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Date:   Tue, 29 Sep 2020 15:24:11 +0100
From:   Mark Rutland <mark.rutland@arm.com>
To:     Marco Elver <elver@google.com>
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Subject: Re: [PATCH v3 01/10] mm: add Kernel Electric-Fence infrastructure
Message-ID: <20200929142411.GC53442@C02TD0UTHF1T.local>
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 <20200921132611.1700350-2-elver@google.com>
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On Mon, Sep 21, 2020 at 03:26:02PM +0200, Marco Elver wrote:
> From: Alexander Potapenko <glider@google.com>
> 
> This adds the Kernel Electric-Fence (KFENCE) infrastructure. KFENCE is a
> low-overhead sampling-based memory safety error detector of heap
> use-after-free, invalid-free, and out-of-bounds access errors.
> 
> KFENCE is designed to be enabled in production kernels, and has near
> zero performance overhead. Compared to KASAN, KFENCE trades performance
> for precision. The main motivation behind KFENCE's design, is that with
> enough total uptime KFENCE will detect bugs in code paths not typically
> exercised by non-production test workloads. One way to quickly achieve a
> large enough total uptime is when the tool is deployed across a large
> fleet of machines.
> 
> KFENCE objects each reside on a dedicated page, at either the left or
> right page boundaries. The pages to the left and right of the object
> page are "guard pages", whose attributes are changed to a protected
> state, and cause page faults on any attempted access to them. Such page
> faults are then intercepted by KFENCE, which handles the fault
> gracefully by reporting a memory access error. To detect out-of-bounds
> writes to memory within the object's page itself, KFENCE also uses
> pattern-based redzones. The following figure illustrates the page
> layout:
> 
>   ---+-----------+-----------+-----------+-----------+-----------+---
>      | xxxxxxxxx | O :       | xxxxxxxxx |       : O | xxxxxxxxx |
>      | xxxxxxxxx | B :       | xxxxxxxxx |       : B | xxxxxxxxx |
>      | x GUARD x | J : RED-  | x GUARD x | RED-  : J | x GUARD x |
>      | xxxxxxxxx | E :  ZONE | xxxxxxxxx |  ZONE : E | xxxxxxxxx |
>      | xxxxxxxxx | C :       | xxxxxxxxx |       : C | xxxxxxxxx |
>      | xxxxxxxxx | T :       | xxxxxxxxx |       : T | xxxxxxxxx |
>   ---+-----------+-----------+-----------+-----------+-----------+---
> 
> Guarded allocations are set up based on a sample interval (can be set
> via kfence.sample_interval). After expiration of the sample interval, a
> guarded allocation from the KFENCE object pool is returned to the main
> allocator (SLAB or SLUB). At this point, the timer is reset, and the
> next allocation is set up after the expiration of the interval.

From other sub-threads it sounds like these addresses are not part of
the linear/direct map. Having kmalloc return addresses outside of the
linear map is going to break anything that relies on virt<->phys
conversions, and is liable to make DMA corrupt memory. There were
problems of that sort with VMAP_STACK, and this is why kvmalloc() is
separate from kmalloc().

Have you tested with CONFIG_DEBUG_VIRTUAL? I'd expect that to scream.

I strongly suspect this isn't going to be safe unless you always use an
in-place carevout from the linear map (which could be the linear alias
of a static carevout).

[...]

> +static __always_inline void *kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags)
> +{
> +	return static_branch_unlikely(&kfence_allocation_key) ? __kfence_alloc(s, size, flags) :
> +								      NULL;
> +}

Minor (unrelated) nit, but this would be easier to read as:

static __always_inline void *kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags)
{
	if (static_branch_unlikely(&kfence_allocation_key))
		return __kfence_alloc(s, size, flags);
	return NULL;
}

Thanks,
Mark.