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

Alexandre Ghiti
2025-04-08T09:09:19Z

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@sifive.com>
> Signed-off-by: Deepak Gupta <debug@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@rivosinc.com>
> +:Date: 12 January 2024
> +
> +=========================================================
> +Shadow stack to protect function returns on RISC-V Linux
> +=========================================================
> +
> +This document briefly describes the interface provided to userspace by Linux
> +to enable shadow stack for user mode applications on RISV-V

s/RISV-V/RISC-V and the final dot

> +
> +1. Feature Overview
> +--------------------
> +
> +Memory corruption issues usually result in to crashes, however when in hands of

s/in to/into

> +an adversary and if used creatively can result into variety security issues.

"into a variety of"

> +
> +One of those security issues can be code re-use attacks on program where
> +adversary can use corrupt return addresses present on stack and chain them
> +together to perform return oriented programming (ROP) and thus compromising
> +control flow integrity (CFI) of the program.
> +
> +Return addresses live on stack and thus in read-write memory and thus are
> +susceptible to corruption and allows an adversary to reach any program counter

s/and allows/which allows

> +(PC) in address space. On RISC-V ``zicfiss`` extension provides an alternate
> +stack termed as shadow stack on which return addresses can be safely placed in
> +prolog of the function and retrieved in epilog. ``zicfiss`` extension makes
> +following changes:
> +
> +- PTE encodings for shadow stack virtual memory
> + An earlier reserved encoding in first stage translation i.e.
> + PTE.R=0, PTE.W=1, PTE.X=0 becomes PTE encoding for shadow stack pages.
> +
> +- ``sspush x1/x5`` instruction pushes (stores) ``x1/x5`` to shadow stack.
> +
> +- ``sspopchk x1/x5`` instruction pops (loads) from shadow stack and compares
> + with ``x1/x5`` and if un-equal, CPU raises ``software check exception`` with
> + ``*tval = 3``
> +
> +Compiler toolchain makes sure that function prologue have ``sspush x1/x5`` to
> +save return address on shadow stack in addition to regular stack. Similarly
> +function epilogs have ``ld x5, offset(x2)`` followed by ``sspopchk x5`` to
> +ensure that popped value from regular stack matches with popped value from
> +shadow stack.
> +
> +2. Shadow stack protections and linux memory manager
> +-----------------------------------------------------
> +
> +As mentioned earlier, shadow stack get new page table encodings and thus have

s/shadow stack/shadow stacks

> +some special properties assigned to them and instructions that operate on them
> +as below:
> +
> +- Regular stores to shadow stack memory raises access store faults. This way
> + shadow stack memory is protected from stray inadvertant writes.

s/inadvertant/inadvertent

> +
> +- Regular loads to shadow stack memory are allowed. This allows stack trace
> + utilities or backtrace functions to read true callstack (not tampered).
> +
> +- Only shadow stack instructions can generate shadow stack load or shadow stack
> + store.
> +
> +- Shadow stack load / shadow stack store on read-only memory raises AMO/store
> + page fault. Thus both ``sspush x1/x5`` and ``sspopchk x1/x5`` will raise AMO/
> + store page fault. This simplies COW handling in kernel During fork, kernel

s/During/during

> + can convert shadow stack pages into read-only memory (as it does for regular
> + read-write memory) and as soon as subsequent ``sspush`` or ``sspopchk`` in
> + userspace is encountered, then kernel can perform COW.
> +
> +- Shadow stack load / shadow stack store on read-write, read-write-execute
> + memory raises an access fault. This is a fatal condition because shadow stack
> + should never be operating on read-write, read-write-execute memory.
> +
> +3. ELF and psABI
> +-----------------
> +
> +Toolchain sets up :c:macro:`GNU_PROPERTY_RISCV_FEATURE_1_BCFI` for property
> +:c:macro:`GNU_PROPERTY_RISCV_FEATURE_1_AND` in notes section of the object file.
> +
> +4. Linux enabling
> +------------------
> +
> +User space programs can have multiple shared objects loaded in its address space
> +and it's a difficult task to make sure all the dependencies have been compiled
> +with support of shadow stack. Thus it's left to dynamic loader to enable
> +shadow stack for the program.
> +
> +5. prctl() enabling
> +--------------------
> +
> +:c:macro:`PR_SET_SHADOW_STACK_STATUS` / :c:macro:`PR_GET_SHADOW_STACK_STATUS` /
> +:c:macro:`PR_LOCK_SHADOW_STACK_STATUS` are three prctls added to manage shadow
> +stack enabling for tasks. prctls are arch agnostic and returns -EINVAL on other
> +arches.
> +
> +* prctl(PR_SET_SHADOW_STACK_STATUS, unsigned long arg)
> +
> +If arg1 :c:macro:`PR_SHADOW_STACK_ENABLE` and if CPU supports ``zicfiss`` then
> +kernel will enable shadow stack for the task. Dynamic loader can issue this
> +:c:macro:`prctl` once it has determined that all the objects loaded in address
> +space have support for shadow stack. Additionally if there is a
> +:c:macro:`dlopen` to an object which wasn't compiled with ``zicfiss``, dynamic
> +loader can issue this prctl with arg1 set to 0 (i.e.
> +:c:macro:`PR_SHADOW_STACK_ENABLE` being clear)
> +
> +* prctl(PR_GET_SHADOW_STACK_STATUS, unsigned long *arg)
> +
> +Returns current status of indirect branch tracking. If enabled it'll return
> +:c:macro:`PR_SHADOW_STACK_ENABLE`.
> +
> +* prctl(PR_LOCK_SHADOW_STACK_STATUS, unsigned long arg)
> +
> +Locks current status of shadow stack enabling on the task. User space may want
> +to run with strict security posture and wouldn't want loading of objects
> +without ``zicfiss`` support in it and thus would want to disallow disabling of
> +shadow stack on current task. In that case user space can use this prctl to
> +lock current settings.
> +
> +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.

> +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.