RE: [PATCH] ACPI / OSL: Fix rcu synchronization logic

[Zheng, Lv]

Hi, Borislav

> From: Borislav Petkov [mailto:bp@xxxxxxxxx]
> Subject: Re: [PATCH] ACPI / OSL: Fix rcu synchronization logic
> 
> On Mon, Jan 09, 2017 at 05:56:09PM +0800, Lv Zheng wrote:
> > The rcu synchronization logic is originally provided to protect
> > apei_read()/apei_write() as in the APEI drivers, there is NMI event source
> > requiring non spinlock based synchronization mechanism.
> >
> > After that, ACPI developers think FADT registers may also require same
> > facility, so they moved the RCU stuffs to generic ACPI layer.
> >
> > So now non-task-context ACPI map lookup is only protected by RCU.
> >
> > This triggers problem as acpi_os_map_memory()/acpi_os_unmap_memory() can be
> > used to map/unmap tables as long as to map/unmap ACPI registers. When it is
> > used for the ACPI tables, the caller could invoke this very early. When it
> > is invoked earlier than workqueue_init() and later than
> > check_early_ioremp_leak(), invoking synchronize_rcu_expedited() can cause a
> > kernel hang.
> >
> > Actually this facility is only used to protect non-task-context ACPI map
> > lookup, and such mappings are only introduced by
> > acpi_os_map_generic_address(). So before it is invoked, there is no need to
> > invoke synchronize_rcu_expedited().
> >
> > Suggested-by: Huang Ying <ying.huang@xxxxxxxxx>
> > Signed-off-by: Lv Zheng <lv.zheng@xxxxxxxxx>
> > Cc: Huang Ying <ying.huang@xxxxxxxxx>
> > Cc: Borislav Petkov <bp@xxxxxxxxx>
> 
> Whatever we end up applying, I'd like to have this thing tagged properly
> - I didn't bisect for 2 days for nothing:

Sure, this is just an RFC.
If it can be fixed in RCU layer, we don't need this workaround.
IMO, as list_add_rcu() is allowed at that stage, synchronize_rcu_expedited() should also be allowed.

> 
> Reported-and-tested-by: Borislav Petkov <bp@xxxxxxx>
> 

Thanks for the test.

> Also, below's the other patch, I think you should copy the detailed
> explanation about what happens from its commit message so that we have
> it somewhere.
> 
> Also, to your patch add:
> 
> Fixes: 174cc7187e6f ("ACPICA: Tables: Back port acpi_get_table_with_size() and
> early_acpi_os_unmap_memory() from Linux kernel")
> Link: https://lkml.kernel.org/r/4034dde8-ffc1-18e2-f40c-00cf37471793@xxxxxxxxx
> 

Sure. Thanks.

> (I've added the link to the second mail in the thread because my first
> one didn't end up on lkml due to attachment size, most likely).
> 
> > ---
> >  drivers/acpi/osl.c |    5 ++++-
> >  1 file changed, 4 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/acpi/osl.c b/drivers/acpi/osl.c
> > index a404ff4..3d93633 100644
> > --- a/drivers/acpi/osl.c
> > +++ b/drivers/acpi/osl.c
> > @@ -77,6 +77,7 @@ static int (*__acpi_os_prepare_extended_sleep)(u8 sleep_state, u32 val_a,
> >  static bool acpi_os_initialized;
> >  unsigned int acpi_sci_irq = INVALID_ACPI_IRQ;
> >  bool acpi_permanent_mmap = false;
> > +bool acpi_synchronize_rcu = false;
> 
> ERROR: do not initialise globals to false
> #54: FILE: drivers/acpi/osl.c:80:
> +bool acpi_synchronize_rcu = false;
> 

Thanks for pointing this out.
Also a "static" is needed or a declaration in header file is needed for this variable.
Maybe we should move all acpi ioremap stuffs to a single file.
It grows big now and contains many hidden logics.
It will be cleaner to have it maintained in a separate file with more comments.

