From: Josh Poimboeuf <jpoimboe@redhat.com>
To: Kees Cook <keescook@chromium.org>
Cc: linux-kernel@vger.kernel.org,
Peter Zijlstra <peterz@infradead.org>,
PaX Team <pageexec@freemail.hu>, Jann Horn <jannh@google.com>,
Eric Biggers <ebiggers3@gmail.com>,
Christoph Hellwig <hch@infradead.org>,
"axboe@kernel.dk" <axboe@kernel.dk>,
James Bottomley <James.Bottomley@hansenpartnership.com>,
Elena Reshetova <elena.reshetova@intel.com>,
Hans Liljestrand <ishkamiel@gmail.com>,
David Windsor <dwindsor@gmail.com>,
"x86@kernel.org" <x86@kernel.org>, Ingo Molnar <mingo@kernel.org>,
Arnd Bergmann <arnd@arndb.de>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
"David S. Miller" <davem@davemloft.net>,
Rik van Riel <riel@redhat.com>,
linux-arch <linux-arch@vger.kernel.org>,
"kernel-hardening@lists.openwall.com"
<kernel-hardening@lists.openwall.com>
Subject: Re: [PATCH v2 2/2] x86, refcount: Implement fast refcount overflow protection
Date: Mon, 1 May 2017 11:30:09 -0500 [thread overview]
Message-ID: <20170501163009.kbemdhpsabdrsfex@treble> (raw)
In-Reply-To: <1493160997-126108-3-git-send-email-keescook@chromium.org>
> +#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
next prev parent reply other threads:[~2017-05-01 16:31 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-04-25 22:56 [PATCH v2 0/2] x86, refcount: Implement fast refcount overflow Kees Cook
2017-04-25 22:56 ` [PATCH v2 1/2] x86, asm: Add suffix macro for GEN_*_RMWcc() Kees Cook
2017-04-25 22:56 ` [PATCH v2 2/2] x86, refcount: Implement fast refcount overflow protection Kees Cook
2017-04-26 0:25 ` Jann Horn
2017-04-26 3:52 ` Kees Cook
2017-04-27 1:31 ` kbuild test robot
2017-04-27 20:22 ` Kees Cook
2017-05-01 15:54 ` Josh Poimboeuf
2017-05-01 17:28 ` Kees Cook
2017-05-01 22:33 ` Josh Poimboeuf
2017-05-01 16:30 ` Josh Poimboeuf [this message]
2017-05-01 17:36 ` Kees Cook
2017-05-01 22:45 ` Josh Poimboeuf
2017-04-26 2:01 ` [PATCH v2 0/2] x86, refcount: Implement fast refcount overflow PaX Team
2017-04-26 3:59 ` Kees Cook
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