[PATCH v12 27/28] riscv: Documentation for shadow stack on riscv

[Deepak Gupta]

On Tue, Apr 08, 2025 at 10:48:08AM +0200, Alexandre Ghiti wrote:
>
>On 14/03/2025 22:39, Deepak Gupta wrote:
>>Adding documentation on shadow stack for user mode on riscv and kernel
>>interfaces exposed so that user tasks can enable it.
>>
>>Reviewed-by: Zong Li <zong.li at sifive.com>
>>Signed-off-by: Deepak Gupta <debug at rivosinc.com>
>>---
>>  Documentation/arch/riscv/index.rst   |   1 +
>>  Documentation/arch/riscv/zicfiss.rst | 176 +++++++++++++++++++++++++++++++++++
>>  2 files changed, 177 insertions(+)
>>
>>diff --git a/Documentation/arch/riscv/index.rst b/Documentation/arch/riscv/index.rst
>>index be7237b69682..e240eb0ceb70 100644
>>--- a/Documentation/arch/riscv/index.rst
>>+++ b/Documentation/arch/riscv/index.rst
>>@@ -15,6 +15,7 @@ RISC-V architecture
>>      vector
>>      cmodx
>>      zicfilp
>>+    zicfiss
>>      features
>>diff --git a/Documentation/arch/riscv/zicfiss.rst b/Documentation/arch/riscv/zicfiss.rst
>>new file mode 100644
>>index 000000000000..5ba389f15b3f
>>--- /dev/null
>>+++ b/Documentation/arch/riscv/zicfiss.rst
>>@@ -0,0 +1,176 @@
>>+.. SPDX-License-Identifier: GPL-2.0
>>+
>>+:Author: Deepak Gupta <debug at rivosinc.com>
>>+:Date:   12 January 2024
>>+
>>+=========================================================
>>+Shadow stack to protect function returns on RISC-V Linux
>>+=========================================================

<... snipped ..>

>>+
>>+5. violations related to returns with shadow stack enabled
>>+-----------------------------------------------------------
>>+
>>+Pertaining to shadow stack, CPU raises software check exception in following
>>+condition:
>>+
>>+- On execution of ``sspopchk x1/x5``, ``x1/x5`` didn't match top of shadow
>>+  stack. If mismatch happens then cpu does ``*tval = 3`` and raise software
>>+  check exception.
>>+
>>+Linux kernel will treat this as :c:macro:`SIGSEV`` with code =
>>+:c:macro:`SEGV_CPERR` and follow normal course of signal delivery.
>>+
>>+6. Shadow stack tokens
>>+-----------------------
>>+Regular stores on shadow stacks are not allowed and thus can't be tampered
>>+with via arbitrary stray writes due to bugs. Method of pivoting / switching to
>>+shadow stack is simply writing to csr ``CSR_SSP`` changes active shadow stack.
>
>
>I don't understand the end of this sentence.

I'll rephrase it to make it readable and understandable.

>
>
>>+This can be problematic because usually value to be written to ``CSR_SSP`` will
>>+be loaded somewhere in writeable memory and thus allows an adversary to
>>+corruption bug in software to pivot to an any address in shadow stack range.
>
>
>Remove "an"
>
>
>>+Shadow stack tokens can help mitigate this problem by making sure that:
>>+
>>+- When software is switching away from a shadow stack, shadow stack pointer
>>+  should be saved on shadow stack itself and call it ``shadow stack token``
>>+
>>+- When software is switching to a shadow stack, it should read the
>>+  ``shadow stack token`` from shadow stack pointer and verify that
>>+  ``shadow stack token`` itself is pointer to shadow stack itself.
>>+
>>+- Once the token verification is done, software can perform the write to
>>+  ``CSR_SSP`` to switch shadow stack.
>>+
>>+Here software can be user mode task runtime itself which is managing various
>>+contexts as part of single thread. Software can be kernel as well when kernel
>>+has to deliver a signal to user task and must save shadow stack pointer. Kernel
>>+can perform similar procedure by saving a token on user shadow stack itself.
>>+This way whenever :c:macro:`sigreturn` happens, kernel can read the token and
>>+verify the token and then switch to shadow stack. Using this mechanism, kernel
>>+helps user task so that any corruption issue in user task is not exploited by
>>+adversary by arbitrarily using :c:macro:`sigreturn`. Adversary will have to
>>+make sure that there is a ``shadow stack token`` in addition to invoking
>>+:c:macro:`sigreturn`
>>+
>>+7. Signal shadow stack
>>+-----------------------
>>+Following structure has been added to sigcontext for RISC-V::
>>+
>>+    struct __sc_riscv_cfi_state {
>>+        unsigned long ss_ptr;
>>+    };
>>+
>>+As part of signal delivery, shadow stack token is saved on current shadow stack
>>+itself and updated pointer is saved away in :c:macro:`ss_ptr` field in
>>+:c:macro:`__sc_riscv_cfi_state` under :c:macro:`sigcontext`. Existing shadow
>>+stack allocation is used for signal delivery. During :c:macro:`sigreturn`,
>>+kernel will obtain :c:macro:`ss_ptr` from :c:macro:`sigcontext` and verify the
>>+saved token on shadow stack itself and switch shadow stack.
>>