CVE-2026-74751

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

riscv: lib: Fix ZBB strnlen reading past count boundary

The ZBB-optimized strnlen loop loads one word ahead before checking the

aligned boundary:

    REG_L   t1, SZREG(t0)       // load next word

    addi    t0, t0, SZREG       // advance

    orc.b   t1, t1

    bgeu    t0, t4, 4f          // boundary check AFTER load

where t4 = (s + count) & -SZREG.  When s is aligned and count is a

multiple of SZREG, t4 equals s + count and the loop loads a full word

starting at exactly s + count.  If s + count falls on a page boundary

with the next page unmapped, this faults.

Fix by computing the aligned boundary from the last valid byte

(s + count - 1) instead of s + count.  This makes the loop stop at the

word containing the last valid byte rather than potentially loading the

word after it.  The count == 0 case is already handled by the beqz

early exit.

Also add a pre-loop guard (bgeu t0, t4) for the case where all valid

bytes fit within the first word.  With the adjusted boundary, t4 can

equal t0, and entering the loop with stale register state from the

first-word processing would produce incorrect results.

The final minu clamp ensures the result is still correct when the last

loaded word extends past s + count - 1 within the same aligned word.