linux-kernel - Re: [PATCH 04/12] __wr_after_init: x86_64: __wr

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Date:   Thu, 20 Dec 2018 15:20:29 -0200
From:   Thiago Jung Bauermann <bauerman@...ux.ibm.com>
To:     Igor Stoppa <igor.stoppa@...il.com>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Matthew Wilcox <willy@...radead.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Mimi Zohar <zohar@...ux.vnet.ibm.com>, igor.stoppa@...wei.com,
        Nadav Amit <nadav.amit@...il.com>,
        Kees Cook <keescook@...omium.org>,
        linux-integrity@...r.kernel.org,
        kernel-hardening@...ts.openwall.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 04/12] __wr_after_init: x86_64: __wr_op


Hello Igor,

> +/*
> + * The following two variables are statically allocated by the linker
> + * script at the the boundaries of the memory region (rounded up to
> + * multiples of PAGE_SIZE) reserved for __wr_after_init.
> + */
> +extern long __start_wr_after_init;
> +extern long __end_wr_after_init;
> +
> +static inline bool is_wr_after_init(unsigned long ptr, __kernel_size_t size)
> +{
> +	unsigned long start = (unsigned long)&__start_wr_after_init;
> +	unsigned long end = (unsigned long)&__end_wr_after_init;
> +	unsigned long low = ptr;
> +	unsigned long high = ptr + size;
> +
> +	return likely(start <= low && low <= high && high <= end);
> +}
> +
> +void *__wr_op(unsigned long dst, unsigned long src, __kernel_size_t len,
> +	      enum wr_op_type op)
> +{
> +	temporary_mm_state_t prev;
> +	unsigned long offset;
> +	unsigned long wr_poking_addr;
> +
> +	/* Confirm that the writable mapping exists. */
> +	if (WARN_ONCE(!wr_ready, "No writable mapping available"))
> +		return (void *)dst;
> +
> +	if (WARN_ONCE(op >= WR_OPS_NUMBER, "Invalid WR operation.") ||
> +	    WARN_ONCE(!is_wr_after_init(dst, len), "Invalid WR range."))
> +		return (void *)dst;
> +
> +	offset = dst - (unsigned long)&__start_wr_after_init;
> +	wr_poking_addr = wr_poking_base + offset;
> +	local_irq_disable();
> +	prev = use_temporary_mm(wr_poking_mm);
> +
> +	if (op == WR_MEMCPY)
> +		copy_to_user((void __user *)wr_poking_addr, (void *)src, len);
> +	else if (op == WR_MEMSET)
> +		memset_user((void __user *)wr_poking_addr, (u8)src, len);
> +
> +	unuse_temporary_mm(prev);
> +	local_irq_enable();
> +	return (void *)dst;
> +}

There's a lot of casting back and forth between unsigned long and void *
(also in the previous patch). Is there a reason for that? My impression
is that there would be less casts if variables holding addresses were
declared as void * in the first place. In that case, it wouldn't hurt to
have an additional argument in __rw_op() to carry the byte value for the
WR_MEMSET operation.

> +
> +#define TB (1UL << 40)
> +
> +struct mm_struct *copy_init_mm(void);
> +void __init wr_poking_init(void)
> +{
> +	unsigned long start = (unsigned long)&__start_wr_after_init;
> +	unsigned long end = (unsigned long)&__end_wr_after_init;
> +	unsigned long i;
> +	unsigned long wr_range;
> +
> +	wr_poking_mm = copy_init_mm();
> +	if (WARN_ONCE(!wr_poking_mm, "No alternate mapping available."))
> +		return;
> +
> +	wr_range = round_up(end - start, PAGE_SIZE);
> +
> +	/* Randomize the poking address base*/
> +	wr_poking_base = TASK_UNMAPPED_BASE +
> +		(kaslr_get_random_long("Write Rare Poking") & PAGE_MASK) %
> +		(TASK_SIZE - (TASK_UNMAPPED_BASE + wr_range));
> +
> +	/*
> +	 * Place 64TB of kernel address space within 128TB of user address
> +	 * space, at a random page aligned offset.
> +	 */
> +	wr_poking_base = (((unsigned long)kaslr_get_random_long("WR Poke")) &
> +			  PAGE_MASK) % (64 * _BITUL(40));

You're setting wr_poking_base twice in a row? Is this an artifact from
rebase?

--
Thiago Jung Bauermann
IBM Linux Technology Center