libxg: shrink variable scope in xc_core_arch_map_p2m_list_rw()

Commit Message

Jan Beulich June 14, 2023, 7:02 a.m. UTC

This in particular allows to drop a dead assignment to "ptes" from near
the end of the function.

Coverity ID: 1532314
Fixes: bd7a29c3d0b9 ("tools/libs/ctrl: fix xc_core_arch_map_p2m() to support linear p2m table")
Signed-off-by: Jan Beulich <[e-mail]>
---
v2: Much bigger change to limit the scope of "ptes" and other variables.

Comments

Anthony PERARD June 14, 2023, 3:08 p.m. UTC | #1

On Wed, Jun 14, 2023 at 09:02:56AM +0200, Jan Beulich wrote:
> This in particular allows to drop a dead assignment to "ptes" from near
> the end of the function.
> 
> Coverity ID: 1532314
> Fixes: bd7a29c3d0b9 ("tools/libs/ctrl: fix xc_core_arch_map_p2m() to support linear p2m table")
> Signed-off-by: Jan Beulich <[e-mail]>
> ---
> v2: Much bigger change to limit the scope of "ptes" and other variables.

The change of scope of all variables isn't too hard to review with
--word-diff option and they all look fine.

> --- a/tools/libs/guest/xg_core_x86.c
> +++ b/tools/libs/guest/xg_core_x86.c
> @@ -169,18 +169,21 @@ xc_core_arch_map_p2m_list_rw(xc_interfac
>          if ( !mfns )
>          {
>              ERROR("Cannot allocate memory for array of %u mfns", idx);
> +        out_unmap:
> +            munmap(ptes, n_pages * PAGE_SIZE);
>              goto out;
>          }
>  

I guess it's not that great to have the label out_unmap in the middle of
the for loop (at least it's near the beginning), but at least that mean
that the mapping need to be gone once out of the loop. So if someone
edit the for loop and introduce a `goto out` instead of `goto out_unmap`
there's just a potential leak rather than potential use-after-free or
double-free, so I guess that better.

So:
Acked-by: Anthony PERARD <[e-mail]>

Cheers,

Patch

--- a/tools/libs/guest/xg_core_x86.c
+++ b/tools/libs/guest/xg_core_x86.c
@@ -92,11 +92,9 @@  xc_core_arch_map_p2m_list_rw(xc_interfac
                              uint64_t p2m_cr3)
 {
     uint64_t p2m_vaddr, p2m_end, mask, off;
-    xen_pfn_t p2m_mfn, mfn, saved_mfn, max_pfn;
-    uint64_t *ptes = NULL;
+    xen_pfn_t p2m_mfn, saved_mfn;
     xen_pfn_t *mfns = NULL;
-    unsigned int fpp, n_pages, level, n_levels, shift,
-                 idx_start, idx_end, idx, saved_idx;
+    unsigned int fpp, level, n_levels, idx_start, idx_end, saved_idx;
 
     p2m_vaddr = GET_FIELD(live_shinfo, arch.p2m_vaddr, dinfo->guest_width);
     fpp = PAGE_SIZE / dinfo->guest_width;
@@ -152,8 +150,10 @@  xc_core_arch_map_p2m_list_rw(xc_interfac
 
     for ( level = n_levels; level > 0; level-- )
     {
-        n_pages = idx_end - idx_start + 1;
-        ptes = xc_map_foreign_pages(xch, dom, PROT_READ, mfns, n_pages);
+        unsigned int n_pages = idx_end - idx_start + 1;
+        uint64_t *ptes = xc_map_foreign_pages(xch, dom, PROT_READ, mfns, n_pages);
+        unsigned int shift, idx;
+
         if ( !ptes )
         {
             PERROR("Failed to map %u page table pages for p2m list", n_pages);
@@ -169,18 +169,21 @@  xc_core_arch_map_p2m_list_rw(xc_interfac
         if ( !mfns )
         {
             ERROR("Cannot allocate memory for array of %u mfns", idx);
+        out_unmap:
+            munmap(ptes, n_pages * PAGE_SIZE);
             goto out;
         }
 
         for ( idx = idx_start; idx <= idx_end; idx++ )
         {
-            mfn = (ptes[idx] & 0x000ffffffffff000ULL) >> PAGE_SHIFT;
+            xen_pfn_t mfn = (ptes[idx] & 0x000ffffffffff000ULL) >> PAGE_SHIFT;
+
             if ( mfn == 0 )
             {
                 ERROR("Bad mfn %#lx during page table walk for vaddr %#" PRIx64 " at level %d of p2m list",
                       mfn, off + ((uint64_t)idx << shift), level);
                 errno = ERANGE;
-                goto out;
+                goto out_unmap;
             }
             mfns[idx - idx_start] = mfn;
 
@@ -197,6 +200,8 @@  xc_core_arch_map_p2m_list_rw(xc_interfac
 
         if ( level == 2 )
         {
+            xen_pfn_t max_pfn;
+
             if ( saved_idx == idx_end )
                 saved_idx++;
             max_pfn = ((xen_pfn_t)saved_idx << 9) * fpp;
@@ -210,7 +215,6 @@  xc_core_arch_map_p2m_list_rw(xc_interfac
         }
 
         munmap(ptes, n_pages * PAGE_SIZE);
-        ptes = NULL;
         off = p2m_vaddr & ((mask >> shift) << shift);
     }
 
@@ -218,8 +222,6 @@  xc_core_arch_map_p2m_list_rw(xc_interfac
 
  out:
     free(mfns);
-    if ( ptes )
-        munmap(ptes, n_pages * PAGE_SIZE);
 
     return NULL;
 }