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Syed Abdul Khaliq
fd11acab3d
The directory and Rock Ridge / SUSP walk in iso9660_dir() derives several
lengths directly from attacker-controlled on-disk fields without validating
them. libfsimage is used by pygrub, which parses the filesystem of an
untrusted guest disk image from dom0, so these are reachable across a trust
boundary.
Five related problems are addressed:
* The directory record loop advances by
idr = (char *)idr + idr->length.l
and only stops on length.l == 0. A record whose length is smaller than
the fixed part of the on-disk layout cannot hold its own mandatory
fields, yet the body still reads name_len/extent/size and computes the
System Use area length from it. Require length to cover at least the
fixed record (sizeof(*idr) - sizeof(idr->name)) before entering the body.
This is CVE-2026-42494.
* The System Use area length is computed before the inner loop as
rr_len = idr->length.l - idr->name_len.l
- sizeof(struct iso_directory_record) + sizeof(idr->name);
in unsigned arithmetic. If length.l is smaller than name_len.l plus the
fixed record size, rr_len underflows to a huge value and the whole SUSP
walk runs off the directory buffer. Guard the subtraction and treat such
records as having no System Use area.
This is CVE-2026-42495.
* Inside the loop, each entry is consumed with
rr_len -= rr_ptr.rr->len;
rr_ptr.ptr += rr_ptr.rr->len;
with no lower or upper bound on the entry's own len byte. A len of 0
spins forever; a len greater than the remaining rr_len underflows it and
walks past the buffer. Validate 4 <= len <= rr_len at the top of the
loop and stop on violation: a structurally broken entry stream cannot be
advanced reliably, so continuing is not meaningful.
This is CVE-2026-62423.
* The NM handler subtracted the 5-byte SUSP/NM header from len without a
lower-bound check, underflowing name_len (the original report). The
generic check above only guarantees len >= 4; NM has an extra flags byte,
so keep an NM-specific len >= 5 check.
This is CVE-2026-62424.
* The CE continuation resets rr_ptr/rr_len from ce.offset and ce.size, both
image-controlled, into the fixed single-sector RRCONT_BUF with no bounds
check. Reject a window that does not fit in the buffer.
This is CVE-2026-62425.
This is XSA-497.
Signed-off-by: Syed Abdul Khaliq <abdul@bugqore.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>