[PATCH v3] f2fs: validate ACL entry sizes in f2fs_acl_from_disk()

[Zhang Cen <rollkingzzc@gmail.com>]

f2fs_acl_count() only validates the aggregate ACL xattr length. A
malformed ACL can still place ACL_USER or ACL_GROUP in a slot that only
contains struct f2fs_acl_entry_short bytes, and f2fs_acl_from_disk()
then reads entry->e_id before verifying that a full entry fits.

Require a short entry before reading e_tag and e_perm, and require a
full entry before reading e_id for ACL_USER and ACL_GROUP. Return
-EFSCORRUPTED from these new truncated-entry checks, while keeping the
pre-existing -EINVAL paths unchanged.

Validation reproduced this kernel report:
KASAN slab-out-of-bounds in __f2fs_get_acl+0x6fb/0x7e0
RIP: 0033:0x7f4b835ea7aa
The buggy address belongs to the object at ffff888114589960 which belongs
to the cache kmalloc-8 of size 8
The buggy address is located 0 bytes to the right of allocated 8-byte
region [ffff888114589960, ffff888114589968)
Read of size 4
Call trace:
  dump_stack_lvl+0x66/0xa0 (?:?)
  print_report+0xce/0x630 (?:?)
  __f2fs_get_acl+0x6fb/0x7e0 (fs/f2fs/acl.c:169)
  srso_alias_return_thunk+0x5/0xfbef5 (?:?)
  __virt_addr_valid+0x224/0x430 (?:?)
  kasan_report+0xe0/0x110 (?:?)
  __f2fs_get_acl+0x5/0x7e0 (fs/f2fs/acl.c:169)
  __get_acl+0x281/0x380 (?:?)
  vfs_get_acl+0x10b/0x190 (?:?)
  do_get_acl+0x2a/0x410 (?:?)
  do_get_acl+0x9/0x410 (?:?)
  do_getxattr+0xe8/0x260 (?:?)
  filename_getxattr+0xd1/0x140 (?:?)
  do_getname+0x2d/0x2d0 (?:?)
  path_getxattrat+0x16c/0x200 (?:?)
  lock_release+0xc8/0x290 (?:?)
  cgroup_update_frozen+0x9d/0x320 (?:?)
  lockdep_hardirqs_on_prepare+0xea/0x1a0 (?:?)
  trace_hardirqs_on+0x1a/0x170 (?:?)
  _raw_spin_unlock_irq+0x28/0x50 (?:?)
  do_syscall_64+0x115/0x6a0 (arch/x86/entry/syscall_64.c:87)
  entry_SYSCALL_64_after_hwframe+0x77/0x7f (?:?)

Fixes: af48b85b8cd3 ("f2fs: add xattr and acl functionalities")
Assisted-by: Codex:gpt-5.5
Signed-off-by: Zhang Cen <rollkingzzc@gmail.com>
---
Changes in v3:
- Keep the pre-existing -EINVAL error paths unchanged.
- Return -EFSCORRUPTED only from the newly added truncated-entry checks.

Changes in v2:
- Wrap the new ACL entry-size checks in unlikely().
- Return -EFSCORRUPTED for malformed on-disk ACL blobs.

 fs/f2fs/acl.c | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/fs/f2fs/acl.c b/fs/f2fs/acl.c
index fa8d81a30fb91..d3253549173e6 100644
--- a/fs/f2fs/acl.c
+++ b/fs/f2fs/acl.c
@@ -47,6 +47,7 @@ static inline int f2fs_acl_count(size_t size)
 static struct posix_acl *f2fs_acl_from_disk(const char *value, size_t size)
 {
 	int i, count;
+	int err = -EINVAL;
 	struct posix_acl *acl;
 	struct f2fs_acl_header *hdr = (struct f2fs_acl_header *)value;
 	struct f2fs_acl_entry *entry = (struct f2fs_acl_entry *)(hdr + 1);
@@ -70,8 +71,11 @@ static struct posix_acl *f2fs_acl_from_disk(const char *value, size_t size)
 
 	for (i = 0; i < count; i++) {
 
-		if ((char *)entry > end)
+		if (unlikely((char *)entry +
+				sizeof(struct f2fs_acl_entry_short) > end)) {
+			err = -EFSCORRUPTED;
 			goto fail;
+		}
 
 		acl->a_entries[i].e_tag  = le16_to_cpu(entry->e_tag);
 		acl->a_entries[i].e_perm = le16_to_cpu(entry->e_perm);
@@ -86,6 +90,11 @@ static struct posix_acl *f2fs_acl_from_disk(const char *value, size_t size)
 			break;
 
 		case ACL_USER:
+			if (unlikely((char *)entry +
+					sizeof(struct f2fs_acl_entry) > end)) {
+				err = -EFSCORRUPTED;
+				goto fail;
+			}
 			acl->a_entries[i].e_uid =
 				make_kuid(&init_user_ns,
 						le32_to_cpu(entry->e_id));
@@ -93,6 +102,11 @@ static struct posix_acl *f2fs_acl_from_disk(const char *value, size_t size)
 					sizeof(struct f2fs_acl_entry));
 			break;
 		case ACL_GROUP:
+			if (unlikely((char *)entry +
+					sizeof(struct f2fs_acl_entry) > end)) {
+				err = -EFSCORRUPTED;
+				goto fail;
+			}
 			acl->a_entries[i].e_gid =
 				make_kgid(&init_user_ns,
 						le32_to_cpu(entry->e_id));
@@ -108,7 +122,7 @@ static struct posix_acl *f2fs_acl_from_disk(const char *value, size_t size)
 	return acl;
 fail:
 	posix_acl_release(acl);
-	return ERR_PTR(-EINVAL);
+	return ERR_PTR(err);
 }
 
 static void *f2fs_acl_to_disk(struct f2fs_sb_info *sbi,
-- 
2.43.0