Re: [PATCH 1/2] xen/hypercall: fix hypercall fallback code forvery old hypervisors

[Jan Beulich]

>>> On 30.10.12 at 16:44, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> wrote:
> On Mon, Oct 29, 2012 at 10:08:17AM -0400, Konrad Rzeszutek Wilk wrote:
>> From: Jan Beulich <jbeulich@xxxxxxxx>
>> 
>> While copying the argument structures in HYPERVISOR_event_channel_op()
>> and HYPERVISOR_physdev_op() into the local variable is sufficiently
>> safe even if the actual structure is smaller than the container one,
>> copying back eventual output values the same way isn't: This may
>> collide with on-stack variables (particularly "rc") which may change
>> between the first and second memcpy() (i.e. the second memcpy() could
>> discard that change).
>> 
>> Move the fallback code into out-of-line functions, and handle all of
>> the operations known by this old a hypervisor individually: Some don't
>> require copying back anything at all, and for the rest use the
>> individual argument structures' sizes rather than the container's.
>> 
>> Reported-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>> [v2: Reduce #define/#undef usage in HYPERVISOR_physdev_op_compat().]
>> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> 
> And it looks like I get 
> 
> ERROR: "HYPERVISOR_event_channel_op_compat" [drivers/xen/xen-evtchn.ko] 
> undefined!
> 
> when I build xen-evtchn as module. Jan did you encounter this issue on 
> 2.6.18?

No - the event channel driver there can't be built as a module, and
for the forward ported kernels I apparently never tried to build with
a compat setting of 3.0.2 or less (and I didn't care that much either
because the oldest we're actually concerned about is 3.0.4 to cover
some of those very old EC2 systems; I'll add the export at the right
point nevertheless).

