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Date:   Mon, 8 Mar 2021 13:38:30 -0800
From:   Kees Cook <keescook@chromium.org>
To:     Topi Miettinen <toiwoton@gmail.com>
Cc:     linux-hardening@vger.kernel.org, akpm@linux-foundation.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Andy Lutomirski <luto@kernel.org>,
        Jann Horn <jannh@google.com>,
        Linux API <linux-api@vger.kernel.org>,
        Matthew Wilcox <willy@infradead.org>,
        Mike Rapoport <rppt@kernel.org>, Vlad Rezki <urezki@gmail.com>
Subject: Re: [PATCH v3] mm/vmalloc: randomize vmalloc() allocations
Message-ID: <202103081329.461F3D035@keescook>
References: <20210215202634.5121-1-toiwoton@gmail.com>
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In-Reply-To: <20210215202634.5121-1-toiwoton@gmail.com>
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On Mon, Feb 15, 2021 at 10:26:34PM +0200, Topi Miettinen wrote:
> Memory mappings inside kernel allocated with vmalloc() are in
> predictable order and packed tightly toward the low addresses, except
> for per-cpu areas which start from top of the vmalloc area. With
> new kernel boot parameter 'randomize_vmalloc=1', the entire area is
> used randomly to make the allocations less predictable and harder to
> guess for attackers. Also module and BPF code locations get randomized
> (within their dedicated and rather small area though) and if
> CONFIG_VMAP_STACK is enabled, also kernel thread stack locations.
> 
> On 32 bit systems this may cause problems due to increased VM
> fragmentation if the address space gets crowded.
> 
> On all systems, it will reduce performance and increase memory and
> cache usage due to less efficient use of page tables and inability to
> merge adjacent VMAs with compatible attributes. On x86_64 with 5 level
> page tables, in the worst case, additional page table entries of up to
> 4 pages are created for each mapping, so with small mappings there's
> considerable penalty.
> 
> Without randomize_vmalloc=1:
> $ grep -v kernel_clone /proc/vmallocinfo
> 0xffffc90000000000-0xffffc90000009000   36864 irq_init_percpu_irqstack+0x176/0x1c0 vmap
> 0xffffc90000009000-0xffffc9000000b000    8192 acpi_os_map_iomem+0x2ac/0x2d0 phys=0x000000001ffe1000 ioremap
> 0xffffc9000000c000-0xffffc9000000f000   12288 acpi_os_map_iomem+0x2ac/0x2d0 phys=0x000000001ffe0000 ioremap
> 0xffffc9000000f000-0xffffc90000011000    8192 hpet_enable+0x31/0x4a4 phys=0x00000000fed00000 ioremap
> 0xffffc90000011000-0xffffc90000013000    8192 gen_pool_add_owner+0x49/0x130 pages=1 vmalloc
> 0xffffc90000013000-0xffffc90000015000    8192 gen_pool_add_owner+0x49/0x130 pages=1 vmalloc
> 0xffffc90000015000-0xffffc90000017000    8192 gen_pool_add_owner+0x49/0x130 pages=1 vmalloc
> 0xffffc90000021000-0xffffc90000023000    8192 gen_pool_add_owner+0x49/0x130 pages=1 vmalloc
> 0xffffc90000023000-0xffffc90000025000    8192 acpi_os_map_iomem+0x2ac/0x2d0 phys=0x00000000fed00000 ioremap
> 0xffffc90000025000-0xffffc90000027000    8192 memremap+0x19c/0x280 phys=0x00000000000f5000 ioremap
> 0xffffc90000031000-0xffffc90000036000   20480 pcpu_create_chunk+0xe8/0x260 pages=4 vmalloc
