Messages in this thread | | | Date | Mon, 24 Jul 2017 11:07:10 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH v7 3/3] x86/refcount: Implement fast refcount overflow protection |
| |
* Kees Cook <keescook@chromium.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
| |