CVE-2026-99018: riscv: entry: gate shadow call stack reload on sstatus.SPP, not tp

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

riscv: entry: gate shadow call stack reload on sstatus.SPP, not tp

Under CONFIGSHADOWCALLSTACK, handleexception() reloads the kernel shadow call stack pointer (gp) only when the trap came from userspace. It made that decision with scsloadcurrentiftaskchanged, which compares the trap-time tp (read back from sscratch into s5) against the current tp:

beq \prev, tp, skipscs scsloadcurrent skipscs:

tp is a user-writable register. A user task that sets tp to the linear-map address of its own taskstruct (&current) before trapping makes the comparison succeed (can achieve by brute force with KASLR enable under our test), so the kernel skips scsloadcurrent and keeps running with the gp that userspace left in place. Under CONFIGSHADOWCALLSTACK gp is the shadow call stack pointer and loadglobalpointer is a no-op, so the compiler-emitted "sd ra, -8(gp)" / "ld ra, -8(gp)" in every non-leaf function then write to, and return through, an attacker-controlled address -- an unprivileged kernel arbitrary-write and control-flow-hijack primitive. The poisoned gp is saved to taskstruct.scssp on return, so the reload is skipped on every subsequent entry as well.

Decide "came from userspace" from the hardware sstatus.SPP bit (already in s1 at this point) instead of from tp, mirroring the check that retfromexception already uses a few instructions later. SPP is written by hardware on trap entry and cannot be forged from U-mode. Legitimate behaviour is unchanged: user entries (SPP=0) reload the task shadow call stack, nested kernel entries (SPP=1) skip it.