fs/ntfs: Implement attribute verification

It was possible to read OOB when an attribute had a size that exceeded
the allocated buffer. This resolves that by making sure all attributes
that get read are fully in the allocated space by implementing
a function to validate them.

Defining the offsets in include/grub/ntfs.h but they are only used in
the validation function and not across the rest of the NTFS code.

Signed-off-by: B Horn <b@horn.uk>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

grub-core/fs/ntfs.c

@@ -70,6 +70,149 @@ res_attr_data_len (void *res_attr_ptr)
   return u32at (res_attr_ptr, 0x10);
 }
 
+/*
+ * Check if the attribute is valid and doesn't exceed the allocated region.
+ * This accounts for resident and non-resident data.
+ *
+ * This is based off the documentation from the linux-ntfs project:
+ * https://flatcap.github.io/linux-ntfs/ntfs/concepts/attribute_header.html
+ */
+static bool
+validate_attribute (grub_uint8_t *attr, void *end)
+{
+  grub_size_t attr_size = 0;
+  grub_size_t min_size = 0;
+  grub_size_t spare = (grub_uint8_t *) end - attr;
+  /*
+   * Just used as a temporary variable to try and deal with cases where someone
+   * tries to overlap fields.
+   */
+  grub_size_t curr = 0;
+
+  /* Need verify we can entirely read the attributes header. */
+  if (attr + GRUB_NTFS_ATTRIBUTE_HEADER_SIZE >= (grub_uint8_t *) end)
+    goto fail;
+
+  /*
+   * So, the rest of this code uses a 16bit int for the attribute length but
+   * from reading the all the documentation I could find it says this field is
+   * actually 32bit. But let's be consistent with the rest of the code.
+   *
+   * https://elixir.bootlin.com/linux/v6.10.7/source/fs/ntfs3/ntfs.h#L370
+   */
+  attr_size = u16at (attr, GRUB_NTFS_ATTRIBUTE_LENGTH);
+
+  if (attr_size > spare)
+    goto fail;
+
+  /* Not an error case, just reached the end of the attributes. */
+  if (attr_size == 0)
+    return false;
+
+  /*
+   * Extra validation by trying to calculate a minimum possible size for this
+   * attribute. +8 from the size of the resident data struct which is the
+   * minimum that can be added.
+   */
+  min_size = GRUB_NTFS_ATTRIBUTE_HEADER_SIZE + 8;
+
+  if (min_size > attr_size)
+    goto fail;
+
+  /* Is the data is resident (0) or not (1). */
+  if (attr[GRUB_NTFS_ATTRIBUTE_RESIDENT] == 0)
+    {
+      /* Read the offset and size of the attribute. */
+      curr = u16at (attr, GRUB_NTFS_ATTRIBUTE_RES_OFFSET);
+      curr += u32at (attr, GRUB_NTFS_ATTRIBUTE_RES_LENGTH);
+      if (curr > min_size)
+	min_size = curr;
+    }
+  else
+    {
+      /*
+       * If the data is non-resident, the minimum size is 64 which is where
+       * the data runs start. We already have a minimum size of 24. So, just
+       * adding 40 to get to the real value.
+       */
+      min_size += 40;
+      if (min_size > attr_size)
+	goto fail;
+      /* If the compression unit size is > 0, +8 bytes*/
+      if (u16at (attr, GRUB_NTFS_ATTRIBUTE_COMPRESSION_UNIT_SIZE) > 0)
+	min_size += 8;
+
+      /*
+       * Need to consider the data runs now. Each member of the run has byte
+       * that describes the size of the data length and offset. Each being
+       * 4 bits in the byte.
+       */
+      curr = u16at (attr, GRUB_NTFS_ATTRIBUTE_DATA_RUNS);
+
+      if (curr + 1 > min_size)
+	min_size = curr + 1;
+
+      if (min_size > attr_size)
+	goto fail;
+
+      /*
+       * Each attribute can store multiple data runs which are stored
+       * continuously in the attribute. They exist as one header byte
+       * with up to 14 bytes following it depending on the lengths.
+       * We stop when we hit a header that is just a NUL byte.
+       *
+       * https://flatcap.github.io/linux-ntfs/ntfs/concepts/data_runs.html
+       */
+      while (attr[curr] != 0)
+	{
+	  /*
+	   * We stop when we hit a header that is just a NUL byte. The data
+	   * run header is stored as a single byte where the top 4 bits refer
+	   * to the number of bytes used to store the total length of the
+	   * data run, and the number of bytes used to store the offset.
+	   * These directly follow the header byte, so we use them to update
+	   * the minimum size.
+	   */
+	  min_size += (attr[curr] & 0x7) + ((attr[curr] >> 4) & 0x7);
+	  curr += min_size;
+	  min_size++;
+	  if (min_size > attr_size)
+	    goto fail;
+	}
+    }
+
+  /* Name offset, doing this after data residence checks. */
+  if (u16at (attr, GRUB_NTFS_ATTRIBUTE_NAME_OFFSET) != 0)
+    {
+      curr = u16at (attr, GRUB_NTFS_ATTRIBUTE_NAME_OFFSET);
+      /*
+       * Multiple the name length by 2 as its UTF-16. Can be zero if this in an
+       * unamed attribute.
+       */
+      curr += attr[GRUB_NTFS_ATTRIBUTE_NAME_LENGTH] * 2;
+      if (curr > min_size)
+	min_size = curr;
+    }
+
+  /* Padded to 8 bytes. */
+  if (min_size % 8 != 0)
+    min_size += 8 - (min_size % 8);
+
+  /*
+   * At this point min_size should be exactly attr_size but being flexible
+   * here to avoid any issues.
+   */
+  if (min_size > attr_size)
+    goto fail;
+
+  return true;
+
+ fail:
+  grub_dprintf ("ntfs", "spare=%" PRIuGRUB_SIZE " min_size=%" PRIuGRUB_SIZE " attr_size=%" PRIuGRUB_SIZE "\n",
+		spare, min_size, attr_size);
+  return false;
+}
+
 /* Return the next attribute if it exists, otherwise return NULL. */
 static grub_uint8_t *
 next_attribute (grub_uint8_t *curr_attribute, void *end)
@@ -84,6 +227,8 @@ next_attribute (grub_uint8_t *curr_attribute, void *end)
     return NULL;
 
