[PATCH v2] efivarfs: Fix slab-out-of-bounds in efivarfs_d_compare

[linan666]

From: Li Nan <linan122@xxxxxxxxxx>

Observed on kernel 6.6 (present on master as well):

  BUG: KASAN: slab-out-of-bounds in memcmp+0x98/0xd0
  Call trace:
   kasan_check_range+0xe8/0x190
   __asan_loadN+0x1c/0x28
   memcmp+0x98/0xd0
   efivarfs_d_compare+0x68/0xd8
   __d_lookup_rcu_op_compare+0x178/0x218
   __d_lookup_rcu+0x1f8/0x228
   d_alloc_parallel+0x150/0x648
   lookup_open.isra.0+0x5f0/0x8d0
   open_last_lookups+0x264/0x828
   path_openat+0x130/0x3f8
   do_filp_open+0x114/0x248
   do_sys_openat2+0x340/0x3c0
   __arm64_sys_openat+0x120/0x1a0

If dentry->d_name.len < EFI_VARIABLE_GUID_LEN , 'guid' can become
negative, leadings to oob. The issue can be triggered as below:

  T1			T2
  lookup_open
   ->lookup
    simple_lookup
     d_add
     // invalid dentry is added to hash list

			lookup_open
			 d_alloc_parallel
			  __d_lookup_rcu
			   __d_lookup_rcu_op_compare
			    hlist_bl_for_each_entry_rcu
			    // invalid dentry can be retrieved
			     ->d_compare
			      efivarfs_d_compare

Fix it by checking len before cmp.

Fixes: da27a24383b2 ("efivarfs: guid part of filenames are case-insensitive")
Signed-off-by: Li Nan <linan122@xxxxxxxxxx>
Signed-off-by: Wu Guanghao <wuguanghao3@xxxxxxxxxx>
---
v2: optimize commit message

 fs/efivarfs/super.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/efivarfs/super.c b/fs/efivarfs/super.c
index 23ff4e873651..c30d758e303a 100644
--- a/fs/efivarfs/super.c
+++ b/fs/efivarfs/super.c
@@ -152,7 +152,7 @@ static int efivarfs_d_compare(const struct dentry *dentry,
 {
 	int guid = len - EFI_VARIABLE_GUID_LEN;
 
-	if (name->len != len)
+	if (name->len != len || len <= EFI_VARIABLE_GUID_LEN)
 		return 1;
 
 	/* Case-sensitive compare for the variable name */
-- 
2.39.2