Re: [PATCH] x86/hvm: Fix double free from vlapic_init() early error path

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 31/03/2021 14:49, Roger Pau Monné wrote:
> On Wed, Mar 31, 2021 at 02:31:25PM +0100, Andrew Cooper wrote:
>> vlapic_init()'s caller calls vlapic_destroy() on error.  Therefore, the error
>> path from __map_domain_page_global() failing would doubly free
>> vlapic->regs_page.
>>
>> Rework vlapic_destroy() to be properly idempotent, introducing the necessary
>> UNMAP_DOMAIN_PAGE_GLOBAL() and FREE_DOMHEAP_PAGE() wrappers.
>>
>> Rearrange vlapic_init() to group all trivial initialisation, and leave all
>> cleanup to the caller, in line with our longer term plans.
>>
>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>> ---
>> CC: Jan Beulich <JBeulich@xxxxxxxx>
>> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>> CC: Wei Liu <wl@xxxxxxx>
>> ---
>>  xen/arch/x86/hvm/vlapic.c     | 11 ++++-------
>>  xen/include/xen/domain_page.h |  5 +++++
>>  xen/include/xen/mm.h          |  6 ++++++
>>  3 files changed, 15 insertions(+), 7 deletions(-)
>>
>> diff --git a/xen/arch/x86/hvm/vlapic.c b/xen/arch/x86/hvm/vlapic.c
>> index 5e21fb4937..da030f41b5 100644
>> --- a/xen/arch/x86/hvm/vlapic.c
>> +++ b/xen/arch/x86/hvm/vlapic.c
>> @@ -1608,7 +1608,9 @@ int vlapic_init(struct vcpu *v)
>>          return 0;
>>      }
>>  
>> +    /* Trivial init. */
>>      vlapic->pt.source = PTSRC_lapic;
>> +    spin_lock_init(&vlapic->esr_lock);
>>  
>>      vlapic->regs_page = alloc_domheap_page(v->domain, MEMF_no_owner);
>>      if ( !vlapic->regs_page )
>> @@ -1616,17 +1618,12 @@ int vlapic_init(struct vcpu *v)
>>  
>>      vlapic->regs = __map_domain_page_global(vlapic->regs_page);
>>      if ( vlapic->regs == NULL )
>> -    {
>> -        free_domheap_page(vlapic->regs_page);
>>          return -ENOMEM;
>> -    }
>>  
>>      clear_page(vlapic->regs);
>>  
>>      vlapic_reset(vlapic);
>>  
>> -    spin_lock_init(&vlapic->esr_lock);
>> -
>>      tasklet_init(&vlapic->init_sipi.tasklet, vlapic_init_sipi_action, v);
>>  
>>      if ( v->vcpu_id == 0 )
>> @@ -1645,8 +1642,8 @@ void vlapic_destroy(struct vcpu *v)
>>      tasklet_kill(&vlapic->init_sipi.tasklet);
>>      TRACE_0D(TRC_HVM_EMUL_LAPIC_STOP_TIMER);
>>      destroy_periodic_time(&vlapic->pt);
>> -    unmap_domain_page_global(vlapic->regs);
>> -    free_domheap_page(vlapic->regs_page);
>> +    UNMAP_DOMAIN_PAGE_GLOBAL(vlapic->regs);
> I think you need to check whether vlapic->regs_page is NULL here...
>
>> +    FREE_DOMHEAP_PAGE(vlapic->regs_page);
>>  }
>>  
>>  /*
>> diff --git a/xen/include/xen/domain_page.h b/xen/include/xen/domain_page.h
>> index a182d33b67..0cb7f2aad3 100644
>> --- a/xen/include/xen/domain_page.h
>> +++ b/xen/include/xen/domain_page.h
>> @@ -77,4 +77,9 @@ static inline void unmap_domain_page_global(const void 
>> *va) {};
>>      (p) = NULL;                     \
>>  } while ( false )
>>  
>> +#define UNMAP_DOMAIN_PAGE_GLOBAL(p) do {   \
>> +    unmap_domain_page_global(p);           \
>> +    (p) = NULL;                            \
>> +} while ( false )
>> +
>>  #endif /* __XEN_DOMAIN_PAGE_H__ */
>> diff --git a/xen/include/xen/mm.h b/xen/include/xen/mm.h
>> index 667f9dac83..c274e2eac4 100644
>> --- a/xen/include/xen/mm.h
>> +++ b/xen/include/xen/mm.h
>> @@ -85,6 +85,12 @@ bool scrub_free_pages(void);
>>  } while ( false )
>>  #define FREE_XENHEAP_PAGE(p) FREE_XENHEAP_PAGES(p, 0)
>>  
>> +#define FREE_DOMHEAP_PAGES(p, o) do { \
>> +    free_domheap_pages(p, o);         \
> ...as both unmap_domain_page_global and free_domheap_pages don't
> support being passed a NULL pointer.
>
> Passing such NULL pointer will result in unmap_domain_page_global
> hitting an assert AFAICT, and free_domheap_pages will try to use
> page_get_owner(NULL).

Urgh - very good points.

Do we perhaps want to take the opportunity to make these functions
tolerate NULL, to simplify all cleanup code across the hypervisor?

~Andrew