efivarfs: Fix slab-out-of-bounds in efivarfs_d_compare

jira VULN-155159
jira VULN-155158
cve CVE-2025-39817
commit-author Li Nan <linan122@huawei.com>
commit a6358f8

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 by parallel
lookups using invalid filename:

  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
			      // oob

Fix it by checking 'guid' before cmp.

Fixes: da27a24 ("efivarfs: guid part of filenames are case-insensitive")
	Signed-off-by: Li Nan <linan122@huawei.com>
	Signed-off-by: Wu Guanghao <wuguanghao3@huawei.com>
	Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
(cherry picked from commit a6358f8)
	Signed-off-by: Jonathan Maple <jmaple@ciq.com>

Author: PlaidCat

fs/efivarfs/super.c

@@ -51,6 +51,10 @@ static int efivarfs_d_compare(const struct dentry *dentry,
 {
 	int guid = len - EFI_VARIABLE_GUID_LEN;
 
+	/* Parallel lookups may produce a temporary invalid filename */
+	if (guid <= 0)
+		return 1;
+
 	if (name->len != len)
 		return 1;