Re: [PATCH v3] ntfs: validate resident attribute values on lookup

[Hyunchul Lee]

Hi DaeMyung,

2026년 5월 26일 (화) 오후 10:31, DaeMyung Kang <charsyam@xxxxxxxxx>님이 작성:
>
> ntfs_attr_find() and ntfs_external_attr_find() check that generic
> resident attribute values fit in their attribute records and that
> fixed-size resident values are large enough. For variable-length resident
> formats, however, the fixed part is not enough: embedded length fields
> can still point callers past the resident value.
>
> A crafted image can set a small resident $FILE_NAME value_length while
> leaving file_name_length large. Callers then trust file_name_length and
> read past the resident value when converting or comparing the name. This
> was reproduced with a crafted image under KASAN as a slab-out-of-bounds
> read from the kmalloc-1k MFT record copy. The stack included
> ntfs_lookup(), ntfs_iget(), ntfs_read_locked_inode(), ntfs_attr_name_get(),
> ntfs_ucstonls(), and utf16s_to_utf8s().
>
> Add a shared resident attribute value validator and use it before a
> lookup path can return an attribute, including the AT_UNUSED enumeration
> case where callers inspect returned attributes directly. The helper
> validates resident value bounds, minimum value sizes, and the
> variable-length $FILE_NAME field, while preserving the fixed-size checks
> for attributes such as $VOLUME_INFORMATION and $EA_INFORMATION. Log
> corruption at the rejection point with the attribute name where known.
>
> Reject non-resident $FILE_NAME records too: the format requires
> $FILE_NAME to be resident and callers treat returned records as
> resident.
>
> Type-specific format validation for $VOLUME_NAME is intentionally not
> added to this helper, even though $VOLUME_NAME is a variable-length
> resident attribute. Existing callers such as ntfs_write_volume_label()
> treat a failed AT_VOLUME_NAME lookup as an absent label and
> unconditionally add a new record afterwards. If the helper rejected
> odd-length labels at lookup time, the corrupt record would stay in
> place and the caller would append a new record next to it, producing
> two $VOLUME_NAME attributes on disk. Caller-side cleanup must precede
> any lookup-time rejection of malformed $VOLUME_NAME payloads, so the
> type-specific check is left for a follow-up patch. The generic resident
> bounds checks (value_offset / value_length / attr_len) still apply to
> $VOLUME_NAME, and its entry in the type-name table is kept so that
> those failures log with the attribute name.
>
> Do not add $INDEX_ROOT-specific value validation in this change. Testing
> with stricter checks showed an existing ntfs_ir_truncate() shrink
> ordering bug: the resident value_length is shrunk before the root index
> allocated_size is updated, so a relookup can observe allocated_size
> beyond the now-smaller resident value and fail. Fix that ordering before
> adding the $INDEX_ROOT-specific minimum-size and structural checks.
>
> Fixes: 6ceb4cc81ef3 ("ntfs: add bound checking to ntfs_attr_find")
> Signed-off-by: DaeMyung Kang <charsyam@xxxxxxxxx>
> ---
> Patch 1/2 of the previous series ("ntfs: free link name from
> ntfs_name_cache") has been applied to ntfs-next; this v3 contains only
> the remaining patch, with the resident attribute validation reworked per
> review feedback.
>
> Changes since v2:
> - Drop $INDEX_ROOT-specific validation from this patch, including both
>   the minimum-size table entry and the structural checks. QEMU testing
>   with xfstests generic/001 showed that those checks expose an existing
>   ntfs_ir_truncate() shrink ordering bug, where value_length is reduced
>   before allocated_size is synchronized. The same test passes when
>   allocated_size is lowered before shrinking value_length, with the
>   $INDEX_ROOT-specific validation enabled. That ordering issue will be
>   fixed in a separate patch, after which the $INDEX_ROOT-specific
>   validation can be reintroduced in a follow-up.
>
> Changes since v1:
> - Refactor resident attribute validation into a shared helper
>   ntfs_attr_value_is_valid() used by both ntfs_attr_find() and
>   ntfs_external_attr_find(), and remove the duplicated inline check
>   from ntfs_external_attr_find().
> - Split out resident header/value bounds extraction into
>   ntfs_resident_attr_value_get(), returning a small view struct so the
>   top-level orchestration reads as: get view -> common min check ->
>   per-type format check.
> - Validate before the AT_UNUSED enumeration path in ntfs_attr_find()
>   can return the attribute to callers that inspect it directly.
> - Reject non-resident $FILE_NAME records: the format requires
>   $FILE_NAME to be resident and callers treat returned records as
>   resident.
> - Type-specific format validation for $VOLUME_NAME is intentionally
>   not added; see the commit message for the caller-side reason.
>   Generic resident bounds checks still apply, and the $VOLUME_NAME
>   entry in the type-name table is kept so that those failures log
>   with a readable attribute name.
> - Log corruption at the rejection point with the attribute type name
>   via a new ntfs_attr_type_name() helper, matching the existing
>   attribute-name corruption message in ntfs_attr_find().
>
>  fs/ntfs/attrib.c | 148 +++++++++++++++++++++++++++++++++++++++++++------------
>  1 file changed, 117 insertions(+), 31 deletions(-)
>
> diff --git a/fs/ntfs/attrib.c b/fs/ntfs/attrib.c
> index 421c6cdcbb53..382d1b6e877e 100644
> --- a/fs/ntfs/attrib.c
> +++ b/fs/ntfs/attrib.c
> @@ -595,6 +595,112 @@ static u32 ntfs_resident_attr_min_value_length(const __le32 type)
>         }
>  }
>
