Re: [Xen-devel] [PATCH] xen/arm: p2m: Prevent deadlock when using memaccess

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Wed, 28 Feb 2018, Julien Grall wrote:
> Commit 7d623b358a4 "arm/mem_access: Add long-descriptor based gpt"
> assumed the read-write lock can be taken recursively. However, this
> assumption is wrong and will lead to deadlock when the lock is
> contended.
> 
> To avoid the nested lock, rework the locking in get_page_from_gva and
> p2m_mem_access_check_and_get_page. The latter will now be called without
> the p2m lock. The new locking in p2m_mem_accces_check_and_get_page will
> not cover the translation of the VA to an IPA.
> 
> This is fine because we can't promise that the stage-1 page-table have
> changed behind our back (they are under guest control). Modification in
> the stage-2 page-table can now happen, but I can't issue any potential
> issue here except with the break-before-make sequence used when updating
> page-table. gva_to_ipa may fail if the sequence is executed at the same
> on another CPU. In that case we would fallback in the software lookup
> path.
> 
> Signed-off-by: Julien Grall <julien.grall@xxxxxxx>


Hi Julien,

At first glance the patch looks OK, but could you please add more
information on the recursive lock sequence that this patch is trying to
solve? Specifically, how Xen can get into a situation where it tries to
acquire the p2m lock twice recursively? I realize the comment at the
bottom says "access_guest_memory_by_ipa in guest_walk_(sd|ld)" but I
would appreciate more details. Thanks!


> ---
>     This patch should be backported to Xen 4.10. There are other
>     potential optimization that I am working on. Although, I don't think
>     they are backport material.
> ---
>  xen/arch/arm/mem_access.c | 8 ++++++--
>  xen/arch/arm/p2m.c        | 4 ++--
>  xen/include/asm-arm/p2m.h | 4 ----
>  3 files changed, 8 insertions(+), 8 deletions(-)
> 
> diff --git a/xen/arch/arm/mem_access.c b/xen/arch/arm/mem_access.c
> index 0f2cbb81d3..11c2b03b7b 100644
> --- a/xen/arch/arm/mem_access.c
> +++ b/xen/arch/arm/mem_access.c
> @@ -126,7 +126,7 @@ p2m_mem_access_check_and_get_page(vaddr_t gva, unsigned 
> long flag,
>           * is not mapped.
>           */
>          if ( guest_walk_tables(v, gva, &ipa, &perms) < 0 )
> -            goto err;
> +            return NULL;
>  
>          /*
>           * Check permissions that are assumed by the caller. For instance in
> @@ -139,11 +139,13 @@ p2m_mem_access_check_and_get_page(vaddr_t gva, unsigned 
> long flag,
>           * test for execute permissions this check can be left out.
>           */
>          if ( (flag & GV2M_WRITE) && !(perms & GV2M_WRITE) )
> -            goto err;
> +            return NULL;
>      }
>  
>      gfn = gaddr_to_gfn(ipa);
>  
> +    p2m_read_lock(p2m);
> +
>      /*
>       * We do this first as this is faster in the default case when no
>       * permission is set on the page.
> @@ -216,6 +218,8 @@ p2m_mem_access_check_and_get_page(vaddr_t gva, unsigned 
> long flag,
>          page = NULL;
>  
>  err:
> +    p2m_read_unlock(p2m);
> +
>      return page;
>  }
>  
> diff --git a/xen/arch/arm/p2m.c b/xen/arch/arm/p2m.c
> index 65e8b9c6ea..5de82aafe1 100644
> --- a/xen/arch/arm/p2m.c
> +++ b/xen/arch/arm/p2m.c
> @@ -1449,11 +1449,11 @@ struct page_info *get_page_from_gva(struct vcpu *v, 
> vaddr_t va,
>      }
>  
>  err:
> +    p2m_read_unlock(p2m);
> +
>      if ( !page && p2m->mem_access_enabled )
>          page = p2m_mem_access_check_and_get_page(va, flags, v);
>  
> -    p2m_read_unlock(p2m);
> -
>      return page;
>  }
>  
> diff --git a/xen/include/asm-arm/p2m.h b/xen/include/asm-arm/p2m.h
> index a0abc84ed8..45ef2cd58b 100644
> --- a/xen/include/asm-arm/p2m.h
> +++ b/xen/include/asm-arm/p2m.h
> @@ -23,10 +23,6 @@ extern void memory_type_changed(struct domain *);
>  struct p2m_domain {
>      /*
>       * Lock that protects updates to the p2m.
> -     *
> -     * Please note that we use this lock in a nested way by calling
> -     * access_guest_memory_by_ipa in guest_walk_(sd|ld). This must be
> -     * considered in the future implementation.
>       */
>      rwlock_t lock;
>  
> -- 
> 2.11.0
> 

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