[PATCH 4.14 023/146] x86/pti: Map the vsyscall page if needed

Greg Kroah-Hartman
2018-01-01T15:10:04Z

From: Andy Lutomirski <luto@kernel.org>

commit 85900ea51577e31b186e523c8f4e068c79ecc7d3 upstream.

Make VSYSCALLs work fully in PTI mode by mapping them properly to the user
space visible page tables.

[ tglx: Hide unused functions (Patch by Arnd Bergmann) ]

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: David Laight <David.Laight@aculab.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
arch/x86/entry/vsyscall/vsyscall_64.c | 6 +--
arch/x86/include/asm/vsyscall.h | 1
arch/x86/mm/pti.c | 65 ++++++++++++++++++++++++++++++++++
3 files changed, 69 insertions(+), 3 deletions(-)

--- a/arch/x86/entry/vsyscall/vsyscall_64.c
+++ b/arch/x86/entry/vsyscall/vsyscall_64.c
@@ -344,14 +344,14 @@ int in_gate_area_no_mm(unsigned long add
* vsyscalls but leave the page not present. If so, we skip calling
* this.
*/
-static void __init set_vsyscall_pgtable_user_bits(void)
+void __init set_vsyscall_pgtable_user_bits(pgd_t *root)
{
pgd_t *pgd;
p4d_t *p4d;
pud_t *pud;
pmd_t *pmd;

- pgd = pgd_offset_k(VSYSCALL_ADDR);
+ pgd = pgd_offset_pgd(root, VSYSCALL_ADDR);
set_pgd(pgd, __pgd(pgd_val(*pgd) | _PAGE_USER));
p4d = p4d_offset(pgd, VSYSCALL_ADDR);
#if CONFIG_PGTABLE_LEVELS >= 5
@@ -373,7 +373,7 @@ void __init map_vsyscall(void)
vsyscall_mode == NATIVE
? PAGE_KERNEL_VSYSCALL
: PAGE_KERNEL_VVAR);
- set_vsyscall_pgtable_user_bits();
+ set_vsyscall_pgtable_user_bits(swapper_pg_dir);
}

BUILD_BUG_ON((unsigned long)__fix_to_virt(VSYSCALL_PAGE) !=
--- a/arch/x86/include/asm/vsyscall.h
+++ b/arch/x86/include/asm/vsyscall.h
@@ -7,6 +7,7 @@

#ifdef CONFIG_X86_VSYSCALL_EMULATION
extern void map_vsyscall(void);
+extern void set_vsyscall_pgtable_user_bits(pgd_t *root);

/*
* Called on instruction fetch fault in vsyscall page.
--- a/arch/x86/mm/pti.c
+++ b/arch/x86/mm/pti.c
@@ -38,6 +38,7 @@

#include <asm/cpufeature.h>
#include <asm/hypervisor.h>
+#include <asm/vsyscall.h>
#include <asm/cmdline.h>
#include <asm/pti.h>
#include <asm/pgtable.h>
@@ -223,6 +224,69 @@ static pmd_t *pti_user_pagetable_walk_pm
return pmd_offset(pud, address);
}

+#ifdef CONFIG_X86_VSYSCALL_EMULATION
+/*
+ * Walk the shadow copy of the page tables (optionally) trying to allocate
+ * page table pages on the way down. Does not support large pages.
+ *
+ * Note: this is only used when mapping *new* kernel data into the
+ * user/shadow page tables. It is never used for userspace data.
+ *
+ * Returns a pointer to a PTE on success, or NULL on failure.
+ */
+static __init pte_t *pti_user_pagetable_walk_pte(unsigned long address)
+{
+ gfp_t gfp = (GFP_KERNEL | __GFP_NOTRACK | __GFP_ZERO);
+ pmd_t *pmd = pti_user_pagetable_walk_pmd(address);
+ pte_t *pte;
+
+ /* We can't do anything sensible if we hit a large mapping. */
+ if (pmd_large(*pmd)) {
+ WARN_ON(1);
+ return NULL;
+ }
+
+ if (pmd_none(*pmd)) {
+ unsigned long new_pte_page = __get_free_page(gfp);
+ if (!new_pte_page)
+ return NULL;
+
+ if (pmd_none(*pmd)) {
+ set_pmd(pmd, __pmd(_KERNPG_TABLE | __pa(new_pte_page)));
+ new_pte_page = 0;
+ }
+ if (new_pte_page)
+ free_page(new_pte_page);
+ }
+
+ pte = pte_offset_kernel(pmd, address);
+ if (pte_flags(*pte) & _PAGE_USER) {
+ WARN_ONCE(1, "attempt to walk to user pte\n");
+ return NULL;
+ }
+ return pte;
+}
+
+static void __init pti_setup_vsyscall(void)
+{
+ pte_t *pte, *target_pte;
+ unsigned int level;
+
+ pte = lookup_address(VSYSCALL_ADDR, &level);
+ if (!pte || WARN_ON(level != PG_LEVEL_4K) || pte_none(*pte))
+ return;
+
+ target_pte = pti_user_pagetable_walk_pte(VSYSCALL_ADDR);
+ if (WARN_ON(!target_pte))
+ return;
+
+ *target_pte = *pte;
+ set_vsyscall_pgtable_user_bits(kernel_to_user_pgdp(swapper_pg_dir));
+}
+#else
+static void __init pti_setup_vsyscall(void) { }
+#endif
+
static void __init
pti_clone_pmds(unsigned long start, unsigned long end, pmdval_t clear)
{
@@ -319,4 +383,5 @@ void __init pti_init(void)
pti_clone_user_shared();
pti_clone_entry_text();
pti_setup_espfix64();
+ pti_setup_vsyscall();
}