> 0xffffc90000043000-0xffffc90000047000   16384 n_tty_open+0x11/0xe0 pages=3 vmalloc
> 0xffffc90000211000-0xffffc90000232000  135168 crypto_scomp_init_tfm+0xc6/0xf0 pages=32 vmalloc
> 0xffffc90000232000-0xffffc90000253000  135168 crypto_scomp_init_tfm+0x67/0xf0 pages=32 vmalloc
> 0xffffc900005a9000-0xffffc900005ba000   69632 pcpu_create_chunk+0x7b/0x260 pages=16 vmalloc
> 0xffffc900005ba000-0xffffc900005cc000   73728 pcpu_create_chunk+0xb2/0x260 pages=17 vmalloc
> 0xffffe8ffffc00000-0xffffe8ffffe00000 2097152 pcpu_get_vm_areas+0x0/0x2290 vmalloc
> 
> With randomize_vmalloc=1, the allocations are randomized:
> $ grep -v kernel_clone /proc/vmallocinfo
> 0xffffc9759d443000-0xffffc9759d445000    8192 hpet_enable+0x31/0x4a4 phys=0x00000000fed00000 ioremap
> 0xffffccf1e9f66000-0xffffccf1e9f68000    8192 gen_pool_add_owner+0x49/0x130 pages=1 vmalloc
> 0xffffcd2fc02a4000-0xffffcd2fc02a6000    8192 gen_pool_add_owner+0x49/0x130 pages=1 vmalloc
> 0xffffcdaefb898000-0xffffcdaefb89b000   12288 acpi_os_map_iomem+0x2ac/0x2d0 phys=0x000000001ffe0000 ioremap
> 0xffffcef8074c3000-0xffffcef8074cc000   36864 irq_init_percpu_irqstack+0x176/0x1c0 vmap
> 0xffffcf725ca2e000-0xffffcf725ca4f000  135168 crypto_scomp_init_tfm+0xc6/0xf0 pages=32 vmalloc
> 0xffffd0efb25e1000-0xffffd0efb25f2000   69632 pcpu_create_chunk+0x7b/0x260 pages=16 vmalloc
> 0xffffd27054678000-0xffffd2705467c000   16384 n_tty_open+0x11/0xe0 pages=3 vmalloc
> 0xffffd2adf716e000-0xffffd2adf7180000   73728 pcpu_create_chunk+0xb2/0x260 pages=17 vmalloc
> 0xffffd4ba5fb6b000-0xffffd4ba5fb6d000    8192 acpi_os_map_iomem+0x2ac/0x2d0 phys=0x000000001ffe1000 ioremap
> 0xffffded126192000-0xffffded126194000    8192 memremap+0x19c/0x280 phys=0x00000000000f5000 ioremap
> 0xffffe01a4dbcd000-0xffffe01a4dbcf000    8192 gen_pool_add_owner+0x49/0x130 pages=1 vmalloc
> 0xffffe4b649952000-0xffffe4b649954000    8192 acpi_os_map_iomem+0x2ac/0x2d0 phys=0x00000000fed00000 ioremap
> 0xffffe71ed592a000-0xffffe71ed592c000    8192 gen_pool_add_owner+0x49/0x130 pages=1 vmalloc
> 0xffffe7dc5824f000-0xffffe7dc58270000  135168 crypto_scomp_init_tfm+0x67/0xf0 pages=32 vmalloc
> 0xffffe8f4f9800000-0xffffe8f4f9a00000 2097152 pcpu_get_vm_areas+0x0/0x2290 vmalloc
> 0xffffe8f4f9a19000-0xffffe8f4f9a1e000   20480 pcpu_create_chunk+0xe8/0x260 pages=4 vmalloc
> 
> With CONFIG_VMAP_STACK, also kernel thread stacks are placed in
> vmalloc area and therefore they also get randomized (only one example
> line from /proc/vmallocinfo shown for brevity):
> 
> unrandomized:
> 0xffffc90000018000-0xffffc90000021000   36864 kernel_clone+0xf9/0x560 pages=8 vmalloc
> 
> randomized:
> 0xffffcb57611a8000-0xffffcb57611b1000   36864 kernel_clone+0xf9/0x560 pages=8 vmalloc
> 
> CC: Andrew Morton <akpm@linux-foundation.org>
> CC: Andy Lutomirski <luto@kernel.org>
> CC: Jann Horn <jannh@google.com>
> CC: Kees Cook <keescook@chromium.org>
> CC: Linux API <linux-api@vger.kernel.org>
> CC: Matthew Wilcox <willy@infradead.org>
> CC: Mike Rapoport <rppt@kernel.org>
> CC: Vlad Rezki <urezki@gmail.com>
> Signed-off-by: Topi Miettinen <toiwoton@gmail.com>

