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Subject[RFC 9/9] x86/mm: Implement PR_SET/GET_TAGGED_ADDR_CTRL with LAM
Date
Provide prctl() interface to enabled LAM for user addresses. Depending
how many tag bits requested it may result in enabling LAM_U57 or
LAM_U48.

If LAM_U48 is enabled, the process is no longer able to use full address
space on 5-level paging machine and gets limited to 47-bit VA.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
arch/x86/include/asm/processor.h | 10 +++
arch/x86/kernel/process_64.c | 145 +++++++++++++++++++++++++++++++
2 files changed, 155 insertions(+)

diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index 82a08b585818..49fac2cc4329 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -810,6 +810,16 @@ extern void start_thread(struct pt_regs *regs, unsigned long new_ip,
extern int get_tsc_mode(unsigned long adr);
extern int set_tsc_mode(unsigned int val);

+#ifdef CONFIG_X86_64
+long set_tagged_addr_ctrl(unsigned long flags,
+ int __user *nr_bits, int __user *offset);
+long get_tagged_addr_ctrl(int __user *nr_bits, int __user *offset);
+#define SET_TAGGED_ADDR_CTRL(flags, nr_bits, offset) \
+ set_tagged_addr_ctrl(flags, nr_bits, offset)
+#define GET_TAGGED_ADDR_CTRL(nr_bits, offset) \
+ get_tagged_addr_ctrl(nr_bits, offset)
+#endif
+
DECLARE_PER_CPU(u64, msr_misc_features_shadow);

#ifdef CONFIG_CPU_SUP_AMD
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index df342bedea88..99b87f0e1bc7 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -837,3 +837,148 @@ unsigned long KSTK_ESP(struct task_struct *task)
{
return task_pt_regs(task)->sp;
}
+
+/*
+ * Control the relaxed ABI allowing tagged user addresses into the kernel.
+ */
+static unsigned int tagged_addr_disabled;
+
+static bool lam_u48_allowed(void)
+{
+ struct mm_struct *mm = current->mm;
+
+ if (!full_va_allowed(mm))
+ return true;
+
+ return find_vma(mm, DEFAULT_MAP_WINDOW) == NULL;
+}
+
+#define LAM_U48_BITS 15
+#define LAM_U57_BITS 6
+
+long set_tagged_addr_ctrl(unsigned long flags,
+ int __user *nr_bits, int __user *offset)
+{
+ int val;
+
+ if (in_32bit_syscall())
+ return -EINVAL;
+ if (flags & ~PR_TAGGED_ADDR_ENABLE)
+ return -EINVAL;
+ if (!boot_cpu_has(X86_FEATURE_LAM))
+ return -ENOTSUPP;
+
+ /* Disable LAM */
+ if (!(flags & PR_TAGGED_ADDR_ENABLE)) {
+ clear_thread_flag(TIF_LAM_U48);
+ clear_thread_flag(TIF_LAM_U57);
+
+ /* Update CR3 */
+ switch_mm(current->mm, current->mm, current);
+
+ return 0;
+ }
+
+ /*
+ * nr_bits == NULL || offset == NULL assumes ARM TBI (nr_bits == 8,
+ * offset == 56). LAM cannot provide this.
+ */
+ if (!nr_bits || !offset)
+ return -EINVAL;
+
+ /*
+ * Do not allow the enabling of the tagged address ABI if globally
+ * disabled via sysctl abi.tagged_addr_disabled.
+ */
+ if (tagged_addr_disabled)
+ return -EINVAL;
+
+ if (get_user(val, nr_bits))
+ return -EFAULT;
+ if (val > LAM_U48_BITS || val < 1)
+ return -EINVAL;
+ if (val > LAM_U57_BITS && !lam_u48_allowed())
+ return -EINVAL;
+
+ val = val > LAM_U57_BITS ? LAM_U48_BITS : LAM_U57_BITS;
+ if (put_user(val, nr_bits) || put_user(63 - val, offset))
+ return -EFAULT;
+
+ if (val == LAM_U57_BITS) {
+ clear_thread_flag(TIF_LAM_U48);
+ set_thread_flag(TIF_LAM_U57);
+ if (current->mm->context.lam == LAM_NONE)
+ current->mm->context.lam = LAM_U57;
+ } else {
+ clear_thread_flag(TIF_LAM_U57);
+ set_thread_flag(TIF_LAM_U48);
+
+ /*
+ * Do not allow to create a mapping above 47 bit.
+ *
+ * It's one way road: once a thread of the process enabled
+ * LAM_U48, no thread can ever create mapping above 47 bit.
+ * Even the LAM got disabled later.
+ */
+ current->mm->context.lam = LAM_U48;
+ }
+
+ /* Update CR3 */
+ switch_mm(current->mm, current->mm, current);
+
+ return 0;
+}
+
+long get_tagged_addr_ctrl(int __user *nr_bits, int __user *offset)
+{
+ if (in_32bit_syscall())
+ return -EINVAL;
+
+ if (test_thread_flag(TIF_LAM_U57)) {
+ if (nr_bits && put_user(LAM_U57_BITS, nr_bits))
+ return -EFAULT;
+ if (offset && put_user(63 - LAM_U57_BITS, offset))
+ return -EFAULT;
+ } else if (test_thread_flag(TIF_LAM_U48)) {
+ if (nr_bits && put_user(LAM_U48_BITS, nr_bits))
+ return -EFAULT;
+ if (offset && put_user(63 - LAM_U48_BITS, offset))
+ return -EFAULT;
+ } else {
+ int max_bits = lam_u48_allowed() ? LAM_U48_BITS : LAM_U57_BITS;
+
+ /* Report maximum tag size */
+ if (nr_bits && put_user(max_bits, nr_bits))
+ return -EFAULT;
+ return 0;
+ }
+
+ return PR_TAGGED_ADDR_ENABLE;
+}
+
+/*
+ * 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 struct ctl_table tagged_addr_sysctl_table[] = {
+ {
+ .procname = "tagged_addr_disabled",
+ .mode = 0644,
+ .data = &tagged_addr_disabled,
+ .maxlen = sizeof(int),
+ .proc_handler = proc_dointvec_minmax,
+ .extra1 = SYSCTL_ZERO,
+ .extra2 = SYSCTL_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);
--
2.26.2
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