> >  /*
> >   * This list of permanent mappings is for memory that may be accessed from
> > @@ -378,7 +379,8 @@ static void acpi_os_drop_map_ref(struct acpi_ioremap *map)
> >  static void acpi_os_map_cleanup(struct acpi_ioremap *map)
> >  {
> >  	if (!map->refcount) {
> > -		synchronize_rcu_expedited();
> > +		if (acpi_synchronize_rcu)
> > +			synchronize_rcu_expedited();
> >  		acpi_unmap(map->phys, map->virt);
> >  		kfree(map);
> >  	}
> > @@ -444,6 +446,7 @@ int acpi_os_map_generic_address(struct acpi_generic_address *gas)
> >  	if (!virt)
> >  		return -EIO;
> >
> > +	acpi_synchronize_rcu = true;
> >  	return 0;
> >  }
> >  EXPORT_SYMBOL(acpi_os_map_generic_address);
> > --
> 
> ---
> From: Borislav Petkov <bp@xxxxxxx>
> Date: Mon, 9 Jan 2017 10:54:21 +0100
> Subject: [PATCH] iommu/amd: Comment out acpi_put_table() for now
> MIME-Version: 1.0
> Content-Type: text/plain; charset=UTF-8
> Content-Transfer-Encoding: 8bit
> 
> We're calling this too early and we land in RCU which is uninitialized
> yet:
> 
>   early_amd_iommu_init()
>   |-> acpi_put_table(ivrs_base);
>   |-> acpi_tb_put_table(table_desc);
>   |-> acpi_tb_invalidate_table(table_desc);
>   |-> acpi_tb_release_table(...)
>   |-> acpi_os_unmap_memory
>   |-> acpi_os_unmap_iomem
>   |-> acpi_os_map_cleanup
>   |-> synchronize_rcu_expedited   <-- the kernel/rcu/tree_exp.h version with CONFIG_PREEMPT_RCU=y
> 
> Now that function goes and sends IPIs, i.e., schedule_work() but this is
> too early - we haven't even done workqueue_init().
> 
> Actually, from looking at the callstack, we do
> kernel_init_freeable()->native_smp_prepare_cpus() and workqueue_init()
> comes next.
> 
> So let's choose the lesser of two evils - leak a little ACPI memory -
> instead of freezing early at boot. Took me a while to bisect this :-\
> 
> Signed-off-by: Borislav Petkov <bp@xxxxxxx>
> Fixes: 6b11d1d67713 ("ACPI / osl: Remove acpi_get_table_with_size()/early_acpi_os_unmap_memory()
> users")
> Cc: Bob Moore <robert.moore@xxxxxxxxx>
> Cc: JÃrg RÃdel <joro@xxxxxxxxxx>
> Cc: Linux ACPI <linux-acpi@xxxxxxxxxxxxxxx>
> Cc: Lv Zheng <lv.zheng@xxxxxxxxx>
> Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
> Cc: "Rafael J. Wysocki" <rafael.j.wysocki@xxxxxxxxx>
> Cc: "Rafael J. Wysocki" <rafael@xxxxxxxxxx>
> ---
>  drivers/iommu/amd_iommu_init.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/drivers/iommu/amd_iommu_init.c b/drivers/iommu/amd_iommu_init.c
> index 6799cf9713f7..b7c228002ec7 100644
> --- a/drivers/iommu/amd_iommu_init.c
> +++ b/drivers/iommu/amd_iommu_init.c
> @@ -2337,8 +2337,14 @@ static int __init early_amd_iommu_init(void)
> 
>  out:
>  	/* Don't leak any ACPI memory */
> +
> +	/*
> +	 * Temporarily avoid doing that because we're called too early and
> +	 * acpi_put_table() ends up in RCU (see acpi_os_map_cleanup()) which is
> +	 * not initialized yet.
>  	acpi_put_table(ivrs_base);
>  	ivrs_base = NULL;
> +	*/
> 
>  	return ret;
>  }
> --
> 2.11.0
> 
> 
> --
> Regards/Gruss,
>     Boris.
> 
> Good mailing practices for 400: avoid top-posting and trim the reply.