linux-kernel - Re: [PATCH v2 2/2] x86, refcount: Implement fast refcount overflow protection

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Message-ID: <20170501163009.kbemdhpsabdrsfex@treble>
Date:   Mon, 1 May 2017 11:30:09 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        PaX Team <pageexec@...email.hu>, Jann Horn <jannh@...gle.com>,
        Eric Biggers <ebiggers3@...il.com>,
        Christoph Hellwig <hch@...radead.org>,
        "axboe@...nel.dk" <axboe@...nel.dk>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        Elena Reshetova <elena.reshetova@...el.com>,
        Hans Liljestrand <ishkamiel@...il.com>,
        David Windsor <dwindsor@...il.com>,
        "x86@...nel.org" <x86@...nel.org>, Ingo Molnar <mingo@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "David S. Miller" <davem@...emloft.net>,
        Rik van Riel <riel@...hat.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v2 2/2] x86, refcount: Implement fast refcount overflow
 protection

> +#define __REFCOUNT_EXCEPTION(size)			\
> +	".if "__stringify(size)" == 4\n\t"		\
> +	".pushsection .text.refcount_overflow\n"	\
> +	".elseif "__stringify(size)" == -4\n\t"		\
> +	".pushsection .text.refcount_underflow\n"	\
> +	".else\n"					\
> +	".error \"invalid size\"\n"			\
> +	".endif\n"					\
> +	"111:\tlea %[counter],%%"_ASM_CX"\n\t"		\
> +	"int $"__stringify(X86_REFCOUNT_VECTOR)"\n"	\
> +	"222:\n\t"					\
> +	".popsection\n"					\
> +	"333:\n"					\
> +	_ASM_EXTABLE(222b, 333b)

The 'size' argument doesn't seem to correspond to an actual size of
anything.  Its value '4' or '-4' only seems to indicate whether it's an
overflow or an underflow.

Also there's some inconsistent use of "\n\t" on some lines, with "\n" on
others.

> +dotraplinkage void do_refcount_error(struct pt_regs *regs, long error_code)
> +{
> +	const char *str = NULL;
> +
> +	BUG_ON(!(regs->flags & X86_EFLAGS_SF));
> +
> +#define range_check(size, dir, type, value)				   \
> +	do {								   \
> +		if ((unsigned long)__##size##_##dir##_start <= regs->ip && \
> +		    regs->ip < (unsigned long)__##size##_##dir##_end) {	   \
> +			*(type *)regs->cx = (value);			   \
> +			str = #size " " #dir;				   \
> +		}							   \
> +	} while (0)

An interrupt was used, not a faulting exception, so regs->ip refers to
the address *after* the 'int' instruction.  So the beginning of the
range should be exclusive, and the end of the range should be inclusive,
like:

> +		if ((unsigned long)__##size##_##dir##_start < regs->ip &&  \
> +		    regs->ip <= (unsigned long)__##size##_##dir##_end) {   \

> +
> +	/*
> +	 * Reset to INT_MAX in both cases to attempt to let system
> +	 * continue operating.
> +	 */
> +	range_check(refcount,   overflow,  int, INT_MAX);
> +	range_check(refcount,   underflow, int, INT_MAX);

I think "range_check" doesn't adequately describe the macro.  In
addition to checking, it has a subtle side effect: it updates the
counter value with INT_MAX.

It's not clear why the 'size' argument has its name.  Also, three of the
arguments are always called with the same value.  Anyway I suspect the
code would be more readable if it were open coded without the macro.

> +#ifdef CONFIG_FAST_REFCOUNT
> +static DEFINE_RATELIMIT_STATE(refcount_ratelimit, 15 * HZ, 3);
> +
> +void refcount_error_report(struct pt_regs *regs, const char *kind)
> +{
> +	do_send_sig_info(SIGKILL, SEND_SIG_FORCED, current, true);
> +
> +	if (!__ratelimit(&refcount_ratelimit))
> +		return;
> +
> +	pr_emerg("%s detected in: %s:%d, uid/euid: %u/%u\n",
> +		kind ? kind : "refcount error",
> +		current->comm, task_pid_nr(current),
> +		from_kuid_munged(&init_user_ns, current_uid()),
> +		from_kuid_munged(&init_user_ns, current_euid()));
> +	print_symbol(KERN_EMERG "refcount error occurred at: %s\n",
> +		instruction_pointer(regs));
> +	preempt_disable();
> +	show_regs(regs);
> +	preempt_enable();
> +}

Why is preemption disabled before calling show_regs()?

> +EXPORT_SYMBOL(refcount_error_report);

Why is this exported?  It looks like it's only called internally from
traps.c.

-- 
Josh