EFI: avoid OOB config file reads

The message emitted by pre_parse() pretty clearly states the original
intention. Yet what it said wasn't done, and would have been unfriendly to
the user. Hence accesses past the allocated buffer were possible. Insert a
terminating NUL immediately past the data read, to then drop the no longer
applicable message.

NB: The iscntrl() check of just the last byte is more strict than what
pre_parse() would accept without issuing its prior message, yet I'd like
to keep the new logic reasonably simple. Config files shouldn't be huge,
and we shouldn't be _that_ short of memory (or we'd fail elsewhere pretty
soon).

Fixes: bf6501a62e ("x86-64: EFI boot code")
Reported-by: Kamil Frankowicz <kamil.frankowicz@cert.pl>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>

xen/common/efi/boot.c

@@ -833,8 +833,9 @@ static bool __init read_file(EFI_FILE_HANDLE dir_handle, CHAR16 *name,
     what = L"Allocation";
     file->addr = min(1UL << (32 + PAGE_SHIFT),
                      HYPERVISOR_VIRT_END - DIRECTMAP_VIRT_START);
+    /* For config files allocate an extra byte to put a NUL there. */
     ret = efi_bs->AllocatePages(AllocateMaxAddress, EfiLoaderData,
-                                PFN_UP(size), &file->addr);
+                                PFN_UP(size + (file == &cfg)), &file->addr);
     if ( EFI_ERROR(ret) )
         goto fail;
 
@@ -853,6 +854,9 @@ static bool __init read_file(EFI_FILE_HANDLE dir_handle, CHAR16 *name,
 
     efi_arch_flush_dcache_area(file->ptr, file->size);
 
+    if ( file == &cfg )
+        file->str[file->size] = 0;
+
     return true;
 
  fail:
@@ -878,6 +882,23 @@ static bool __init read_section(const EFI_LOADED_IMAGE *image,
 
     file->ptr = ptr;
 
+    /* For cfg file, if necessary allocate space to put an extra NUL there. */
+    if ( file == &cfg && file->size && !iscntrl(file->str[file->size - 1]) )
+    {
+        EFI_PHYSICAL_ADDRESS addr;
+        EFI_STATUS ret = efi_bs->AllocatePages(AllocateMaxAddress,
+                                               EfiLoaderData,
+                                               PFN_UP(file->size + 1), &addr);
+
+        if ( EFI_ERROR(ret) )
+            return false;
+
+        memcpy((void *)addr, ptr, file->size);
+        file->addr = addr;
+        file->need_to_free = true;
+        file->str[file->size] = 0;
+    }
+
     handle_file_info(name, file, options);
 
     return true;
@@ -906,9 +927,6 @@ static void __init pre_parse(const struct file *file)
         else
             start = 0;
     }
-    if ( file->size && end[-1] )
-         PrintStr(L"No newline at end of config file,"
-                   " last line will be ignored.\r\n");
 }
 
 static void __init init_secure_boot_mode(void)