Re: [PATCH 03/22] libxc: Fix range checking in xc_dom_pfn_to_ptr etc.

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 11/06/13 19:20, Ian Jackson wrote:
> * Ensure that xc_dom_pfn_to_ptr (when called with count==0) does not
>   return a previously-allocated block which is entirely before the
>   requested pfn (!)
>
> * Provide a version of xc_dom_pfn_to_ptr, xc_dom_pfn_to_ptr_retcount,
>   which provides the length of the mapped region via an out parameter.
>
> * Change xc_dom_vaddr_to_ptr to always provide the length of the
>   mapped region and change the call site in xc_dom_binloader.c to
>   check it.  The call site in xc_dom_load_elf_symtab will be corrected
>   in a forthcoming patch, and for now ignores the returned length.
>
> This is part of the fix to a security issue, XSA-55.
>
> Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>

Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

>
> v5: This patch is new in v5 of the series.
> ---
>  tools/libxc/xc_dom.h           |   16 +++++++++++++---
>  tools/libxc/xc_dom_binloader.c |   11 ++++++++++-
>  tools/libxc/xc_dom_core.c      |   13 +++++++++++++
>  tools/libxc/xc_dom_elfloader.c |    3 ++-
>  4 files changed, 38 insertions(+), 5 deletions(-)
>
> diff --git a/tools/libxc/xc_dom.h b/tools/libxc/xc_dom.h
> index 316c5cb..ad6fdd4 100644
> --- a/tools/libxc/xc_dom.h
> +++ b/tools/libxc/xc_dom.h
> @@ -291,6 +291,8 @@ int xc_dom_alloc_segment(struct xc_dom_image *dom,
>  
>  void *xc_dom_pfn_to_ptr(struct xc_dom_image *dom, xen_pfn_t first,
>                          xen_pfn_t count);
> +void *xc_dom_pfn_to_ptr_retcount(struct xc_dom_image *dom, xen_pfn_t first,
> +                                 xen_pfn_t count, xen_pfn_t *count_out);
>  void xc_dom_unmap_one(struct xc_dom_image *dom, xen_pfn_t pfn);
>  void xc_dom_unmap_all(struct xc_dom_image *dom);
>  
> @@ -318,13 +320,21 @@ static inline void *xc_dom_seg_to_ptr(struct xc_dom_image *dom,
>  }
>  
>  static inline void *xc_dom_vaddr_to_ptr(struct xc_dom_image *dom,
> -                                        xen_vaddr_t vaddr)
> +                                        xen_vaddr_t vaddr,
> +                                        size_t *safe_region_out)
>  {
>      unsigned int page_size = XC_DOM_PAGE_SIZE(dom);
>      xen_pfn_t page = (vaddr - dom->parms.virt_base) / page_size;
>      unsigned int offset = (vaddr - dom->parms.virt_base) % page_size;
> -    void *ptr = xc_dom_pfn_to_ptr(dom, page, 0);
> -    return (ptr ? (ptr + offset) : NULL);
> +    xen_pfn_t safe_region_count;
> +    void *ptr;
> +
> +    *safe_region_out = 0;
> +    ptr = xc_dom_pfn_to_ptr_retcount(dom, page, 0, &safe_region_count);
> +    if ( ptr == NULL )
> +        return ptr;
> +    *safe_region_out = (safe_region_count << XC_DOM_PAGE_SHIFT(dom)) - offset;
> +    return ptr;
>  }
>  
>  static inline xen_pfn_t xc_dom_p2m_host(struct xc_dom_image *dom, xen_pfn_t pfn)
> diff --git a/tools/libxc/xc_dom_binloader.c b/tools/libxc/xc_dom_binloader.c
> index c14727c..d2de04c 100644
> --- a/tools/libxc/xc_dom_binloader.c
> +++ b/tools/libxc/xc_dom_binloader.c
> @@ -249,6 +249,7 @@ static int xc_dom_load_bin_kernel(struct xc_dom_image *dom)
>      char *image = dom->kernel_blob;
>      char *dest;