Jan

>> ---
>>  arch/x86/include/asm/xen/hypercall.h |   21 +++------
>>  drivers/xen/Makefile                 |    2 +-
>>  drivers/xen/fallback.c               |   78 
> ++++++++++++++++++++++++++++++++++
>>  3 files changed, 86 insertions(+), 15 deletions(-)
>>  create mode 100644 drivers/xen/fallback.c
>> 
>> diff --git a/arch/x86/include/asm/xen/hypercall.h 
> b/arch/x86/include/asm/xen/hypercall.h
>> index 59c226d..c812360 100644
>> --- a/arch/x86/include/asm/xen/hypercall.h
>> +++ b/arch/x86/include/asm/xen/hypercall.h
>> @@ -359,18 +359,14 @@ HYPERVISOR_update_va_mapping(unsigned long va, pte_t 
> new_val,
>>  		return _hypercall4(int, update_va_mapping, va,
>>  				   new_val.pte, new_val.pte >> 32, flags);
>>  }
>> +int __must_check HYPERVISOR_event_channel_op_compat(int, void *);
>>  
>>  static inline int
>>  HYPERVISOR_event_channel_op(int cmd, void *arg)
>>  {
>>  	int rc = _hypercall2(int, event_channel_op, cmd, arg);
>> -	if (unlikely(rc == -ENOSYS)) {
>> -		struct evtchn_op op;
>> -		op.cmd = cmd;
>> -		memcpy(&op.u, arg, sizeof(op.u));
>> -		rc = _hypercall1(int, event_channel_op_compat, &op);
>> -		memcpy(arg, &op.u, sizeof(op.u));
>> -	}
>> +	if (unlikely(rc == -ENOSYS))
>> +		rc = HYPERVISOR_event_channel_op_compat(cmd, arg);
>>  	return rc;
>>  }
>>  
>> @@ -386,17 +382,14 @@ HYPERVISOR_console_io(int cmd, int count, char *str)
>>  	return _hypercall3(int, console_io, cmd, count, str);
>>  }
>>  
>> +int __must_check HYPERVISOR_physdev_op_compat(int, void *);
>> +
>>  static inline int
>>  HYPERVISOR_physdev_op(int cmd, void *arg)
>>  {
>>  	int rc = _hypercall2(int, physdev_op, cmd, arg);
>> -	if (unlikely(rc == -ENOSYS)) {
>> -		struct physdev_op op;
>> -		op.cmd = cmd;
>> -		memcpy(&op.u, arg, sizeof(op.u));
>> -		rc = _hypercall1(int, physdev_op_compat, &op);
>> -		memcpy(arg, &op.u, sizeof(op.u));
>> -	}
>> +	if (unlikely(rc == -ENOSYS))
>> +		rc = HYPERVISOR_physdev_op_compat(cmd, arg);
>>  	return rc;
>>  }
>>  
>> diff --git a/drivers/xen/Makefile b/drivers/xen/Makefile
>> index 0e863703..46de6cd 100644
>> --- a/drivers/xen/Makefile
>> +++ b/drivers/xen/Makefile
>> @@ -2,7 +2,7 @@ ifneq ($(CONFIG_ARM),y)
>>  obj-y	+= manage.o balloon.o
>>  obj-$(CONFIG_HOTPLUG_CPU)		+= cpu_hotplug.o
>>  endif
>> -obj-y	+= grant-table.o features.o events.o
>> +obj-y	+= grant-table.o features.o events.o fallback.o
>>  obj-y	+= xenbus/
>>  
>>  nostackp := $(call cc-option, -fno-stack-protector)
>> diff --git a/drivers/xen/fallback.c b/drivers/xen/fallback.c
>> new file mode 100644
>> index 0000000..0bdc468
>> --- /dev/null
>> +++ b/drivers/xen/fallback.c
>> @@ -0,0 +1,78 @@
>> +#include <linux/kernel.h>
>> +#include <linux/string.h>
>> +#include <linux/bug.h>
>> +#include <asm/hypervisor.h>
>> +#include <asm/xen/hypercall.h>
>> +
>> +int HYPERVISOR_event_channel_op_compat(int cmd, void *arg)
>> +{
>> +	struct evtchn_op op;
>> +	int rc;
>> +
>> +	op.cmd = cmd;
>> +	memcpy(&op.u, arg, sizeof(op.u));
>> +	rc = _hypercall1(int, event_channel_op_compat, &op);
>> +
>> +	switch (cmd) {
>> +	case EVTCHNOP_close:
>> +	case EVTCHNOP_send:
>> +	case EVTCHNOP_bind_vcpu:
>> +	case EVTCHNOP_unmask:
>> +		/* no output */
>> +		break;
>> +
>> +#define COPY_BACK(eop) \
>> +	case EVTCHNOP_##eop: \
>> +		memcpy(arg, &op.u.eop, sizeof(op.u.eop)); \
>> +		break
>> +
>> +	COPY_BACK(bind_interdomain);
>> +	COPY_BACK(bind_virq);
>> +	COPY_BACK(bind_pirq);
>> +	COPY_BACK(status);
>> +	COPY_BACK(alloc_unbound);
>> +	COPY_BACK(bind_ipi);
>> +#undef COPY_BACK
>> +
>> +	default:
>> +		WARN_ON(rc != -ENOSYS);
>> +		break;
>> +	}
>> +
>> +	return rc;
>> +}
>> +
>> +int HYPERVISOR_physdev_op_compat(int cmd, void *arg)
>> +{
>> +	struct physdev_op op;
>> +	int rc;
>> +
>> +	op.cmd = cmd;
>> +	memcpy(&op.u, arg, sizeof(op.u));
>> +	rc = _hypercall1(int, physdev_op_compat, &op);
>> +
>> +	switch (cmd) {
>> +	case PHYSDEVOP_IRQ_UNMASK_NOTIFY:
>> +	case PHYSDEVOP_set_iopl:
>> +	case PHYSDEVOP_set_iobitmap:
>> +	case PHYSDEVOP_apic_write:
>> +		/* no output */
>> +		break;
>> +
>> +#define COPY_BACK(pop, fld) \
>> +	case PHYSDEVOP_##pop: \
>> +		memcpy(arg, &op.u.fld, sizeof(op.u.fld)); \
>> +		break
>> +
>> +	COPY_BACK(irq_status_query, irq_status_query);
>> +	COPY_BACK(apic_read, apic_op);
>> +	COPY_BACK(ASSIGN_VECTOR, irq_op);
>> +#undef COPY_BACK
>> +
>> +	default:
>> +		WARN_ON(rc != -ENOSYS);
>> +		break;
>> +	}
>> +
>> +	return rc;
>> +}
>> -- 
>> 1.7.7.6


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