Thanks for working on this! I'd like to see this in the kernel, even if
it's only for the more paranoia crowd. Have you done any workload
measurements to see how much of a hit this has in the real world?

> ---
> v2: retry allocation from other end of vmalloc space in case of
> failure (Matthew Wilcox), improve commit message and documentation
> v3: randomize also percpu allocations (pcpu_get_vm_areas())
> ---
>  .../admin-guide/kernel-parameters.txt         | 23 ++++++++++++
>  mm/vmalloc.c                                  | 36 +++++++++++++++++--
>  2 files changed, 56 insertions(+), 3 deletions(-)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index c722ec19cd00..38d6b5728ccc 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -4024,6 +4024,29 @@
>  
>  	ramdisk_start=	[RAM] RAM disk image start address
>  
> +	randomize_vmalloc= [KNL] Randomize vmalloc() allocations. With 1,
> +			the entire vmalloc() area is used randomly to
> +			make the allocations less predictable and
> +			harder to guess for attackers. Also module and
> +			BPF code locations get randomized (within
> +			their dedicated and rather small area though)
> +			and if CONFIG_VMAP_STACK is enabled, also
> +			kernel thread stack locations.
> +
> +			On 32 bit systems this may cause problems due
> +			to increased VM fragmentation if the address
> +			space gets crowded.
> +
> +			On all systems, it will reduce performance and
> +			increase memory and cache usage due to less
> +			efficient use of page tables and inability to
> +			merge adjacent VMAs with compatible
> +			attributes. On x86_64 with 5 level page
> +			tables, in the worst case, additional page
> +			table entries of up to 4 pages are created for
> +			each mapping, so with small mappings there's
> +			considerable penalty.
> +
>  	random.trust_cpu={on,off}
>  			[KNL] Enable or disable trusting the use of the
>  			CPU's random number generator (if available) to
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 4d88fe5a277a..1e8e0ee1925f 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -34,6 +34,7 @@
>  #include <linux/bitops.h>
>  #include <linux/rbtree_augmented.h>
>  #include <linux/overflow.h>
> +#include <linux/random.h>
>  
>  #include <linux/uaccess.h>
>  #include <asm/tlbflush.h>
> @@ -1089,6 +1090,17 @@ adjust_va_to_fit_type(struct vmap_area *va,
>  	return 0;
>  }
>  
> +static int randomize_vmalloc = 0;

This should be __ro_after_init, and even better, it should be a static
branch so there's no performance hit at all for the "disabled" case.
I recommend:

static DEFINE_STATIC_KEY_FALSE_RO(randomize_vmalloc);

static int __init set_randomize_vmalloc(char *str)
{
       int ret;
       bool bool_result;

       ret = kstrtobool(buf, &bool_result);
       if (ret)
               return ret;

       if (bool_result)
               static_branch_enable(&randomize_kstack_offset);
       else
               static_branch_disable(&randomize_kstack_offset);
       return 1;
}
__setup("randomize_vmalloc=", set_randomize_vmalloc);


> +
>  /*
>   * Returns a start address of the newly allocated area, if success.
>   * Otherwise a vend is returned that indicates failure.
> @@ -1162,7 +1174,7 @@ static struct vmap_area *alloc_vmap_area(unsigned long size,
>  				int node, gfp_t gfp_mask)
>  {
>  	struct vmap_area *va, *pva;
> -	unsigned long addr;
> +	unsigned long addr, voffset;
>  	int purged = 0;
>  	int ret;
>  
> @@ -1217,11 +1229,24 @@ static struct vmap_area *alloc_vmap_area(unsigned long size,
>  	if (pva && __this_cpu_cmpxchg(ne_fit_preload_node, NULL, pva))
>  		kmem_cache_free(vmap_area_cachep, pva);
>  
> +	/* Randomize allocation */
> +	if (randomize_vmalloc) {

if (static_branch_unlikely(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);
> +
> +	if (unlikely(addr == vend) && voffset)
> +		/* Retry randomization from other end */
> +		addr = __alloc_vmap_area(size, align, vstart, vstart + voffset + size);
>  	spin_unlock(&free_vmap_area_lock);
>  
>  	if (unlikely(addr == vend))
> @@ -3256,7 +3281,12 @@ struct vm_struct **pcpu_get_vm_areas(const unsigned long *offsets,
>  	start = offsets[area];
>  	end = start + sizes[area];
>  
> -	va = pvm_find_va_enclose_addr(vmalloc_end);
> +	if (randomize_vmalloc)
> +		va = pvm_find_va_enclose_addr(vmalloc_start +
> +					      (get_random_long() &
> +					       (roundup_pow_of_two(vmalloc_end - vmalloc_start) - 1)));
> +	else
> +		va = pvm_find_va_enclose_addr(vmalloc_end);
>  	base = pvm_determine_end_from_reverse(&va, align) - end;
>  
>  	while (true) {
> -- 
> 2.30.0
> 

But otherwise, yes please. It's a simple change that makes per-boot
order of vmalloc allocations unpredictable. I'm for it! :)

-- 
Kees Cook