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From: Mark Rutland <mark.rutland@arm.com>
To: linux-arm-kernel@lists.infradead.org
Cc: arnd@arndb.de, catalin.marinas@arm.com, cdall@linaro.org,
	kvmarm@lists.cs.columbia.edu, linux-arch@vger.kernel.org,
	marc.zyngier@arm.com, mark.rutland@arm.com,
	suzuki.poulose@arm.com, will.deacon@arm.com, yao.qi@arm.com,
	kernel-hardening@lists.openwall.com,
	linux-kernel@vger.kernel.org, awallis@codeaurora.org
Subject: [PATCHv2 07/12] arm64: expose user PAC bit positions via ptrace
Date: Mon, 27 Nov 2017 16:38:01 +0000
Message-ID: <20171127163806.31435-8-mark.rutland@arm.com> (raw)
In-Reply-To: <20171127163806.31435-1-mark.rutland@arm.com>

When pointer authentication is in use, data/instruction pointers have a
number of PAC bits inserted into them. The number and position of these
bits depends on the configured TCR_ELx.TxSZ and whether tagging is
enabled. ARMv8.3 allows tagging to differ for instruction and data
pointers.

For userspace debuggers to unwind the stack and/or to follow pointer
chains, they need to be able to remove the PAC bits before attempting to
use a pointer.

This patch adds a new structure with masks describing the location of
the PAC bits in userspace instruction and data pointers (i.e. those
addressable via TTBR0), which userspace can query via PTRACE_GETREGSET.
By clearing these bits from pointers, userspace can acquire the PAC-less
versions.

This new regset is exposed when the kernel is built with (user) pointer
authentication support, and the feature is enabled. Otherwise, it is
hidden.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Yao Qi <yao.qi@arm.com>
---
 arch/arm64/include/asm/pointer_auth.h |  8 ++++++++
 arch/arm64/include/uapi/asm/ptrace.h  |  7 +++++++
 arch/arm64/kernel/ptrace.c            | 38 +++++++++++++++++++++++++++++++++++
 include/uapi/linux/elf.h              |  1 +
 4 files changed, 54 insertions(+)

diff --git a/arch/arm64/include/asm/pointer_auth.h b/arch/arm64/include/asm/pointer_auth.h
index 964da0c3dc48..b08ebec4b806 100644
--- a/arch/arm64/include/asm/pointer_auth.h
+++ b/arch/arm64/include/asm/pointer_auth.h
@@ -16,9 +16,11 @@
 #ifndef __ASM_POINTER_AUTH_H
 #define __ASM_POINTER_AUTH_H
 
+#include <linux/bitops.h>
 #include <linux/random.h>
 
 #include <asm/cpufeature.h>
+#include <asm/memory.h>
 #include <asm/sysreg.h>
 
 #ifdef CONFIG_ARM64_POINTER_AUTHENTICATION
@@ -71,6 +73,12 @@ static inline void ptrauth_keys_dup(struct ptrauth_keys *old,
 	*new = *old;
 }
 
+/*
+ * The EL0 pointer bits used by a pointer authentication code.
+ * This is dependent on TBI0 being enabled, or bits 63:56 would also apply.
+ */
+#define ptrauth_pac_mask() 	GENMASK(54, VA_BITS)
+
 #define mm_ctx_ptrauth_init(ctx) \
 	ptrauth_keys_init(&(ctx)->ptrauth_keys)
 
diff --git a/arch/arm64/include/uapi/asm/ptrace.h b/arch/arm64/include/uapi/asm/ptrace.h
index 98c4ce55d9c3..4994d718771a 100644
--- a/arch/arm64/include/uapi/asm/ptrace.h
+++ b/arch/arm64/include/uapi/asm/ptrace.h
@@ -228,6 +228,13 @@ struct user_sve_header {
 		  SVE_PT_SVE_OFFSET + SVE_PT_SVE_SIZE(vq, flags)	\
 		: SVE_PT_FPSIMD_OFFSET + SVE_PT_FPSIMD_SIZE(vq, flags))
 
+/* pointer authentication masks (NT_ARM_PAC_MASK) */
+
+struct user_pac_mask {
+	__u64		data_mask;
+	__u64		insn_mask;
+};
+
 #endif /* __ASSEMBLY__ */
 
 #endif /* _UAPI__ASM_PTRACE_H */
diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
index 7c44658b316d..6901dce44c8d 100644
--- a/arch/arm64/kernel/ptrace.c
+++ b/arch/arm64/kernel/ptrace.c
@@ -44,6 +44,7 @@
 #include <asm/cpufeature.h>
 #include <asm/debug-monitors.h>
 #include <asm/pgtable.h>
+#include <asm/pointer_auth.h>
 #include <asm/stacktrace.h>
 #include <asm/syscall.h>
 #include <asm/traps.h>
@@ -951,6 +952,30 @@ static int sve_set(struct task_struct *target,
 