> +static const char *ntfs_attr_type_name(const __le32 type)
> +{
> +       switch (type) {
> +       case AT_STANDARD_INFORMATION:
> +               return "$STANDARD_INFORMATION";
> +       case AT_FILE_NAME:
> +               return "$FILE_NAME";
> +       case AT_VOLUME_NAME:
> +               return "$VOLUME_NAME";
> +       case AT_VOLUME_INFORMATION:
> +               return "$VOLUME_INFORMATION";
> +       case AT_INDEX_ROOT:
> +               return "$INDEX_ROOT";
> +       case AT_EA_INFORMATION:
> +               return "$EA_INFORMATION";
> +       default:
> +               return NULL;
> +       }
> +}
> +
> +static bool ntfs_file_name_attr_value_is_valid(const u8 *value, const u32 value_length)
> +{
> +       const struct file_name_attr *fn;
> +       u32 file_name_size;
> +
> +       if (value_length < ntfs_resident_attr_min_value_length(AT_FILE_NAME))
> +               return false;
> +
> +       fn = (const struct file_name_attr *)value;
> +       file_name_size = fn->file_name_length * sizeof(__le16);
> +
> +       return file_name_size <=
> +                       value_length - offsetof(struct file_name_attr, file_name);
> +}
> +
> +struct ntfs_resident_attr_value {
> +       const u8 *data;
> +       u32 len;
> +};
> +
> +static bool ntfs_resident_attr_value_get(const struct attr_record *a,
> +                                        struct ntfs_resident_attr_value *value)
> +{
> +       u32 attr_len;
> +       u16 value_offset;
> +
> +       attr_len = le32_to_cpu(a->length);
> +       if (attr_len < offsetof(struct attr_record, data.resident.reserved) +
> +                       sizeof(a->data.resident.reserved))
> +               return false;
> +
> +       value->len = le32_to_cpu(a->data.resident.value_length);
> +       value_offset = le16_to_cpu(a->data.resident.value_offset);
> +
> +       if (value->len > attr_len || value_offset > attr_len - value->len)
> +               return false;
> +
> +       value->data = (const u8 *)a + value_offset;
> +       return true;
> +}
> +
> +static bool ntfs_attr_value_is_valid(struct ntfs_volume *vol,
> +                                    const struct attr_record *a,
> +                                    const u64 mft_no)
> +{
> +       struct ntfs_resident_attr_value value;
> +       const char *type_name;
> +       u32 min_len;
> +
> +       if (a->non_resident) {
> +               if (a->type != AT_FILE_NAME)
> +                       return true;
> +               ntfs_error(vol->sb,
> +                          "Corrupt non-resident $FILE_NAME attribute in MFT record %llu\n",
> +                          mft_no);
> +               return false;
> +       }
> +
> +       if (!ntfs_resident_attr_value_get(a, &value))
> +               goto corrupt;
> +
> +       min_len = ntfs_resident_attr_min_value_length(a->type);
> +       if (min_len && value.len < min_len)
> +               goto corrupt;
> +
> +       switch (a->type) {
> +       case AT_FILE_NAME:
> +               if (!ntfs_file_name_attr_value_is_valid(value.data, value.len))
> +                       goto corrupt;
> +               break;
> +       }
> +       return true;
> +
> +corrupt:
> +       type_name = ntfs_attr_type_name(a->type);
> +       if (type_name)
> +               ntfs_error(vol->sb,
> +                          "Corrupt resident %s attribute in MFT record %llu\n",
> +                          type_name, mft_no);
> +       else
> +               ntfs_error(vol->sb,
> +                          "Corrupt resident %#x attribute in MFT record %llu\n",
> +                          le32_to_cpu(a->type), mft_no);
> +       return false;
> +}
> +
>  /*
>   * ntfs_attr_find - find (next) attribute in mft record
>   * @type:      attribute type to find
> @@ -705,8 +811,11 @@ static int ntfs_attr_find(const __le32 type, const __le16 *name,
>                         }
>                 }
>
> -               if (type == AT_UNUSED)
> +               if (type == AT_UNUSED) {
> +                       if (!ntfs_attr_value_is_valid(vol, a, ctx->ntfs_ino->mft_no))
> +                               break;
>                         return 0;
> +               }
>                 if (a->type != type)
>                         continue;
>                 /*
> @@ -747,24 +856,11 @@ static int ntfs_attr_find(const __le32 type, const __le16 *name,
>                         }
>                 }
>
> -                /* Validate attribute's value offset/length */
> -               if (!a->non_resident) {
> -                       u32 min_len;
> -                       u32 value_length = le32_to_cpu(a->data.resident.value_length);
> -                       u16 value_offset = le16_to_cpu(a->data.resident.value_offset);
> -
> -                       if (value_length > le32_to_cpu(a->length) ||
> -                           value_offset > le32_to_cpu(a->length) - value_length)
> -                               break;
> +               if (!ntfs_attr_value_is_valid(vol, a, ctx->ntfs_ino->mft_no))
> +                       break;
>
> -                       min_len = ntfs_resident_attr_min_value_length(a->type);
> -                       if (min_len && value_length < min_len) {
> -                               ntfs_error(vol->sb,
> -                                          "Too small %#x resident attribute value in MFT record %lld\n",
> -                                          le32_to_cpu(a->type), (long long)ctx->ntfs_ino->mft_no);
> -                               break;
> -                       }
> -               } else {
> +               /* Validate non-resident mapping-pairs fields. */
> +               if (a->non_resident) {

