linux-kernel - Re: [RFC] arm64: Enforce gettimeofday vdso structure read ordering

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Message-ID: <20160811155409.GE8232@arm.com>
Date:	Thu, 11 Aug 2016 16:54:10 +0100
From:	Will Deacon <will.deacon@....com>
To:	Christopher Covington <cov@...eaurora.org>
Cc:	Catalin Marinas <catalin.marinas@....com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Nathan Lynch <nathan_lynch@...tor.com>,
	Timur Tabi <timur@...eaurora.org>,
	Brent DeGraaf <bdegraaf@...eaurora.org>
Subject: Re: [RFC] arm64: Enforce gettimeofday vdso structure read ordering

On Thu, Aug 11, 2016 at 11:37:44AM -0400, Christopher Covington wrote:
> From: Brent DeGraaf <bdegraaf@...eaurora.org>
> 
> Prior gettimeofday code register read code is not architecturally
> correct as there is no control flow gating logic enforced
> immediately prior to the required isb.  Introduce explicit
> control-flow logic prior to register read in all cases so that
> the mrs read will always be done after all vdso data elements are
> read, and read all data elements within the protection logic
> provided by the sequence counter.

-ENOPARSE

Can you explain the problem that you're fixing here, please?

> Signed-off-by: Brent DeGraaf <bdegraaf@...eaurora.org>
> ---
>  arch/arm64/include/asm/vdso_datapage.h |   4 +-
>  arch/arm64/kernel/vdso.c               |  11 ++--
>  arch/arm64/kernel/vdso/gettimeofday.S  | 106 +++++++++++++++------------------
>  3 files changed, 56 insertions(+), 65 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/vdso_datapage.h b/arch/arm64/include/asm/vdso_datapage.h
> index 2b9a637..49a0a51 100644
> --- a/arch/arm64/include/asm/vdso_datapage.h
> +++ b/arch/arm64/include/asm/vdso_datapage.h
> @@ -21,6 +21,8 @@
>  #ifndef __ASSEMBLY__
>  
>  struct vdso_data {
> +	__u32 tb_seq_count;	/* Timebase sequence counter */
> +	__u32 use_syscall;
>  	__u64 cs_cycle_last;	/* Timebase at clocksource init */
>  	__u64 raw_time_sec;	/* Raw time */
>  	__u64 raw_time_nsec;
> @@ -30,14 +32,12 @@ struct vdso_data {
>  	__u64 xtime_coarse_nsec;
>  	__u64 wtm_clock_sec;	/* Wall to monotonic time */
>  	__u64 wtm_clock_nsec;
> -	__u32 tb_seq_count;	/* Timebase sequence counter */
>  	/* cs_* members must be adjacent and in this order (ldp accesses) */
>  	__u32 cs_mono_mult;	/* NTP-adjusted clocksource multiplier */
>  	__u32 cs_shift;		/* Clocksource shift (mono = raw) */
>  	__u32 cs_raw_mult;	/* Raw clocksource multiplier */
>  	__u32 tz_minuteswest;	/* Whacky timezone stuff */
>  	__u32 tz_dsttime;
> -	__u32 use_syscall;
>  };
>  
>  #endif /* !__ASSEMBLY__ */
> diff --git a/arch/arm64/kernel/vdso.c b/arch/arm64/kernel/vdso.c
> index 076312b..7751667 100644
> --- a/arch/arm64/kernel/vdso.c
> +++ b/arch/arm64/kernel/vdso.c
> @@ -201,10 +201,12 @@ up_fail:
>   */
>  void update_vsyscall(struct timekeeper *tk)
>  {
> +	register u32 tmp;
>  	u32 use_syscall = strcmp(tk->tkr_mono.clock->name, "arch_sys_counter");
>  
> -	++vdso_data->tb_seq_count;
> -	smp_wmb();
> +	tmp = smp_load_acquire(&vdso_data->tb_seq_count);
> +	++tmp;
> +	smp_store_release(&vdso_data->tb_seq_count, tmp);

This looks busted -- the writes to vdso_data can be reordered before the
update of tb_seq_count.

/confused

Will