Subject: Re: [PATCH v16 02/16] arm64: untag user pointers in access_ok and __uaccess_mask_ptr

From: Catalin Marinas <[email-masked]>
Date: Tue, 11 Jun 2019 15:57:20 +0100

On Mon, Jun 10, 2019 at 06:53:27PM +0100, Catalin Marinas wrote:
> On Mon, Jun 03, 2019 at 06:55:04PM +0200, Andrey Konovalov wrote:
> > diff --git a/arch/arm64/include/asm/uaccess.h b/arch/arm64/include/asm/uaccess.h
> > index e5d5f31c6d36..9164ecb5feca 100644
> > --- a/arch/arm64/include/asm/uaccess.h
> > +++ b/arch/arm64/include/asm/uaccess.h
> > @@ -94,7 +94,7 @@ static inline unsigned long __range_ok(const void __user *addr, unsigned long si
> >  	return ret;
> >  }
> >  
> > -#define access_ok(addr, size)	__range_ok(addr, size)
> > +#define access_ok(addr, size)	__range_ok(untagged_addr(addr), size)
> 
> I'm going to propose an opt-in method here (RFC for now). We can't have
> a check in untagged_addr() since this is already used throughout the
> kernel for both user and kernel addresses (khwasan) but we can add one
> in __range_ok(). The same prctl() option will be used for controlling
> the precise/imprecise mode of MTE later on. We can use a TIF_ flag here
> assuming that this will be called early on and any cloned thread will
> inherit this.

Updated patch, inlining it below. Once we agreed on the approach, I
think Andrey can insert in in this series, probably after patch 2. The
differences from the one I posted yesterday:

- renamed PR_* macros together with get/set variants and the possibility
  to disable the relaxed ABI

- sysctl option - /proc/sys/abi/tagged_addr to disable the ABI globally
  (just the prctl() opt-in, tasks already using it won't be affected)

And, of course, it needs more testing.

---------8<----------------
>From 7c624777a4e545522dec1b34e60f0229cb2bd59f Mon Sep 17 00:00:00 2001
From: Catalin Marinas <[email-masked]>
Date: Tue, 11 Jun 2019 13:03:38 +0100
Subject: [PATCH] arm64: Introduce prctl() options to control the tagged user
 addresses ABI

It is not desirable to relax the ABI to allow tagged user addresses into
the kernel indiscriminately. This patch introduces a prctl() interface
for enabling or disabling the tagged ABI with a global sysctl control
for preventing applications from enabling the relaxed ABI (meant for
testing user-space prctl() return error checking without reconfiguring
the kernel). The ABI properties are inherited by threads of the same
application and fork()'ed children but cleared on execve().

The PR_SET_TAGGED_ADDR_CTRL will be expanded in the future to handle
MTE-specific settings like imprecise vs precise exceptions.

Signed-off-by: Catalin Marinas <[email-masked]>
---
 arch/arm64/include/asm/processor.h   |  6 +++
 arch/arm64/include/asm/thread_info.h |  1 +
 arch/arm64/include/asm/uaccess.h     |  5 ++-
 arch/arm64/kernel/process.c          | 67 ++++++++++++++++++++++++++++
 include/uapi/linux/prctl.h           |  5 +++
 kernel/sys.c                         | 16 +++++++
 6 files changed, 99 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/processor.h b/arch/arm64/include/asm/processor.h
index fcd0e691b1ea..fee457456aa8 100644
--- a/arch/arm64/include/asm/processor.h
+++ b/arch/arm64/include/asm/processor.h
@@ -307,6 +307,12 @@ extern void __init minsigstksz_setup(void);
 /* PR_PAC_RESET_KEYS prctl */
 #define PAC_RESET_KEYS(tsk, arg)	ptrauth_prctl_reset_keys(tsk, arg)
 
+/* PR_TAGGED_ADDR prctl */
+long set_tagged_addr_ctrl(unsigned long arg);
+long get_tagged_addr_ctrl(void);
+#define SET_TAGGED_ADDR_CTRL(arg)	set_tagged_addr_ctrl(arg)
+#define GET_TAGGED_ADDR_CTRL()		get_tagged_addr_ctrl()
+
 /*
  * For CONFIG_GCC_PLUGIN_STACKLEAK
  *
diff --git a/arch/arm64/include/asm/thread_info.h b/arch/arm64/include/asm/thread_info.h
index c285d1ce7186..7263d4c973ce 100644
--- a/arch/arm64/include/asm/thread_info.h
+++ b/arch/arm64/include/asm/thread_info.h
@@ -101,6 +101,7 @@ void arch_release_task_struct(struct task_struct *tsk);
 #define TIF_SVE			23	/* Scalable Vector Extension in use */
 #define TIF_SVE_VL_INHERIT	24	/* Inherit sve_vl_onexec across exec */
 #define TIF_SSBD		25	/* Wants SSB mitigation */
+#define TIF_TAGGED_ADDR		26
 
 #define _TIF_SIGPENDING		(1 << TIF_SIGPENDING)
 #define _TIF_NEED_RESCHED	(1 << TIF_NEED_RESCHED)
diff --git a/arch/arm64/include/asm/uaccess.h b/arch/arm64/include/asm/uaccess.h
index 9164ecb5feca..995b9ea11a89 100644
--- a/arch/arm64/include/asm/uaccess.h
+++ b/arch/arm64/include/asm/uaccess.h
@@ -73,6 +73,9 @@ static inline unsigned long __range_ok(const void __user *addr, unsigned long si
 {
 	unsigned long ret, limit = current_thread_info()->addr_limit;
 
