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From:   Benjamin Gray <bgray@linux.ibm.com>
To:     linuxppc-dev@lists.ozlabs.org
Cc:     ajd@linux.ibm.com, ruscur@russell.cc, linux-kernel@vger.kernel.org,
        linux-hardening@vger.kernel.org, cmr@bluescreens.de,
        Benjamin Gray <bgray@linux.ibm.com>
Subject: [RFC PATCH 06/13] powerpc/dexcr: Add prctl implementation
Date:   Mon, 28 Nov 2022 13:44:51 +1100
Message-Id: <20221128024458.46121-7-bgray@linux.ibm.com>
In-Reply-To: <20221128024458.46121-1-bgray@linux.ibm.com>
References: <20221128024458.46121-1-bgray@linux.ibm.com>
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Adds an initial prctl interface implementation. Unprivileged processes
can query the current prctl setting, including whether an aspect is
implemented by the hardware or is permitted to be modified by a setter
prctl. Editable aspects can be changed by a CAP_SYS_ADMIN privileged
process.

The prctl setting represents what the process itself has requested, and
does not account for any overrides. Either the kernel or a hypervisor
may enforce a different setting for an aspect.

Userspace can access a readonly view of the current DEXCR via SPR 812,
and a readonly view of the aspects enforced by the hypervisor via
SPR 455. A bitwise OR of these two SPRs will give the effective
DEXCR aspect state of the process.

Signed-off-by: Benjamin Gray <bgray@linux.ibm.com>
---
 arch/powerpc/include/asm/processor.h |  13 +++
 arch/powerpc/kernel/dexcr.c          | 133 ++++++++++++++++++++++++++-
 arch/powerpc/kernel/process.c        |   6 ++
 3 files changed, 151 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/include/asm/processor.h b/arch/powerpc/include/asm/processor.h
index 2381217c95dc..4c995258f668 100644
--- a/arch/powerpc/include/asm/processor.h
+++ b/arch/powerpc/include/asm/processor.h
@@ -265,6 +265,9 @@ struct thread_struct {
 	unsigned long   sier2;
 	unsigned long   sier3;
 	unsigned long	hashkeyr;
+	unsigned int	dexcr_override;
+	unsigned int	dexcr_mask;
+	unsigned int	dexcr_forced;
 
 #endif
 };
@@ -338,6 +341,16 @@ extern int set_endian(struct task_struct *tsk, unsigned int val);
 extern int get_unalign_ctl(struct task_struct *tsk, unsigned long adr);
 extern int set_unalign_ctl(struct task_struct *tsk, unsigned int val);
 
+#ifdef CONFIG_PPC_BOOK3S_64
+
+#define PPC_GET_DEXCR_ASPECT(tsk, asp) dexcr_prctl_get((tsk), (asp))
+#define PPC_SET_DEXCR_ASPECT(tsk, asp, val) dexcr_prctl_set((tsk), (asp), (val))
+
+int dexcr_prctl_get(struct task_struct *tsk, unsigned long asp);
+int dexcr_prctl_set(struct task_struct *tsk, unsigned long asp, unsigned long val);
+
+#endif
+
 extern void load_fp_state(struct thread_fp_state *fp);
 extern void store_fp_state(struct thread_fp_state *fp);
 extern void load_vr_state(struct thread_vr_state *vr);
diff --git a/arch/powerpc/kernel/dexcr.c b/arch/powerpc/kernel/dexcr.c
index 11515e67afac..9290beed722a 100644
--- a/arch/powerpc/kernel/dexcr.c
+++ b/arch/powerpc/kernel/dexcr.c
@@ -1,5 +1,8 @@
 #include <linux/cache.h>
+#include <linux/capability.h>
 #include <linux/init.h>
+#include <linux/prctl.h>
+#include <linux/sched.h>
 
 #include <asm/cpu_has_feature.h>
 #include <asm/cputable.h>
@@ -11,6 +14,10 @@
 
 #define DEFAULT_DEXCR	0
 
+/* Allow process configuration of these by default */
+#define DEXCR_PRCTL_EDITABLE (DEXCR_PRO_SBHE | DEXCR_PRO_IBRTPD | \
+			      DEXCR_PRO_SRAPD | DEXCR_PRO_NPHIE)
+
 static int __init dexcr_init(void)
 {
 	if (!early_cpu_has_feature(CPU_FTR_ARCH_31))
@@ -43,5 +50,129 @@ bool is_hashchk_trap(struct pt_regs const *regs)
 
