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Date:   Mon, 23 Apr 2018 19:47:22 -0700 (PDT)
From:   David Rientjes <rientjes@google.com>
To:     Mikulas Patocka <mpatocka@redhat.com>
cc:     Michal Hocko <mhocko@kernel.org>,
        Matthew Wilcox <willy@infradead.org>,
        David Miller <davem@davemloft.net>,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        eric.dumazet@gmail.com, edumazet@google.com,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        mst@redhat.com, jasowang@redhat.com,
        virtualization@lists.linux-foundation.org, dm-devel@redhat.com,
        Vlastimil Babka <vbabka@suse.cz>
Subject: Re: [PATCH v3] kvmalloc: always use vmalloc if CONFIG_DEBUG_SG
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On Mon, 23 Apr 2018, Mikulas Patocka wrote:

> The kvmalloc function tries to use kmalloc and falls back to vmalloc if
> kmalloc fails.
> 
> Unfortunatelly, some kernel code has bugs - it uses kvmalloc and then
> uses DMA-API on the returned memory or frees it with kfree. Such bugs were
> found in the virtio-net driver, dm-integrity or RHEL7 powerpc-specific
> code.
> 
> These bugs are hard to reproduce because kvmalloc falls back to vmalloc
> only if memory is fragmented.
> 
> In order to detect these bugs reliably I submit this patch that changes
> kvmalloc to fall back to vmalloc with 1/2 probability if CONFIG_DEBUG_SG
> is turned on. CONFIG_DEBUG_SG is used, because it makes the DMA API layer
> verify the addresses passed to it, and so the user will get a reliable
> stacktrace.
> 

Why not just do it unconditionally?  Sounds better than "50% of the time 
this will catch bugs".

> Some bugs (such as buffer overflows) are better detected
> with kmalloc code, so we must test the kmalloc path too.
> 
> Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
> 
> ---
>  mm/util.c |   10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> Index: linux-2.6/mm/util.c
> ===================================================================
> --- linux-2.6.orig/mm/util.c	2018-04-23 00:12:05.000000000 +0200
> +++ linux-2.6/mm/util.c	2018-04-23 17:57:02.000000000 +0200
> @@ -14,6 +14,7 @@
>  #include <linux/hugetlb.h>
>  #include <linux/vmalloc.h>
>  #include <linux/userfaultfd_k.h>
> +#include <linux/random.h>
>  
>  #include <asm/sections.h>
>  #include <linux/uaccess.h>
> @@ -404,6 +405,12 @@ void *kvmalloc_node(size_t size, gfp_t f
>  	 */
>  	WARN_ON_ONCE((flags & GFP_KERNEL) != GFP_KERNEL);
>  
> +#ifdef CONFIG_DEBUG_SG
> +	/* Catch bugs when the caller uses DMA API on the result of kvmalloc. */
> +	if (!(prandom_u32_max(2) & 1))
> +		goto do_vmalloc;
> +#endif
> +
>  	/*
>  	 * We want to attempt a large physically contiguous block first because
>  	 * it is less likely to fragment multiple larger blocks and therefore
> @@ -427,6 +434,9 @@ void *kvmalloc_node(size_t size, gfp_t f
>  	if (ret || size <= PAGE_SIZE)
>  		return ret;
>  
> +#ifdef CONFIG_DEBUG_SG
> +do_vmalloc:
> +#endif

You can just do

do_vmalloc: __maybe_unused

>  	return __vmalloc_node_flags_caller(size, node, flags,
>  			__builtin_return_address(0));
>  }
> 
>