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Date: Mon, 24 Jul 2017 11:07:10 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Kees Cook <keescook@...omium.org>
Cc: Peter Zijlstra <peterz@...radead.org>,
Josh Poimboeuf <jpoimboe@...hat.com>,
Christoph Hellwig <hch@...radead.org>,
"Eric W. Biederman" <ebiederm@...ssion.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Jann Horn <jannh@...gle.com>, Eric Biggers <ebiggers3@...il.com>,
Elena Reshetova <elena.reshetova@...el.com>,
Hans Liljestrand <ishkamiel@...il.com>,
Greg KH <gregkh@...uxfoundation.org>,
Alexey Dobriyan <adobriyan@...il.com>,
"Serge E. Hallyn" <serge@...lyn.com>, arozansk@...hat.com,
Davidlohr Bueso <dave@...olabs.net>,
Manfred Spraul <manfred@...orfullife.com>,
"axboe@...nel.dk" <axboe@...nel.dk>,
James Bottomley <James.Bottomley@...senpartnership.com>,
"x86@...nel.org" <x86@...nel.org>, Arnd Bergmann <arnd@...db.de>,
"David S. Miller" <davem@...emloft.net>,
Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org,
linux-arch <linux-arch@...r.kernel.org>,
"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Peter Zijlstra <a.p.zijlstra@...llo.nl>,
Thomas Gleixner <tglx@...utronix.de>,
"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v7 3/3] x86/refcount: Implement fast refcount overflow
protection
* Kees Cook <keescook@...omium.org> wrote:
> +config ARCH_HAS_REFCOUNT
> + bool
> + help
> + An architecture selects this when it has implemented refcount_t
> + using primitizes that provide a faster runtime at the expense
> + of some full refcount state checks. The refcount overflow condition,
> + however, must be retained. Catching overflows is the primary
> + security concern for protecting against bugs in reference counts.
s/primitizes/primitives
also, the 'faster runtime' and the whole explanation reads a bit weird to me,
how about something like:
An architecture selects this when it has implemented refcount_t
using open coded assembly primitives that provide an optimized
refcount_t implementation, possibly at the expense of some full
refcount state checks of CONFIG_REFCOUNT_FULL=y.
The refcount overflow check behavior, however, must be retained.
Catching overflows is the primary security concern for protecting
against bugs in reference counts.
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -55,6 +55,7 @@ config X86
> select ARCH_HAS_KCOV if X86_64
> select ARCH_HAS_MMIO_FLUSH
> select ARCH_HAS_PMEM_API if X86_64
> + select ARCH_HAS_REFCOUNT
> select ARCH_HAS_UACCESS_FLUSHCACHE if X86_64
> select ARCH_HAS_SET_MEMORY
> select ARCH_HAS_SG_CHAIN
Just wonderin, how was the 32-bit kernel tested?
> +/*
> + * Body of refcount error handling: in .text.unlikely, saved into CX the
> + * address of the refcount that has entered a bad state, and trigger an
> + * exception. Fixup address is back in regular execution flow in .text.
I had to read this 4 times to parse it (and even now I'm unsure whether I parsed
it correctly) - could this explanation be transformed to simpler, more
straightforward English?
> + */
> +#define _REFCOUNT_EXCEPTION \
> + ".pushsection .text.unlikely\n" \
> + "111:\tlea %[counter], %%" _ASM_CX "\n" \
> + "112:\t" ASM_UD0 "\n" \
> + ASM_UNREACHABLE \
> + ".popsection\n" \
> + "113:\n" \
> + _ASM_EXTABLE_REFCOUNT(112b, 113b)
Would it be technically possible to use named labels instead of these random
numbered labels?
> + /*
> + * This function has been called because either a negative refcount
> + * value was seen by any of the refcount functions, or a zero
> + * refcount value was seen by refcount_dec().
> + *
> + * If we crossed from INT_MAX to INT_MIN, the OF flag (result
> + * wrapped around) will be set. Additionally, seeing the refcount
> + * reach 0 will set the ZF flag. In each of these cases we want a
> + * report, since it's a boundary condition.
Small nit: 'ZF' stands for 'zero flag' - so we should either write 'zero flag' or
'ZF' - 'ZF flag' is kind of redundant.
> +#else
> +static inline void refcount_error_report(struct pt_regs *regs,
> + const char *msg) { }
By now you should know that for x86 code you should not break lines in such an
ugly fashion, right? :-)
Thanks,
Ingo
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