CVE-2026-90199: fs/ntfs3: reject out-of-range evcn in mi_enum_attr()

In the Linux kernel, the following vulnerability has been resolved:

fs/ntfs3: reject out-of-range evcn in mienumattr()

In mienumattr(), the start/end VCN validation for non-resident attributes is:

if (svcn > evcn + 1) goto out;

When evcn is U64MAX the "evcn + 1" expression wraps to 0 and any svcn passes the check. For evcn values close to U64MAX (but not equal to it) the right-hand side is still a meaningless near-wrap upper bound, so a malformed on-disk attribute with svcn == 0 and evcn near U64MAX can pass mienumattr() unrejected.

VCN (virtual cluster number) is a cluster index, so any valid evcn is bounded by the volume's total cluster count, which ntfs3 holds in sbi->used.bitmap.nbits (set up in ntfsinitfromboot() before any caller of mienumattr() runs). Reject evcn values that fall outside this range.

However, an empty non-resident attribute (no allocated clusters) is legitimately encoded with svcn == 0 and evcn == -1 (U64MAX), e.g. via attr->nres.evcn = cputole64((u64)vcn - 1) with vcn == 0. That sentinel must keep passing, so exclude evcn == U64MAX from the range check. The existing "svcn > evcn + 1" test still tolerates the sentinel ("0 > 0" is false) and continues to require svcn == 0 for it, while the range check rejects every other out-of-range evcn and thereby also defuses the "evcn + 1" wraparound.

svcn does not need its own bound: once evcn < nbits, "svcn > evcn + 1" implies svcn <= nbits.

[almaz.alexandrovich@paragon-software.com: fixed evcn check]