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SubjectRe: [PATCH v19 02/15] arm64: Introduce prctl() options to control the tagged user addresses ABI
On Tue, Jul 23, 2019 at 07:58:39PM +0200, Andrey Konovalov wrote:
> From: Catalin Marinas <catalin.marinas@arm.com>
>
> 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(). A Kconfig
> option allows the overall disabling of the relaxed ABI.
>
> The PR_SET_TAGGED_ADDR_CTRL will be expanded in the future to handle
> MTE-specific settings like imprecise vs precise exceptions.
>
> Reviewed-by: Kees Cook <keescook@chromium.org>
> Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
> Signed-off-by: Andrey Konovalov <andreyknvl@google.com>

Following several discussions on the list and in private, I'm proposing
the update below. I can send it as a patch on top of the current series
since Will has already queued this.

---------------8<-------------------------------------
From 1b3f57ab0c2c51f8b31c19fb34d270e1f3ee57fe Mon Sep 17 00:00:00 2001
From: Catalin Marinas <catalin.marinas@arm.com>
Date: Fri, 9 Aug 2019 15:09:15 +0100
Subject: [PATCH] fixup! arm64: Introduce prctl() options to control the
tagged user addresses ABI

Rename abi.tagged_addr sysctl control to abi.tagged_addr_disabled,
defaulting to 0. Only prevent prctl(PR_TAGGED_ADDR_ENABLE)from being
called when abi.tagged_addr_disabled==1.

Force unused arg* of the new prctl() to 0.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
---
arch/arm64/kernel/process.c | 17 ++++++++++-------
kernel/sys.c | 4 ++++
2 files changed, 14 insertions(+), 7 deletions(-)

diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
index 76b7c55026aa..03689c0beb34 100644
--- a/arch/arm64/kernel/process.c
+++ b/arch/arm64/kernel/process.c
@@ -579,17 +579,22 @@ void arch_setup_new_exec(void)
/*
* Control the relaxed ABI allowing tagged user addresses into the kernel.
*/
-static unsigned int tagged_addr_prctl_allowed = 1;
+static unsigned int tagged_addr_disabled;

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;

+ /*
+ * Do not allow the enabling of the tagged address ABI if globally
+ * disabled via sysctl abi.tagged_addr_disabled.
+ */
+ if (arg & PR_TAGGED_ADDR_ENABLE && tagged_addr_disabled)
+ return -EINVAL;
+
update_thread_flag(TIF_TAGGED_ADDR, arg & PR_TAGGED_ADDR_ENABLE);

return 0;
@@ -597,8 +602,6 @@ long set_tagged_addr_ctrl(unsigned long arg)

long get_tagged_addr_ctrl(void)
{
- if (!tagged_addr_prctl_allowed)
- return -EINVAL;
if (is_compat_task())
return -EINVAL;

@@ -618,9 +621,9 @@ static int one = 1;

static struct ctl_table tagged_addr_sysctl_table[] = {
{
- .procname = "tagged_addr",
+ .procname = "tagged_addr_disabled",
.mode = 0644,
- .data = &tagged_addr_prctl_allowed,
+ .data = &tagged_addr_disabled,
.maxlen = sizeof(int),
.proc_handler = proc_dointvec_minmax,
.extra1 = &zero,
diff --git a/kernel/sys.c b/kernel/sys.c
index c6c4d5358bd3..ec48396b4943 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -2499,9 +2499,13 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
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:
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