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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

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).

Thanks, Roger.