drm/xe: harden adjust_idledly() against divide-by-zero and overflow

Overview

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

drm/xe: harden adjust_idledly() against divide-by-zero and overflow

adjust_idledly() has several corner-case issues flagged during review:

1. If xe_gt_clock_init() failed to recognise the crystal clock, gt->info.timestamp_base is 0, which makes idledly_units_ps also 0. The subsequent DIV_ROUND_CLOSEST(..., idledly_units_ps) is then a divide-by-zero and panics the kernel.

2. The tick-to-ns conversions are done in u32: idledly * idledly_units_ps, (maxcnt - 1) * 1000 Both overflow u32 before DIV_ROUND_CLOSEST() sees them.

3. If IDLE_WAIT_TIME reads back as 0, maxcnt evaluates to 0 and the maxcnt - 1 clamp wraps to 0xFFFFFFFF in u32.

4. The register only stores whole ticks, so the clamped ns value has to be converted to ticks and back. DIV_ROUND_CLOSEST() can round that conversion up past maxcnt:

   maxcnt = 640 ns, one tick = 666664 ps

   clamp: maxcnt - 1 = 639 ns ns -> ticks: 639000 / 666664 = 0.958 -> rounds to 1 tick tick -> ns: 1 * 666664 / 1000 = 667 ns

   667 ns is programmed into RING_IDLEDLY, but 667 >= maxcnt (640), so xe_gt_WARN_ON() fires again on every subsequent init.

Return early if timestamp_base is 0 (the unknown-crystal path). Do the conversions in u64 via the *_ULL() helpers so they cannot wrap. Clamp with a floor (DIV_ROUND_DOWN_ULL) so the programmed delay stays strictly below maxcnt, and guard the maxcnt == 0 case with a zero delay while still writing RING_IDLEDLY so INHIBIT_SWITCH_UNTIL_PREEMPTED is cleared.

v2: Drop the redundant warn on the timestamp_base == 0 path; xe_gt_clock_init() already warns on an unrecognised crystal clock. Keep the early return to avoid the divide-by-zero. - Vinay

v3: Field-mask the RING_IDLEDLY write with REG_FIELD_PREP(IDLE_DELAY, ...) instead of writing the raw tick count, which could clobber INHIBIT_SWITCH_UNTIL_PREEMPTED and reserved bits. Split the inhibit-switch clear from the maxcnt clamp so a set inhibit bit no longer forces a needless delay overwrite when the delay itself is already valid. Use gt_to_xe(gt) instead of gt_to_xe(hwe->gt).

(cherry picked from commit d864065ea25e9d12897c175de9176ce46677e176)

Affected

• Linux >= d2de4410a88ffc6053300d03041cc73b8c85dbaf < 2bcbfc5e7bfa9cc76818c54db698da5bdc13d9ff
• Linux >= d2de4410a88ffc6053300d03041cc73b8c85dbaf < 531d4cd03118224022dc21ae25af052ce2a9e267
• Linux >= d2de4410a88ffc6053300d03041cc73b8c85dbaf < 90f467577ebc28c5bc4ba2f0f1b83a4419fb0740
• Linux 6.15

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.