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Wed, 22 Oct 2025 16:30:04 -0700 (PDT) Received: from debug.ba.rivosinc.com ([64.71.180.162]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-7a274dc12b2sm392646b3a.67.2025.10.22.16.30.02 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 22 Oct 2025 16:30:03 -0700 (PDT) From: Deepak Gupta Date: Wed, 22 Oct 2025 16:29:38 -0700 Subject: [PATCH v22 12/28] riscv: Implements arch agnostic shadow stack prctls Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable Message-Id: <20251022-v5_user_cfi_series-v22-12-fdaa7e4022aa@rivosinc.com> References: <20251022-v5_user_cfi_series-v22-0-fdaa7e4022aa@rivosinc.com> In-Reply-To: <20251022-v5_user_cfi_series-v22-0-fdaa7e4022aa@rivosinc.com> To: Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H. Peter Anvin" , Andrew Morton , "Liam R. Howlett" , Vlastimil Babka , Lorenzo Stoakes , Paul Walmsley , Palmer Dabbelt , Albert Ou , Conor Dooley , Rob Herring , Krzysztof Kozlowski , Arnd Bergmann , Christian Brauner , Peter Zijlstra , Oleg Nesterov , Eric Biederman , Kees Cook , Jonathan Corbet , Shuah Khan , Jann Horn , Conor Dooley , Miguel Ojeda , Alex Gaynor , Boqun Feng , Gary Guo , =?utf-8?q?Bj=C3=B6rn_Roy_Baron?= , Andreas Hindborg , Alice Ryhl , Trevor Gross , Benno Lossin Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-riscv@lists.infradead.org, devicetree@vger.kernel.org, linux-arch@vger.kernel.org, linux-doc@vger.kernel.org, linux-kselftest@vger.kernel.org, alistair.francis@wdc.com, richard.henderson@linaro.org, jim.shu@sifive.com, andybnac@gmail.com, kito.cheng@sifive.com, charlie@rivosinc.com, atishp@rivosinc.com, evan@rivosinc.com, cleger@rivosinc.com, alexghiti@rivosinc.com, samitolvanen@google.com, broonie@kernel.org, rick.p.edgecombe@intel.com, rust-for-linux@vger.kernel.org, Zong Li , Deepak Gupta X-Mailer: b4 0.13.0 Implement architecture agnostic prctls() interface for setting and getting shadow stack status. prctls implemented are PR_GET_SHADOW_STACK_STATUS, PR_SET_SHADOW_STACK_STATUS and PR_LOCK_SHADOW_STACK_STATUS. As part of PR_SET_SHADOW_STACK_STATUS/PR_GET_SHADOW_STACK_STATUS, only PR_SHADOW_STACK_ENABLE is implemented because RISCV allows each mode to write to their own shadow stack using `sspush` or `ssamoswap`. PR_LOCK_SHADOW_STACK_STATUS locks current configuration of shadow stack enabling. Reviewed-by: Zong Li Signed-off-by: Deepak Gupta --- arch/riscv/include/asm/usercfi.h | 16 ++++++ arch/riscv/kernel/process.c | 8 +++ arch/riscv/kernel/usercfi.c | 110 +++++++++++++++++++++++++++++++++++= ++++ 3 files changed, 134 insertions(+) diff --git a/arch/riscv/include/asm/usercfi.h b/arch/riscv/include/asm/user= cfi.h index a16a5dff8b0e..d71093f414df 100644 --- a/arch/riscv/include/asm/usercfi.h +++ b/arch/riscv/include/asm/usercfi.h @@ -7,6 +7,7 @@ =20 #ifndef __ASSEMBLER__ #include +#include =20 struct task_struct; struct kernel_clone_args; @@ -14,6 +15,7 @@ struct kernel_clone_args; #ifdef CONFIG_RISCV_USER_CFI struct cfi_state { unsigned long ubcfi_en : 1; /* Enable for backward cfi. */ + unsigned long ubcfi_locked : 1; unsigned long user_shdw_stk; /* Current user shadow stack pointer */ unsigned long shdw_stk_base; /* Base address of shadow stack */ unsigned long shdw_stk_size; /* size of shadow stack */ @@ -26,6 +28,12 @@ void set_shstk_base(struct task_struct *task, unsigned l= ong shstk_addr, unsigned unsigned long get_shstk_base(struct task_struct *task, unsigned long *size= ); void set_active_shstk(struct task_struct *task, unsigned long shstk_addr); bool is_shstk_enabled(struct task_struct *task); +bool is_shstk_locked(struct task_struct *task); +bool is_shstk_allocated(struct task_struct *task); +void set_shstk_lock(struct task_struct *task); +void set_shstk_status(struct task_struct *task, bool enable); + +#define PR_SHADOW_STACK_SUPPORTED_STATUS_MASK (PR_SHADOW_STACK_ENABLE) =20 #else =20 @@ -41,6 +49,14 @@ bool is_shstk_enabled(struct task_struct *task); =20 #define is_shstk_enabled(task) false =20 +#define is_shstk_locked(task) false + +#define is_shstk_allocated(task) false + +#define set_shstk_lock(task) do {} while (0) + +#define set_shstk_status(task, enable) do {} while (0) + #endif /* CONFIG_RISCV_USER_CFI */ =20 #endif /* __ASSEMBLER__ */ diff --git a/arch/riscv/kernel/process.c b/arch/riscv/kernel/process.c index 72d35adc6e0e..a137d3483646 100644 --- a/arch/riscv/kernel/process.c +++ b/arch/riscv/kernel/process.c @@ -156,6 +156,14 @@ void start_thread(struct