From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Daniel Axtens <dja@axtens.net>
Date: Mon, 18 Jan 2021 14:57:17 +1100
Subject: [PATCH] fs/jfs: Limit the extents that getblk() can consider

getblk() implicitly trusts that treehead->count is an accurate count of
the number of extents. However, that value is read from disk and is not
trustworthy, leading to OOB reads and crashes. I am not sure to what
extent the data read from OOB can influence subsequent program execution.

Require callers to pass in the maximum number of extents for which
they have storage.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
---
 grub-core/fs/jfs.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/grub-core/fs/jfs.c b/grub-core/fs/jfs.c
index 1819899bd..6e81f37da 100644
--- a/grub-core/fs/jfs.c
+++ b/grub-core/fs/jfs.c
@@ -261,13 +261,15 @@ static grub_err_t grub_jfs_lookup_symlink (struct grub_jfs_data *data, grub_uint
 static grub_int64_t
 getblk (struct grub_jfs_treehead *treehead,
 	struct grub_jfs_tree_extent *extents,
+	int max_extents,
 	struct grub_jfs_data *data,
 	grub_uint64_t blk)
 {
   int found = -1;
   int i;
 
-  for (i = 0; i < grub_le_to_cpu16 (treehead->count) - 2; i++)
+  for (i = 0; i < grub_le_to_cpu16 (treehead->count) - 2 &&
+	      i < max_extents; i++)
     {
       if (treehead->flags & GRUB_JFS_TREE_LEAF)
 	{
@@ -302,7 +304,7 @@ getblk (struct grub_jfs_treehead *treehead,
 			   << (grub_le_to_cpu16 (data->sblock.log2_blksz)
 			       - GRUB_DISK_SECTOR_BITS), 0,
 			   sizeof (*tree), (char *) tree))
-	ret = getblk (&tree->treehead, &tree->extents[0], data, blk);
+	ret = getblk (&tree->treehead, &tree->extents[0], 254, data, blk);
       grub_free (tree);
       return ret;
     }
@@ -316,7 +318,7 @@ static grub_int64_t
 grub_jfs_blkno (struct grub_jfs_data *data, struct grub_jfs_inode *inode,
 		grub_uint64_t blk)
 {
-  return getblk (&inode->file.tree, &inode->file.extents[0], data, blk);
+  return getblk (&inode->file.tree, &inode->file.extents[0], 16, data, blk);
 }