[RFC][PATCH v2 11/21] x86/pti: Extend PTI user mappings

Alexandre Chartre
2020-11-16T14:49:28Z

Extend PTI user mappings so that more kernel entry code can be executed
with the user page-table. To do so, we need to map syscall and interrupt
entry code, per cpu offsets (__per_cpu_offset, which is used some in
entry code), the stack canary, and the PTI stack (which is defined per
task).

Signed-off-by: Alexandre Chartre <alexandre.chartre@oracle.com>
---
arch/x86/entry/entry_64.S | 2 --
arch/x86/mm/pti.c | 19 +++++++++++++++++++
kernel/fork.c | 22 ++++++++++++++++++++++
3 files changed, 41 insertions(+), 2 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 6e0b5b010e0b..458af12ed9a1 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -274,7 +274,6 @@ SYM_FUNC_END(__switch_to_asm)
* rbx: kernel thread func (NULL for user thread)
* r12: kernel thread arg
*/
-.pushsection .text, "ax"
SYM_CODE_START(ret_from_fork)
UNWIND_HINT_REGS
movq %rsp, %rdi /* pt_regs */
@@ -284,7 +283,6 @@ SYM_CODE_START(ret_from_fork)
call return_from_fork /* returns with IRQs disabled */
jmp swapgs_restore_regs_and_return_to_usermode
SYM_CODE_END(ret_from_fork)
-.popsection

.macro DEBUG_ENTRY_ASSERT_IRQS_OFF
#ifdef CONFIG_DEBUG_ENTRY
diff --git a/arch/x86/mm/pti.c b/arch/x86/mm/pti.c
index 71ca245d7b38..e4c6cb4a4840 100644
--- a/arch/x86/mm/pti.c
+++ b/arch/x86/mm/pti.c
@@ -449,6 +449,7 @@ static void __init pti_clone_percpu_page(void *addr)
*/
static void __init pti_clone_user_shared(void)
{
+ unsigned long start, end;
unsigned int cpu;

pti_clone_p4d(CPU_ENTRY_AREA_BASE);
@@ -465,7 +466,16 @@ static void __init pti_clone_user_shared(void)
*/
pti_clone_percpu_page(&per_cpu(cpu_tss_rw, cpu));

+ /*
+ * Map fixed_percpu_data to get the stack canary.
+ */
+ if (IS_ENABLED(CONFIG_STACKPROTECTOR))
+ pti_clone_percpu_page(&per_cpu(fixed_percpu_data, cpu));
}
+
+ start = (unsigned long)__per_cpu_offset;
+ end = start + sizeof(__per_cpu_offset);
+ pti_clone_init_pgtable(start, end, PTI_CLONE_PTE);
}

#else /* CONFIG_X86_64 */
@@ -505,6 +515,15 @@ static void pti_clone_entry_text(void)
pti_clone_init_pgtable((unsigned long) __entry_text_start,
(unsigned long) __entry_text_end,
PTI_CLONE_PMD);
+
+ /*
+ * Syscall and interrupt entry code (which is in the noinstr
+ * section) will be entered with the user page-table, so that
+ * code has to be mapped in.
+ */
+ pti_clone_init_pgtable((unsigned long) __noinstr_text_start,
+ (unsigned long) __noinstr_text_end,
+ PTI_CLONE_PMD);
}

/*
diff --git a/kernel/fork.c b/kernel/fork.c
index 6d266388d380..31cd77dbdba3 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -999,6 +999,25 @@ static void mm_init_uprobes_state(struct mm_struct *mm)
#endif
}

+static void mm_map_task(struct mm_struct *mm, struct task_struct *tsk)
+{
+#ifdef CONFIG_PAGE_TABLE_ISOLATION
+ unsigned long addr;
+
+ if (!tsk || !static_cpu_has(X86_FEATURE_PTI))
+ return;
+
+ /*
+ * Map the task stack after the kernel stack into the user
+ * address space, so that this stack can be used when entering
+ * syscall or interrupt from user mode.
+ */
+ BUG_ON(!task_stack_page(tsk));
+ addr = (unsigned long)task_top_of_kernel_stack(tsk);
+ pti_clone_pgtable(mm, addr, addr + KERNEL_STACK_SIZE, PTI_CLONE_PTE);
+#endif
+}
+
static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
struct user_namespace *user_ns)
{
@@ -1043,6 +1062,8 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
if (init_new_context(p, mm))
goto fail_nocontext;

+ mm_map_task(mm, p);
+
mm->user_ns = get_user_ns(user_ns);
return mm;

@@ -1404,6 +1425,7 @@ static int copy_mm(unsigned long clone_flags, struct task_struct *tsk)
vmacache_flush(tsk);

if (clone_flags & CLONE_VM) {
+ mm_map_task(oldmm, tsk);
mmget(oldmm);
mm = oldmm;
goto good_mm;
--
2.18.4