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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
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