infradead.org>
Date:   Wed, 2 Dec 2020 18:53:34 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Topi Miettinen <toiwoton@...il.com>
Cc:     linux-hardening@...r.kernel.org, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andy Lutomirski <luto@...nel.org>,
        Jann Horn <jannh@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Linux API <linux-api@...r.kernel.org>,
        Mike Rapoport <rppt@...nel.org>
Subject: Re: [PATCH] mm/vmalloc: randomize vmalloc() allocations

On Tue, Dec 01, 2020 at 11:45:47PM +0200, Topi Miettinen wrote:
> +	/* Randomize allocation */
> +	if (randomize_vmalloc) {
> +		voffset = get_random_long() & (roundup_pow_of_two(vend - vstart) - 1);
> +		voffset = PAGE_ALIGN(voffset);
> +		if (voffset + size > vend - vstart)
> +			voffset = vend - vstart - size;
> +	} else
> +		voffset = 0;
> +
>  	/*
>  	 * If an allocation fails, the "vend" address is
>  	 * returned. Therefore trigger the overflow path.
>  	 */
> -	addr = __alloc_vmap_area(size, align, vstart, vend);
> +	addr = __alloc_vmap_area(size, align, vstart + voffset, vend);
>  	spin_unlock(&free_vmap_area_lock);

What if there isn't any free address space between vstart+voffset and
vend, but there is free address space between vstart and voffset?
Seems like we should add:

	addr = __alloc_vmap_area(size, align, vstart + voffset, vend);
+	if (!addr)
+		addr = __alloc_vmap_area(size, align, vstart, vend);
	spin_unlock(&free_vmap_area_lock);