 unsigned long get_thread_dexcr(struct thread_struct const *t)
 {
-	return DEFAULT_DEXCR;
+	unsigned long dexcr = DEFAULT_DEXCR;
+
+	/* Apply prctl overrides */
+	dexcr = (dexcr & ~t->dexcr_mask) | t->dexcr_override;
+
+	return dexcr;
+}
+
+static void update_dexcr_on_cpu(void *info)
+{
+	mtspr(SPRN_DEXCR, get_thread_dexcr(&current->thread));
+}
+
+static int dexcr_aspect_get(struct task_struct *task, unsigned int aspect)
+{
+	int ret = 0;
+
+	if (aspect & DEXCR_PRCTL_EDITABLE)
+		ret |= PR_PPC_DEXCR_PRCTL;
+
+	if (aspect & task->thread.dexcr_mask) {
+		if (aspect & task->thread.dexcr_override) {
+			if (aspect & task->thread.dexcr_forced)
+				ret |= PR_PPC_DEXCR_FORCE_SET_ASPECT;
+			else
+				ret |= PR_PPC_DEXCR_SET_ASPECT;
+		} else {
+			ret |= PR_PPC_DEXCR_CLEAR_ASPECT;
+		}
+	}
+
+	return ret;
+}
+
+int dexcr_prctl_get(struct task_struct *task, unsigned long which)
+{
+	switch (which) {
+	case PR_PPC_DEXCR_SBHE:
+		if (!cpu_has_feature(CPU_FTR_DEXCR_SBHE))
+			return -ENODEV;
+		return dexcr_aspect_get(task, DEXCR_PRO_SBHE);
+	case PR_PPC_DEXCR_IBRTPD:
+		if (!cpu_has_feature(CPU_FTR_DEXCR_IBRTPD))
+			return -ENODEV;
+		return dexcr_aspect_get(task, DEXCR_PRO_IBRTPD);
+	case PR_PPC_DEXCR_SRAPD:
+		if (!cpu_has_feature(CPU_FTR_DEXCR_SRAPD))
+			return -ENODEV;
+		return dexcr_aspect_get(task, DEXCR_PRO_SRAPD);
+	case PR_PPC_DEXCR_NPHIE:
+		if (!cpu_has_feature(CPU_FTR_DEXCR_NPHIE))
+			return -ENODEV;
+		return dexcr_aspect_get(task, DEXCR_PRO_NPHIE);
+	default:
+		return -ENODEV;
+	}
+}
+
+static int dexcr_aspect_set(struct task_struct *task, unsigned int aspect, unsigned long ctrl)
+{
+	if (!(aspect & DEXCR_PRCTL_EDITABLE))
+		return -ENXIO;  /* Aspect is not allowed to be changed by prctl */
+
+	if (aspect & task->thread.dexcr_forced)
+		return -EPERM;  /* Aspect has been forced to current state */
+
+	switch (ctrl) {
+	case PR_PPC_DEXCR_SET_ASPECT:
+		task->thread.dexcr_mask |= aspect;
+		task->thread.dexcr_override |= aspect;
+		break;
+	case PR_PPC_DEXCR_FORCE_SET_ASPECT:
+		task->thread.dexcr_mask |= aspect;
+		task->thread.dexcr_override |= aspect;
+		task->thread.dexcr_forced |= aspect;
+		break;
+	case PR_PPC_DEXCR_CLEAR_ASPECT:
+		task->thread.dexcr_mask |= aspect;
+		task->thread.dexcr_override &= ~aspect;
+		break;
+	default:
+		return -ERANGE;
+	}
+
+	return 0;
+}
+
+int dexcr_prctl_set(struct task_struct *task, unsigned long which, unsigned long ctrl)
+{
+	int err = 0;
+
+	if (!capable(CAP_SYS_ADMIN))
+		return -EPERM;
+
+	switch (which) {
+	case PR_PPC_DEXCR_SBHE:
+		if (!cpu_has_feature(CPU_FTR_DEXCR_SBHE))
+			return -ENODEV;
+		err = dexcr_aspect_set(task, DEXCR_PRO_SBHE, ctrl);
+		break;
+	case PR_PPC_DEXCR_IBRTPD:
+		if (!cpu_has_feature(CPU_FTR_DEXCR_IBRTPD))
+			return -ENODEV;
+		err = dexcr_aspect_set(task, DEXCR_PRO_IBRTPD, ctrl);
+		break;
+	case PR_PPC_DEXCR_SRAPD:
+		if (!cpu_has_feature(CPU_FTR_DEXCR_SRAPD))
+			return -ENODEV;
+		err = dexcr_aspect_set(task, DEXCR_PRO_SRAPD, ctrl);
+		break;
+	case PR_PPC_DEXCR_NPHIE:
+		if (!cpu_has_feature(CPU_FTR_DEXCR_NPHIE))
+			return -ENODEV;
+		err = dexcr_aspect_set(task, DEXCR_PRO_NPHIE, ctrl);
+		break;
+	default:
+		return -ENODEV;
+	}
+
+	if (err)
+		return err;
+
+	update_dexcr_on_cpu(NULL);
+
+	return 0;
 }
diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
index 4d7b0c7641d0..a280842750f9 100644
--- a/arch/powerpc/kernel/process.c
+++ b/arch/powerpc/kernel/process.c
@@ -1825,6 +1825,12 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
 #ifdef CONFIG_PPC_BOOK3S_64
 	if (cpu_has_feature(CPU_FTR_DEXCR_NPHIE))
 		p->thread.hashkeyr = current->thread.hashkeyr;
+
+	if (cpu_has_feature(CPU_FTR_ARCH_31)) {
+		p->thread.dexcr_override = current->thread.dexcr_override;
+		p->thread.dexcr_mask = current->thread.dexcr_mask;
+		p->thread.dexcr_forced = current->thread.dexcr_forced;
+	}
 #endif
 	/*
 	 * Run with the current AMR value of the kernel
-- 
2.38.1