iommufd/selftest: Avoid selftest dirty bitmap size wrap

IOMMU_TEST_OP_DIRTY sizes its temporary dirty bitmap from length /
page_size.  Very large selftest ranges can make the DIV_ROUND_UP()
additions wrap before allocation, producing a zero-length allocation while
the later test_bit() loop still walks the original number of bits.

The selftest helper does not need to support unbounded dirty bitmap sizes.
Reject requests that would allocate more than SZ_16M for the temporary
buffer.

Fixes: 79ea4a496ab5 ("iommufd/selftest: Fix buffer read overrrun in the dirty test")
Link: https://patch.msgid.link/r/20260628152331.82122.408afd7b466c.iommufd-test-dirty-bitmap-size-wrap@trailofbits.com
Assisted-by: Codex:gpt-5.5-cyber-preview
Signed-off-by: Samuel Moelius <sam.moelius@trailofbits.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

0 files changed, 0 insertions, 0 deletions