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CVE-2026-99018

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

Description (CNA)

In the Linux kernel, the following vulnerability has been resolved: riscv: entry: gate shadow call stack reload on sstatus.SPP, not tp Under CONFIG_SHADOW_CALL_STACK, handle_exception() reloads the kernel shadow call stack pointer (gp) only when the trap came from userspace. It made that decision with scs_load_current_if_task_changed, which compares the trap-time tp (read back from sscratch into s5) against the current tp: beq \prev, tp, _skip_scs scs_load_current _skip_scs: tp is a user-writable register. A user task that sets tp to the linear-map address of its own task_struct (&current) before trapping makes the comparison succeed (can achieve by brute force with KASLR enable under our test), so the kernel skips scs_load_current and keeps running with the gp that userspace left in place. Under CONFIG_SHADOW_CALL_STACK gp is the shadow call stack pointer and load_global_pointer 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 task_struct.scs_sp 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 ret_from_exception 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.

Affected (CNA)

Vendor / productVersions
Linux / Linuxaffected d1584d791a297aa8ed93503382a682a6ecfc4218 before 6639735367a7d491977e9e5e7afc50b489d81d83; affected d1584d791a297aa8ed93503382a682a6ecfc4218 before 7d1367a53137e379498eb8791295e58854378636; affected d1584d791a297aa8ed93503382a682a6ecfc4218 before 37b4401ee6fa06046446d55e97e8d11784080843; affected d1584d791a297aa8ed93503382a682a6ecfc4218 before 4d0697f192975dab5985e921989be1d85cf580e9
Linux / Linuxaffected 6.7; unaffected 0 before 6.7; unaffected 6.12.113 through 6.12.*; unaffected 6.18.57 through 6.18.*; unaffected 7.2.11 through 7.2.*; unaffected 7.3 through *

CWE

No CWE identifiers supplied.

References