pt_regs *regs, unsigned long = pc, regs->epc =3D pc; regs->sp =3D sp; =20 + /* + * clear shadow stack state on exec. + * libc will set it later via prctl. + */ + set_shstk_status(current, false); + set_shstk_base(current, 0, 0); + set_active_shstk(current, 0); + #ifdef CONFIG_64BIT regs->status &=3D ~SR_UXL; =20 diff --git a/arch/riscv/kernel/usercfi.c b/arch/riscv/kernel/usercfi.c index ec3d78efd6f3..08620bdae696 100644 --- a/arch/riscv/kernel/usercfi.c +++ b/arch/riscv/kernel/usercfi.c @@ -24,6 +24,16 @@ bool is_shstk_enabled(struct task_struct *task) return task->thread_info.user_cfi_state.ubcfi_en; } =20 +bool is_shstk_allocated(struct task_struct *task) +{ + return task->thread_info.user_cfi_state.shdw_stk_base; +} + +bool is_shstk_locked(struct task_struct *task) +{ + return task->thread_info.user_cfi_state.ubcfi_locked; +} + void set_shstk_base(struct task_struct *task, unsigned long shstk_addr, un= signed long size) { task->thread_info.user_cfi_state.shdw_stk_base =3D shstk_addr; @@ -42,6 +52,26 @@ void set_active_shstk(struct task_struct *task, unsigned= long shstk_addr) task->thread_info.user_cfi_state.user_shdw_stk =3D shstk_addr; } =20 +void set_shstk_status(struct task_struct *task, bool enable) +{ + if (!cpu_supports_shadow_stack()) + return; + + task->thread_info.user_cfi_state.ubcfi_en =3D enable ? 1 : 0; + + if (enable) + task->thread.envcfg |=3D ENVCFG_SSE; + else + task->thread.envcfg &=3D ~ENVCFG_SSE; + + csr_write(CSR_ENVCFG, task->thread.envcfg); +} + +void set_shstk_lock(struct task_struct *task) +{ + task->thread_info.user_cfi_state.ubcfi_locked =3D 1; +} + /* * If size is 0, then to be compatible with regular stack we want it to be= as big as * regular stack. Else PAGE_ALIGN it and return back @@ -261,3 +291,83 @@ void shstk_release(struct task_struct *tsk) vm_munmap(base, size); set_shstk_base(tsk, 0, 0); } + +int arch_get_shadow_stack_status(struct task_struct *t, unsigned long __us= er *status) +{ + unsigned long bcfi_status =3D 0; + + if (!cpu_supports_shadow_stack()) + return -EINVAL; + + /* this means shadow stack is enabled on the task */ + bcfi_status |=3D (is_shstk_enabled(t) ? PR_SHADOW_STACK_ENABLE : 0); + + return copy_to_user(status, &bcfi_status, sizeof(bcfi_status)) ? -EFAULT = : 0; +} + +int arch_set_shadow_stack_status(struct task_struct *t, unsigned long stat= us) +{ + unsigned long size =3D 0, addr =3D 0; + bool enable_shstk =3D false; + + if (!cpu_supports_shadow_stack()) + return -EINVAL; + + /* Reject unknown flags */ + if (status & ~PR_SHADOW_STACK_SUPPORTED_STATUS_MASK) + return -EINVAL; + + /* bcfi status is locked and further can't be modified by user */ + if (is_shstk_locked(t)) + return -EINVAL; + + enable_shstk =3D status & PR_SHADOW_STACK_ENABLE; + /* Request is to enable shadow stack and shadow stack is not enabled alre= ady */ + if (enable_shstk && !is_shstk_enabled(t)) { + /* shadow stack was allocated and enable request again + * no need to support such usecase and return EINVAL. + */ + if (is_shstk_allocated(t)) + return -EINVAL; + + size =3D calc_shstk_size(0); + addr =3D allocate_shadow_stack(0, size, 0, false); + if (IS_ERR_VALUE(addr)) + return -ENOMEM; + set_shstk_base(t, addr, size); + set_active_shstk(t, addr + size); + } + + /* + * If a request to disable shadow stack happens, let's go ahead and relea= se it + * Although, if CLONE_VFORKed child did this, then in that case we will e= nd up + * not releasing the shadow stack (because it might be needed in parent).= Although + * we will disable it for VFORKed child. And if VFORKed child tries to en= able again + * then in that case, it'll get entirely new shadow stack because followi= ng condition + * are true + * - shadow stack was not enabled for vforked child + * - shadow stack base was anyways pointing to 0 + * This shouldn't be a big issue because we want parent to have availabil= ity of shadow + * stack whenever VFORKed child releases resources via exit or exec but a= t the same + * time we want VFORKed child to break away and establish new shadow stac= k if it desires + * + */ + if (!enable_shstk) + shstk_release(t); + + set_shstk_status(t, enable_shstk); + return 0; +} + +int arch_lock_shadow_stack_status(struct task_struct *task, + unsigned long arg) +{ + /* If shtstk not supported or not enabled on task, nothing to lock here */ + if (!cpu_supports_shadow_stack() || + !is_shstk_enabled(task) || arg !=3D 0) + return -EINVAL; + + set_shstk_lock(task); + + return 0; +} --=20 2.43.0