RE: [PATCH AUTOSEL 5.5 389/542] clocksource/drivers/hyper-v: Reserve PAGE_SIZE space for tsc page

[Michael Kelley]

This patch does not need to be backported to any stable releases.  It is prep work for guests on Hyper-V ARM64 when the guest page size is 16K or 64K, and that functionality isn't upstream yet.

Michael

> -----Original Message-----
> From: Sasha Levin <sashal@xxxxxxxxxx>
> Sent: Friday, February 14, 2020 7:46 AM
> To: linux-kernel@xxxxxxxxxxxxxxx; stable@xxxxxxxxxxxxxxx
> Cc: Boqun Feng <boqun.feng@xxxxxxxxx>; linux-hyperv@xxxxxxxxxxxxxxx; Michael Kelley
> <mikelley@xxxxxxxxxxxxx>; Daniel Lezcano <daniel.lezcano@xxxxxxxxxx>; Sasha Levin
> <sashal@xxxxxxxxxx>
> Subject: [PATCH AUTOSEL 5.5 389/542] clocksource/drivers/hyper-v: Reserve PAGE_SIZE
> space for tsc page
> 
> From: Boqun Feng <boqun.feng@xxxxxxxxx>
> 
> [ Upstream commit ddc61bbc45017726a2b450350d476b4dc5ae25ce ]
> 
> Currently, the reserved size for a tsc page is 4K, which is enough for
> communicating with hypervisor. However, in the case where we want to
> export the tsc page to userspace (e.g. for vDSO to read the
> clocksource), the tsc page should be at least PAGE_SIZE, otherwise, when
> PAGE_SIZE is larger than 4K, extra kernel data will be mapped into
> userspace, which means leaking kernel information.
> 
> Therefore reserve PAGE_SIZE space for tsc_pg as a preparation for the
> vDSO support of ARM64 in the future. Also, while at it, replace all
> reference to tsc_pg with hv_get_tsc_page() since it should be the only
> interface to access tsc page.
> 
> Signed-off-by: Boqun Feng (Microsoft) <boqun.feng@xxxxxxxxx>
> Cc: linux-hyperv@xxxxxxxxxxxxxxx
> Reviewed-by: Michael Kelley <mikelley@xxxxxxxxxxxxx>
> Signed-off-by: Daniel Lezcano <daniel.lezcano@xxxxxxxxxx>
> Link:
> https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Fr
> %2F20191126021723.4710-1-
> boqun.feng%40gmail.com&amp;data=02%7C01%7Cmikelley%40microsoft.com%7C2f481a9
> 3db624cb4b5e208d7b16691f6%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C6371
> 72926408924092&amp;sdata=%2F1JNMBiq7l1ufSsWphZb%2FMLNep2EIOCzlXcG%2F9foZNw
> %3D&amp;reserved=0
> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
> ---
>  drivers/clocksource/hyperv_timer.c | 11 +++++++----
>  1 file changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/clocksource/hyperv_timer.c b/drivers/clocksource/hyperv_timer.c
> index 287d8d58c21ac..b6ea3a2093c56 100644
> --- a/drivers/clocksource/hyperv_timer.c
> +++ b/drivers/clocksource/hyperv_timer.c
> @@ -307,17 +307,20 @@ EXPORT_SYMBOL_GPL(hv_stimer_global_cleanup);
>  struct clocksource *hyperv_cs;
>  EXPORT_SYMBOL_GPL(hyperv_cs);
> 
> -static struct ms_hyperv_tsc_page tsc_pg __aligned(PAGE_SIZE);
> +static union {
> +	struct ms_hyperv_tsc_page page;
> +	u8 reserved[PAGE_SIZE];
> +} tsc_pg __aligned(PAGE_SIZE);
> 
>  struct ms_hyperv_tsc_page *hv_get_tsc_page(void)
>  {
> -	return &tsc_pg;
> +	return &tsc_pg.page;
>  }
>  EXPORT_SYMBOL_GPL(hv_get_tsc_page);
> 
>  static u64 notrace read_hv_clock_tsc(struct clocksource *arg)
>  {
> -	u64 current_tick = hv_read_tsc_page(&tsc_pg);
> +	u64 current_tick = hv_read_tsc_page(hv_get_tsc_page());
> 
>  	if (current_tick == U64_MAX)
>  		hv_get_time_ref_count(current_tick);
> @@ -372,7 +375,7 @@ static bool __init hv_init_tsc_clocksource(void)
>  		return false;
> 
>  	hyperv_cs = &hyperv_cs_tsc;
> -	phys_addr = virt_to_phys(&tsc_pg);
> +	phys_addr = virt_to_phys(hv_get_tsc_page());
> 
>  	/*
>  	 * The Hyper-V TLFS specifies to preserve the value of reserved
> --
> 2.20.1