>      size_t image_size = dom->kernel_size;
> +    size_t dest_size;
>      uint32_t start_addr;
>      uint32_t load_end_addr;
>      uint32_t bss_end_addr;
> @@ -272,7 +273,15 @@ static int xc_dom_load_bin_kernel(struct xc_dom_image *dom)
>      DOMPRINTF("  text_size: 0x%" PRIx32 "", text_size);
>      DOMPRINTF("  bss_size:  0x%" PRIx32 "", bss_size);
>  
> -    dest = xc_dom_vaddr_to_ptr(dom, dom->kernel_seg.vstart);
> +    dest = xc_dom_vaddr_to_ptr(dom, dom->kernel_seg.vstart, &dest_size);
> +
> +    if ( dest_size < text_size ||
> +         dest_size - text_size < bss_size )
> +    {
> +        DOMPRINTF("%s: mapped region is too small for image", __FUNCTION__);
> +        return -EINVAL;
> +    }
> +
>      memcpy(dest, image + skip, text_size);
>      memset(dest + text_size, 0, bss_size);
>  
> diff --git a/tools/libxc/xc_dom_core.c b/tools/libxc/xc_dom_core.c
> index b92e4a9..cf96bfa 100644
> --- a/tools/libxc/xc_dom_core.c
> +++ b/tools/libxc/xc_dom_core.c
> @@ -351,11 +351,20 @@ int xc_dom_try_gunzip(struct xc_dom_image *dom, void **blob, size_t * size)
>  void *xc_dom_pfn_to_ptr(struct xc_dom_image *dom, xen_pfn_t pfn,
>                          xen_pfn_t count)
>  {
> +    xen_pfn_t count_out_dummy;
> +    return xc_dom_pfn_to_ptr_retcount(dom, pfn, count, &count_out_dummy);
> +}
> +
> +void *xc_dom_pfn_to_ptr_retcount(struct xc_dom_image *dom, xen_pfn_t pfn,
> +                                 xen_pfn_t count, xen_pfn_t *count_out)
> +{
>      struct xc_dom_phys *phys;
>      xen_pfn_t offset;
>      unsigned int page_shift = XC_DOM_PAGE_SHIFT(dom);
>      char *mode = "unset";
>  
> +    *count_out = 0;
> +
>      offset = pfn - dom->rambase_pfn;
>      if ( offset > dom->total_pages || /* multiple checks to avoid overflows */
>           count > dom->total_pages ||
> @@ -386,6 +395,7 @@ void *xc_dom_pfn_to_ptr(struct xc_dom_image *dom, xen_pfn_t pfn,
>                            phys->count);
>                  return NULL;
>              }
> +            *count_out = count;
>          }
>          else
>          {
> @@ -393,6 +403,9 @@ void *xc_dom_pfn_to_ptr(struct xc_dom_image *dom, xen_pfn_t pfn,
>                 just hand out a pointer to it */
>              if ( pfn < phys->first )
>                  continue;
> +            if ( pfn >= phys->first + phys->count )
> +                continue;
> +            *count_out = phys->count - (pfn - phys->first);
>          }
>          return phys->ptr + ((pfn - phys->first) << page_shift);
>      }
> diff --git a/tools/libxc/xc_dom_elfloader.c b/tools/libxc/xc_dom_elfloader.c
> index 6583859..bc92302 100644
> --- a/tools/libxc/xc_dom_elfloader.c
> +++ b/tools/libxc/xc_dom_elfloader.c
> @@ -128,10 +128,11 @@ static int xc_dom_load_elf_symtab(struct xc_dom_image *dom,
>  
>      if ( load )
>      {
> +        size_t allow_size; /* will be used in a forthcoming XSA-55 patch */
>          if ( !dom->bsd_symtab_start )
>              return 0;
>          size = dom->kernel_seg.vend - dom->bsd_symtab_start;
> -        hdr  = xc_dom_vaddr_to_ptr(dom, dom->bsd_symtab_start);
> +        hdr  = xc_dom_vaddr_to_ptr(dom, dom->bsd_symtab_start, &allow_size);
>          *(int *)hdr = size - sizeof(int);
>      }
>      else