 #endif /* CONFIG_ARM64_SVE */
 
+#ifdef CONFIG_ARM64_POINTER_AUTHENTICATION
+static int pac_mask_get(struct task_struct *target,
+			const struct user_regset *regset,
+			unsigned int pos, unsigned int count,
+			void *kbuf, void __user *ubuf)
+{
+	/*
+	 * The PAC bits can differ across data and instruction pointers
+	 * depending on TCR_EL1.TBID*, which we may make use of in future, so
+	 * we expose separate masks.
+	 */
+	unsigned long mask = ptrauth_pac_mask();
+	struct user_pac_mask uregs = {
+		.data_mask = mask,
+		.insn_mask = mask,
+	};
+
+	if (!cpus_have_cap(ARM64_HAS_ADDRESS_AUTH))
+		return -EINVAL;
+
+	return user_regset_copyout(&pos, &count, &kbuf, &ubuf, &uregs, 0, -1);
+}
+#endif /* CONFIG_ARM64_POINTER_AUTHENTICATION */
+
 enum aarch64_regset {
 	REGSET_GPR,
 	REGSET_FPR,
@@ -963,6 +988,9 @@ enum aarch64_regset {
 #ifdef CONFIG_ARM64_SVE
 	REGSET_SVE,
 #endif
+#ifdef CONFIG_ARM64_POINTER_AUTHENTICATION
+	REGSET_PAC_MASK,
+#endif
 };
 
 static const struct user_regset aarch64_regsets[] = {
@@ -1032,6 +1060,16 @@ static const struct user_regset aarch64_regsets[] = {
 		.get_size = sve_get_size,
 	},
 #endif
+#ifdef CONFIG_ARM64_POINTER_AUTHENTICATION
+	[REGSET_PAC_MASK] = {
+		.core_note_type = NT_ARM_PAC_MASK,
+		.n = sizeof(struct user_pac_mask) / sizeof(u64),
+		.size = sizeof(u64),
+		.align = sizeof(u64),
+		.get = pac_mask_get,
+		/* this cannot be set dynamically */
+	},
+#endif
 };
 
 static const struct user_regset_view user_aarch64_view = {
diff --git a/include/uapi/linux/elf.h b/include/uapi/linux/elf.h
index bb6836986200..4ca58005e04a 100644
--- a/include/uapi/linux/elf.h
+++ b/include/uapi/linux/elf.h
@@ -419,6 +419,7 @@ typedef struct elf64_shdr {
 #define NT_ARM_HW_WATCH	0x403		/* ARM hardware watchpoint registers */
 #define NT_ARM_SYSTEM_CALL	0x404	/* ARM system call number */
 #define NT_ARM_SVE	0x405		/* ARM Scalable Vector Extension registers */
+#define NT_ARM_PAC_MASK		0x406	/* ARM pointer authentication code masks */
 #define NT_METAG_CBUF	0x500		/* Metag catch buffer registers */
 #define NT_METAG_RPIPE	0x501		/* Metag read pipeline state */
 #define NT_METAG_TLS	0x502		/* Metag TLS pointer */
-- 
2.11.0

  parent reply index

Thread overview: 26+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-11-27 16:37 [PATCHv2 00/12] ARMv8.3 pointer authentication userspace support Mark Rutland
2017-11-27 16:37 ` [PATCHv2 01/12] asm-generic: mm_hooks: allow hooks to be overridden individually Mark Rutland
2017-11-27 16:37 ` [PATCHv2 02/12] arm64: add pointer authentication register bits Mark Rutland
2017-11-27 16:37 ` [PATCHv2 03/12] arm64/cpufeature: add ARMv8.3 id_aa64isar1 bits Mark Rutland
2017-11-27 16:37 ` [PATCHv2 04/12] arm64/cpufeature: detect pointer authentication Mark Rutland
2017-11-27 16:37 ` [PATCHv2 05/12] arm64: Don't trap host pointer auth use to EL2 Mark Rutland
2018-02-06 12:39   ` Christoffer Dall
2018-02-12 16:00     ` Mark Rutland
2017-11-27 16:38 ` [PATCHv2 06/12] arm64: add basic pointer authentication support Mark Rutland
2018-05-22 19:06   ` Adam Wallis
2017-11-27 16:38 ` Mark Rutland [this message]
2017-11-27 16:38 ` [PATCHv2 08/12] arm64: perf: strip PAC when unwinding userspace Mark Rutland
2017-11-27 16:38 ` [PATCHv2 09/12] arm64/kvm: preserve host HCR_EL2 value Mark Rutland
2018-02-06 12:39   ` Christoffer Dall
2018-04-09 14:57     ` Mark Rutland
2018-04-09 19:03       ` Christoffer Dall
2017-11-27 16:38 ` [PATCHv2 10/12] arm64/kvm: context-switch ptrauth registers Mark Rutland
2018-02-06 12:38   ` Christoffer Dall
2018-03-09 14:28     ` Mark Rutland
2018-04-09 12:58       ` Christoffer Dall
2018-04-09 14:37         ` Mark Rutland
2017-11-27 16:38 ` [PATCHv2 11/12] arm64: enable pointer authentication Mark Rutland
2017-11-27 16:38 ` [PATCHv2 12/12] arm64: docs: document " Mark Rutland
2017-11-28 15:07   ` Andrew Jones
2017-12-04 12:39     ` Mark Rutland
2017-12-04 12:49       ` Andrew Jones

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