[SECURITY PATCH 075/117] fs/sfs: Fix over-read of root object name

[Daniel Kiper]

From: Daniel Axtens <dja@axtens.net>

There's a read of the name of the root object that assumes that the name
is nul-terminated within the root block. This isn't guaranteed - it seems
SFS would require you to read multiple blocks to get a full name in general,
but maybe that doesn't apply to the root object.

Either way, figure out how much space is left in the root block and don't
over-read it. This fixes some OOB reads.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
---
 grub-core/fs/sfs.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/grub-core/fs/sfs.c b/grub-core/fs/sfs.c
index de2b107a4..983e88008 100644
--- a/grub-core/fs/sfs.c
+++ b/grub-core/fs/sfs.c
@@ -373,6 +373,7 @@ grub_sfs_mount (grub_disk_t disk)
   struct grub_sfs_objc *rootobjc;
   char *rootobjc_data = 0;
   grub_uint32_t blk;
+  unsigned int max_len;
 
   data = grub_malloc (sizeof (*data));
   if (!data)
@@ -421,7 +422,13 @@ grub_sfs_mount (grub_disk_t disk)
   data->diropen.data = data;
   data->diropen.cache = 0;
   data->disk = disk;
-  data->label = grub_strdup ((char *) (rootobjc->objects[0].filename));
+
+  /* We only read 1 block of data, so truncate the name if needed. */
+  max_len = ((GRUB_DISK_SECTOR_SIZE << data->log_blocksize)
+            - 24    /* offsetof (struct grub_sfs_objc, objects) */
+            - 25);  /* offsetof (struct grub_sfs_obj, filename) */
+  data->label = grub_zalloc (max_len + 1);
+  grub_strncpy (data->label, (char *) rootobjc->objects[0].filename, max_len);
 
   grub_free (rootobjc_data);
   return data;
-- 
2.11.0