How about moving this validation into ntfs_attr_value_is_valid() as
well? The function name
suggests that it validates both resident and non-resident attributes,
and since the same
validation logic also exists in ntfs_attr_external_find(), moving it
would help prevent
duplication.


>                         u32 min_len;
>                         u16 mp_offset;
>
> @@ -1252,6 +1348,9 @@ static int ntfs_external_attr_find(const __le32 type,
>
>                 ctx->attr = a;
>
> +               if (!ntfs_attr_value_is_valid(vol, a, ctx->ntfs_ino->mft_no))
> +                       break;
> +
>                 if (a->non_resident) {
>                         u32 min_len;
>                         u16 mp_offset;
> @@ -1279,19 +1378,6 @@ static int ntfs_external_attr_find(const __le32 type,
>                         u32 value_length = le32_to_cpu(a->data.resident.value_length);
>                         u16 value_offset = le16_to_cpu(a->data.resident.value_offset);
>
> -                       if (attr_len < offsetof(struct attr_record, data.resident.reserved) +
> -                                       sizeof(a->data.resident.reserved))
> -                               break;
> -                       if (value_length > attr_len || value_offset > attr_len - value_length)
> -                               break;
> -
> -                       value_length = ntfs_resident_attr_min_value_length(a->type);
> -                       if (value_length && le32_to_cpu(a->data.resident.value_length) <
> -                           value_length) {
> -                               pr_err("Too small resident attribute value in MFT record %lld, type %#x\n",
> -                                      (long long)ctx->ntfs_ino->mft_no, a->type);
> -                               break;
> -                       }
>                         if (value_length == val_len &&
>                             !memcmp((u8 *)a + value_offset, val, val_len)) {
>  attr_found:
> --
> 2.43.0



-- 
Thanks,
Hyunchul