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Date: Tue, 25 Apr 2017 12:23:37 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Kees Cook <keescook@...omium.org>
Cc: linux-kernel@...r.kernel.org, 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,
Ingo Molnar <mingo@...nel.org>, Arnd Bergmann <arnd@...db.de>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Jann Horn <jann@...jh.net>, davem@...emloft.net,
linux-arch@...r.kernel.org, kernel-hardening@...ts.openwall.com,
PaX Team <pageexec@...email.hu>
Subject: Re: [PATCH] x86/refcount: Implement fast refcount_t handling
On Fri, Apr 21, 2017 at 03:09:39PM -0700, Kees Cook wrote:
> +static __always_inline void refcount_inc(refcount_t *r)
> +{
> + asm volatile(LOCK_PREFIX "incl %0\n\t"
> + REFCOUNT_CHECK_OVERFLOW(4)
> + : [counter] "+m" (r->refs.counter)
> + : : "cc", "cx");
> +}
> +
> +static __always_inline void refcount_dec(refcount_t *r)
> +{
> + asm volatile(LOCK_PREFIX "decl %0\n\t"
> + REFCOUNT_CHECK_UNDERFLOW(4)
> + : [counter] "+m" (r->refs.counter)
> + : : "cc", "cx");
> +}
> +dotraplinkage void do_refcount_error(struct pt_regs *regs, long error_code)
> +{
> + const char *str = NULL;
> +
> + BUG_ON(!(regs->flags & X86_EFLAGS_OF));
> +
> +#define range_check(size, direction, type, value) \
> + if ((unsigned long)__##size##_##direction##_start <= regs->ip && \
> + regs->ip < (unsigned long)__##size##_##direction##_end) { \
> + *(type *)regs->cx = value; \
> + str = #size " " #direction; \
> + }
> +
> + range_check(refcount, overflow, int, INT_MAX)
> + range_check(refcount, underflow, int, INT_MIN)
> +
> +#undef range_check
> +
> + BUG_ON(!str);
> + do_error_trap(regs, error_code, (char *)str, X86_REFCOUNT_VECTOR,
> + SIGILL);
> +}
> +#endif
So what avoids this:
CPU0 CPU1
lock inc %[val]; # 0x7fffffff
jo 2f
1: ...
lock dec %[val]; # 0x80000000
jo 2f
1: ...
2: mov $0x7fffffff, %[val]
jmp 1b
2: mov $0x80000000, %[val]
jmp 1b
~~~~//~~~~
lock inc %val; #0x80000000
....
lock inc %val; 0xffffffff
lock inc %val; 0x00000000
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