   next += u16at (curr_attribute, 4);
+  if (validate_attribute (next, end) == false)
+    return NULL;
 
   return next;
 }
@@ -290,6 +435,9 @@ find_attr (struct grub_ntfs_attr *at, grub_uint8_t attr)
       /* From this point on pa_end is the end of the buffer */
       at->end = pa_end;
 
+      if (validate_attribute (at->attr_nxt, pa_end) == false)
+	return NULL;
+
       while (at->attr_nxt)
 	{
 	  if ((*at->attr_nxt == attr) || (attr == 0))
@@ -319,6 +467,9 @@ find_attr (struct grub_ntfs_attr *at, grub_uint8_t attr)
 						  + 1));
 	  pa = at->attr_nxt + u16at (pa, 4);
 
+	  if (validate_attribute (pa, pa_end) == true)
+	    pa = NULL;
+
 	  while (pa)
 	    {
 	      if (*pa != attr)
@@ -572,6 +723,8 @@ read_attr (struct grub_ntfs_attr *at, grub_uint8_t *dest, grub_disk_addr_t ofs,
       else
 	vcn = ofs >> (at->mft->data->log_spc + GRUB_NTFS_BLK_SHR);
       pa = at->attr_nxt + u16at (at->attr_nxt, 4);
+      if (validate_attribute (pa, at->attr_end) == false)
+	pa = NULL;
 
       while (pa)
 	{

include/grub/ntfs.h

@@ -91,6 +91,28 @@ enum
 
 #define GRUB_NTFS_ATTRIBUTE_HEADER_SIZE 16
 
+/*
+ * To make attribute validation clearer the offsets for each value in the
+ * attribute headers are defined as macros.
+ *
+ * These offsets are all from:
+ * https://flatcap.github.io/linux-ntfs/ntfs/concepts/attribute_header.html
+ */
+
+/* These offsets are part of the attribute header. */
+#define GRUB_NTFS_ATTRIBUTE_LENGTH      4
+#define GRUB_NTFS_ATTRIBUTE_RESIDENT    8
+#define GRUB_NTFS_ATTRIBUTE_NAME_LENGTH 9
+#define GRUB_NTFS_ATTRIBUTE_NAME_OFFSET 10
+
+/* Offsets for values needed for resident data. */
+#define GRUB_NTFS_ATTRIBUTE_RES_LENGTH  16
+#define GRUB_NTFS_ATTRIBUTE_RES_OFFSET  20
+
+/* Offsets for values needed for non-resident data. */
+#define GRUB_NTFS_ATTRIBUTE_DATA_RUNS             32
+#define GRUB_NTFS_ATTRIBUTE_COMPRESSION_UNIT_SIZE 34
+
 enum
   {
     GRUB_NTFS_AF_ALST		= 1,