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

JIRA: https://issues.redhat.com/browse/RHEL-118459
CVE: CVE-2025-39817

commit a6358f8
Author: Li Nan <linan122@huawei.com>
Date:   Wed Aug 27 15:39:54 2025 +0800

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

    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>

Signed-off-by: CKI Backport Bot <cki-ci-bot+cki-gitlab-backport-bot@redhat.com>

Author: CKI Backport Bot

fs/efivarfs/super.c

@@ -127,6 +127,10 @@ 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;