[PATCH v8 2/2] x86/boot: Improve MBI2 structure check

[Frediano Ziglio <frediano.ziglio@cloud.com>]

Tag structure should contain at least the tag header.
Entire tag structure must be contained inside MBI2 data.

Signed-off-by: Frediano Ziglio <frediano.ziglio@cloud.com>
---
Changes since v6:
- compare against total_size every time to avoid overflows.

Changes since v7:
- check for tag type being MULTIBOOT2_TAG_TYPE_END before checking
  tag size. This allows caller to not fill full tag (type field is
  before size field).
---
 xen/arch/x86/efi/mbi2.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/efi/mbi2.c b/xen/arch/x86/efi/mbi2.c
index 55a1777483..d2f0f0ee61 100644
--- a/xen/arch/x86/efi/mbi2.c
+++ b/xen/arch/x86/efi/mbi2.c
@@ -13,6 +13,7 @@ efi_multiboot2_prelude(uint32_t magic, const multiboot2_fixed_t *mbi)
     EFI_HANDLE ImageHandle = NULL;
     EFI_SYSTEM_TABLE *SystemTable = NULL;
     const char *cmdline = NULL;
+    const void *const mbi_raw = (const void *)mbi;
     bool have_bs = false;
 
     if ( magic != MULTIBOOT2_BOOTLOADER_MAGIC )
@@ -21,8 +22,10 @@ efi_multiboot2_prelude(uint32_t magic, const multiboot2_fixed_t *mbi)
     /* Skip Multiboot2 information fixed part. */
     tag = _p(ROUNDUP((unsigned long)(mbi + 1), MULTIBOOT2_TAG_ALIGN));
 
-    for ( ; (const void *)tag - (const void *)mbi < mbi->total_size &&
-            tag->type != MULTIBOOT2_TAG_TYPE_END;
+    for ( ; (const void *)(tag + 1) - mbi_raw <= mbi->total_size &&
+            tag->type != MULTIBOOT2_TAG_TYPE_END &&
+            tag->size >= sizeof(*tag) &&
+            (const void *)tag + tag->size - mbi_raw <= mbi->total_size;
           tag = _p(ROUNDUP((unsigned long)tag + tag->size,
                    MULTIBOOT2_TAG_ALIGN)) )
     {
-- 
2.34.1