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Subject[PATCH v17 03/15] arm64: Introduce prctl() options to control the tagged user addresses ABI
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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().

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 <catalin.marinas@arm.com>
---
arch/arm64/include/asm/processor.h | 6 +++
arch/arm64/include/asm/thread_info.h | 1 +
arch/arm64/include/asm/uaccess.h | 3 +-
arch/arm64/kernel/process.c | 67 ++++++++++++++++++++++++++++
include/uapi/linux/prctl.h | 5 +++
kernel/sys.c | 16 +++++++
6 files changed, 97 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 f1d032be628a..354a31d2b737 100644
--- a/arch/arm64/include/asm/thread_info.h
+++ b/arch/arm64/include/asm/thread_info.h
@@ -99,6 +99,7 @@ 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 /* Allow tagged user addresses */

#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 df729afca0ba..995b9ea11a89 100644
--- a/arch/arm64/include/asm/uaccess.h
+++ b/arch/arm64/include/asm/uaccess.h
@@ -73,7 +73,8 @@ static inline unsigned long __range_ok(const void __user *addr, unsigned long si
{
unsigned long ret, limit = current_thread_info()->addr_limit;

- addr = untagged_addr(addr);
+ if (test_thread_flag(TIF_TAGGED_ADDR))
+ addr = untagged_addr(addr);

__chk_user_ptr(addr);
asm volatile(
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;
--
2.22.0.rc2.383.gf4fbbf30c2-goog
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