+	if (test_thread_flag(TIF_TAGGED_ADDR))
+		addr = untagged_addr(addr);
+
 	__chk_user_ptr(addr);
 	asm volatile(
 	// A + B <= C + 1 for all A,B,C, in four easy steps:
@@ -94,7 +97,7 @@ static inline unsigned long __range_ok(const void __user *addr, unsigned long si
 	return ret;
 }
 
-#define access_ok(addr, size)	__range_ok(untagged_addr(addr), size)
+#define access_ok(addr, size)	__range_ok(addr, size)
 #define user_addr_max			get_fs
 
 #define _ASM_EXTABLE(from, to)						\
diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
index 3767fb21a5b8..69d0be1fc708 100644
--- a/arch/arm64/kernel/process.c
+++ b/arch/arm64/kernel/process.c
@@ -30,6 +30,7 @@
 #include <linux/kernel.h>
 #include <linux/mm.h>
 #include <linux/stddef.h>
+#include <linux/sysctl.h>
 #include <linux/unistd.h>
 #include <linux/user.h>
 #include <linux/delay.h>
@@ -323,6 +324,7 @@ void flush_thread(void)
 	fpsimd_flush_thread();
 	tls_thread_flush();
 	flush_ptrace_hw_breakpoint(current);
+	clear_thread_flag(TIF_TAGGED_ADDR);
 }
 
 void release_thread(struct task_struct *dead_task)
@@ -552,3 +554,68 @@ void arch_setup_new_exec(void)
 
 	ptrauth_thread_init_user(current);
 }
+
+/*
+ * Control the relaxed ABI allowing tagged user addresses into the kernel.
+ */
+static unsigned int tagged_addr_prctl_allowed = 1;
+
+long set_tagged_addr_ctrl(unsigned long arg)
+{
+	if (!tagged_addr_prctl_allowed)
+		return -EINVAL;
+	if (is_compat_task())
+		return -EINVAL;
+	if (arg & ~PR_TAGGED_ADDR_ENABLE)
+		return -EINVAL;
+
+	if (arg & PR_TAGGED_ADDR_ENABLE)
+		set_thread_flag(TIF_TAGGED_ADDR);
+	else
+		clear_thread_flag(TIF_TAGGED_ADDR);
+
+	return 0;
+}
+
+long get_tagged_addr_ctrl(void)
+{
+	if (!tagged_addr_prctl_allowed)
+		return -EINVAL;
+	if (is_compat_task())
+		return -EINVAL;
+
+	if (test_thread_flag(TIF_TAGGED_ADDR))
+		return PR_TAGGED_ADDR_ENABLE;
+
+	return 0;
+}
+
+/*
+ * Global sysctl to disable the tagged user addresses support. This control
+ * only prevents the tagged address ABI enabling via prctl() and does not
+ * disable it for tasks that already opted in to the relaxed ABI.
+ */
+static int zero;
+static int one = 1;
+
+static struct ctl_table tagged_addr_sysctl_table[] = {
+	{
+		.procname	= "tagged_addr",
+		.mode		= 0644,
+		.data		= &tagged_addr_prctl_allowed,
+		.maxlen		= sizeof(int),
+		.proc_handler	= proc_dointvec_minmax,
+		.extra1		= &zero,
+		.extra2		= &one,
+	},
+	{ }
+};
+
+static int __init tagged_addr_init(void)
+{
+	if (!register_sysctl("abi", tagged_addr_sysctl_table))
+		return -EINVAL;
+	return 0;
+}
+
+core_initcall(tagged_addr_init);
diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
index 094bb03b9cc2..2e927b3e9d6c 100644
--- a/include/uapi/linux/prctl.h
+++ b/include/uapi/linux/prctl.h
@@ -229,4 +229,9 @@ struct prctl_mm_map {
 # define PR_PAC_APDBKEY			(1UL << 3)
 # define PR_PAC_APGAKEY			(1UL << 4)
 
+/* Tagged user address controls for arm64 */
+#define PR_SET_TAGGED_ADDR_CTRL		55
+#define PR_GET_TAGGED_ADDR_CTRL		56
+# define PR_TAGGED_ADDR_ENABLE		(1UL << 0)
+
 #endif /* _LINUX_PRCTL_H */
diff --git a/kernel/sys.c b/kernel/sys.c
index 2969304c29fe..ec48396b4943 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -124,6 +124,12 @@
 #ifndef PAC_RESET_KEYS
 # define PAC_RESET_KEYS(a, b)	(-EINVAL)
 #endif
+#ifndef SET_TAGGED_ADDR_CTRL
+# define SET_TAGGED_ADDR_CTRL(a)	(-EINVAL)
+#endif
+#ifndef GET_TAGGED_ADDR_CTRL
+# define GET_TAGGED_ADDR_CTRL()		(-EINVAL)
+#endif
 
 /*
  * this is where the system-wide overflow UID and GID are defined, for
@@ -2492,6 +2498,16 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
 			return -EINVAL;
 		error = PAC_RESET_KEYS(me, arg2);
 		break;
+	case PR_SET_TAGGED_ADDR_CTRL:
+		if (arg3 || arg4 || arg5)
+			return -EINVAL;
+		error = SET_TAGGED_ADDR_CTRL(arg2);
+		break;
+	case PR_GET_TAGGED_ADDR_CTRL:
+		if (arg2 || arg3 || arg4 || arg5)
+			return -EINVAL;
+		error = GET_TAGGED_ADDR_CTRL();
+		break;
 	default:
 		error = -EINVAL;
 		break;

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