Re: [RFC PATCH v2 11/15] khwasan, mm: perform untagged pointers comparison in krealloc

From: Ingo Molnar <mingo@kernel.org>
To: Andrey Konovalov <andreyknvl@google.com>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>,
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Subject: Re: [RFC PATCH v2 11/15] khwasan, mm: perform untagged pointers comparison in krealloc
Date: Sat, 24 Mar 2018 09:29:47 +0100	[thread overview]
Message-ID: <20180324082947.3isostkpsjraefqt@gmail.com> (raw)
In-Reply-To: <6eb08c160ae23eb890bd937ddf8346ba211df09f.1521828274.git.andreyknvl@google.com>

* Andrey Konovalov <andreyknvl@google.com> wrote:

> The krealloc function checks where the same buffer was reused or a new one
> allocated by comparing kernel pointers. KHWASAN changes memory tag on the
> krealloc'ed chunk of memory and therefore also changes the pointer tag of
> the returned pointer. Therefore we need to perform comparison on untagged
> (with tags reset) pointers to check whether it's the same memory region or
> not.
> 
> Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
> ---
>  mm/slab_common.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index a33e61315ca6..5911f2194cf7 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -1494,7 +1494,7 @@ void *krealloc(const void *p, size_t new_size, gfp_t flags)
>  	}
>  
>  	ret = __do_krealloc(p, new_size, flags);
> -	if (ret && p != ret)
> +	if (ret && khwasan_reset_tag(p) != khwasan_reset_tag(ret))
>  		kfree(p);

Small nit:

If 'reset' here means an all zeroes tag (upper byte) then khwasan_clear_tag() 
might be a slightly easier to read primitive?

Thanks,

	Ingo