linux-kernel - [PATCH 5/6] x86/shstk: don't create the shadow stack for PF_USER

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Message-ID: <20250814101435.GA17362@redhat.com>
Date: Thu, 14 Aug 2025 12:14:35 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>,
	Jens Axboe <axboe@...nel.dk>, Peter Zijlstra <peterz@...radead.org>,
	Rick Edgecombe <rick.p.edgecombe@...el.com>,
	Sohil Mehta <sohil.mehta@...el.com>,
	Thomas Gleixner <tglx@...utronix.de>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org
Subject: [PATCH 5/6] x86/shstk: don't create the shadow stack for
 PF_USER_WORKERs

If a features_enabled(ARCH_SHSTK_SHSTK) userspace thread creates a
PF_USER_WORKER thread, shstk_alloc_thread_stack() allocates the shadow
stack for no reason, the new (kernel) thread will never return to usermode.

Plus the current code doesn't even look correct, in this case fpu_clone()
won't call update_fpu_shstk().

Add the new "bool minimal = !!args->fn" argument (which matches that of
fpu_clone()) to shstk_alloc_thread_stack() and change it to do
reset_thread_features(tsk) if "minimal" is true.

With this patch ssp_get() -> ssp_active(target) should never return true
if target->flags & PF_USER_WORKER.

Signed-off-by: Oleg Nesterov <oleg@...hat.com>
---
 arch/x86/include/asm/shstk.h |  4 ++--
 arch/x86/kernel/process.c    |  2 +-
 arch/x86/kernel/shstk.c      | 11 ++++++++++-
 3 files changed, 13 insertions(+), 4 deletions(-)

diff --git a/arch/x86/include/asm/shstk.h b/arch/x86/include/asm/shstk.h
index 92d449cc352a..dfb2aeebc25f 100644
--- a/arch/x86/include/asm/shstk.h
+++ b/arch/x86/include/asm/shstk.h
@@ -17,7 +17,7 @@ struct thread_shstk {
 long shstk_prctl(struct task_struct *task, int option, unsigned long arg2);
 void reset_thread_features(struct task_struct *task);
 unsigned long shstk_alloc_thread_stack(struct task_struct *p, unsigned long clone_flags,
-				       unsigned long stack_size);
+				       bool minimal, unsigned long stack_size);
 void shstk_free(struct task_struct *p);
 int setup_signal_shadow_stack(struct ksignal *ksig);
 int restore_signal_shadow_stack(void);
@@ -28,7 +28,7 @@ static inline long shstk_prctl(struct task_struct *task, int option,
 			       unsigned long arg2) { return -EINVAL; }
 static inline void reset_thread_features(struct task_struct *task) {}
 static inline unsigned long shstk_alloc_thread_stack(struct task_struct *p,
-						     unsigned long clone_flags,
+						     unsigned long clone_flags, bool minimal,
 						     unsigned long stack_size) { return 0; }
 static inline void shstk_free(struct task_struct *p) {}
 static inline int setup_signal_shadow_stack(struct ksignal *ksig) { return 0; }
diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 1b7960cf6eb0..e932e0e53972 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -209,7 +209,7 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
 	 * is disabled, new_ssp will remain 0, and fpu_clone() will know not to
 	 * update it.
 	 */
-	new_ssp = shstk_alloc_thread_stack(p, clone_flags, args->stack_size);
+	new_ssp = shstk_alloc_thread_stack(p, clone_flags, args->fn, args->stack_size);
 	if (IS_ERR_VALUE(new_ssp))
 		return PTR_ERR((void *)new_ssp);
 
diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c
index e6d3b1371b11..3da22c6f5874 100644
--- a/arch/x86/kernel/shstk.c
+++ b/arch/x86/kernel/shstk.c
@@ -192,11 +192,20 @@ void reset_thread_features(struct task_struct *tsk)
 }
 
 unsigned long shstk_alloc_thread_stack(struct task_struct *tsk, unsigned long clone_flags,
-				       unsigned long stack_size)
+				       bool minimal, unsigned long stack_size)
 {
 	struct thread_shstk *shstk = &tsk->thread.shstk;
 	unsigned long addr, size;
 
+	/*
+	 * Kernel threads cloned from userspace thread never return to
+	 * usermode.
+	 */
+	if (minimal) {
+		reset_thread_features(tsk);
+		return 0;
+	}
+
 	/*
 	 * If shadow stack is not enabled on the new thread, skip any
 	 * switch to a new shadow stack.
-- 
2.25.1.362.g51ebf55