fs/ext2: Rework out-of-bounds read for inline and external extents

Previously, the number of extent entries was not properly capped based
on the actual available space. This could lead to insufficient reads for
external extents, since the computation was based solely on the inline
extent layout.

In this patch, when processing the extent header, we determine whether
the header is stored inline (i.e., at inode->blocks.dir_blocks) or in an
external extent block. We then clamp the number of entries accordingly
(using max_inline_ext for inline extents and max_external_ext for
external extent blocks).

This change ensures that only the valid number of extent entries is
processed, preventing out-of-bound reads and potential filesystem
corruption.

Fixes: 7e2f750 (fs/ext2: Fix out-of-bounds read for inline extents)

Signed-off-by: Michael Chang <mchang@suse.com>
Tested-by: Christian Hesse <mail@eworm.de>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

Author: Michael Chang via Grub-devel

grub-core/fs/ext2.c

@@ -482,7 +482,10 @@ grub_ext2_read_block (grub_fshelp_node_t node, grub_disk_addr_t fileblock)
       int i;
       grub_disk_addr_t ret;
       grub_uint16_t nent;
+      /* maximum number of extent entries in the inode's inline extent area */
       const grub_uint16_t max_inline_ext = sizeof (inode->blocks) / sizeof (*ext) - 1; /* Minus 1 extent header. */
+      /* maximum number of extent entries in the external extent block */
+      const grub_uint16_t max_external_ext = EXT2_BLOCK_SIZE(data) / sizeof (*ext) - 1; /* Minus 1 extent header. */
 
       leaf = grub_ext4_find_leaf (data, (struct grub_ext4_extent_header *) inode->blocks.dir_blocks, fileblock);
       if (! leaf)
@@ -495,8 +498,18 @@ grub_ext2_read_block (grub_fshelp_node_t node, grub_disk_addr_t fileblock)
 
       nent = grub_le_to_cpu16 (leaf->entries);
 
-      if (leaf->depth == 0)
+      /*
+       * Determine the effective number of extent entries (nent) to process:
+       * If the extent header (leaf) is stored inline in the inode’s block
+       * area (i.e. at inode->blocks.dir_blocks), then only max_inline_ext
+       * entries can fit.
+       * Otherwise, if the header was read from an external extent block, use
+       * the larger limit, max_external_ext, based on the full block size.
+       */
+      if (leaf == (struct grub_ext4_extent_header *) inode->blocks.dir_blocks)
 	nent = grub_min (nent, max_inline_ext);
+      else
+	nent = grub_min (nent, max_external_ext);
 
       for (i = 0; i < nent; i++)
         {