x86/ldt: Unmap PTEs for the slot before freeing LDT pages

modify_ldt(2) leaves the old LDT mapped after switching over to the new
one. The old LDT gets freed and the pages can be re-used.

Leaving the mapping in place can have security implications. The mapping is
present in the userspace page tables and Meltdown-like attacks can read
these freed and possibly reused pages.

It's relatively simple to fix: unmap the old LDT and flush TLB before
freeing the old LDT memory.

This further allows to avoid flushing the TLB in map_ldt_struct() as the
slot is unmapped and flushed by unmap_ldt_struct() or has never been mapped
at all.

[ tglx: Massaged changelog and removed the needless line breaks ]

Fixes: f55f0501cbf6 ("x86/pti: Put the LDT in its own PGD if PTI is on")
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: bp@alien8.de
Cc: hpa@zytor.com
Cc: dave.hansen@linux.intel.com
Cc: luto@kernel.org
Cc: peterz@infradead.org
Cc: boris.ostrovsky@oracle.com
Cc: jgross@suse.com
Cc: bhe@redhat.com
Cc: willy@infradead.org
Cc: linux-mm@kvack.org
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20181026122856.66224-3-kirill.shutemov@linux.intel.com

diff --git a/arch/x86/kernel/ldt.c b/arch/x86/kernel/ldt.c
index ab18e08..18e4525 100644 (file)
--- a/arch/x86/kernel/ldt.c
+++ b/arch/x86/kernel/ldt.c
@@ -199,14 +199,6 @@ static void sanity_check_ldt_mapping(struct mm_struct *mm)
 /*
  * If PTI is enabled, this maps the LDT into the kernelmode and
  * usermode tables for the given mm.
- *
- * There is no corresponding unmap function.  Even if the LDT is freed, we
- * leave the PTEs around until the slot is reused or the mm is destroyed.
- * This is harmless: the LDT is always in ordinary memory, and no one will
- * access the freed slot.
- *
- * If we wanted to unmap freed LDTs, we'd also need to do a flush to make
- * it useful, and the flush would slow down modify_ldt().
  */
 static int
 map_ldt_struct(struct mm_struct *mm, struct ldt_struct *ldt, int slot)
@@ -214,8 +206,8 @@ map_ldt_struct(struct mm_struct *mm, struct ldt_struct *ldt, int slot)
        unsigned long va;
        bool is_vmalloc;
        spinlock_t *ptl;
+       int i, nr_pages;
        pgd_t *pgd;
-       int i;
 
        if (!static_cpu_has(X86_FEATURE_PTI))
                return 0;
@@ -238,7 +230,9 @@ map_ldt_struct(struct mm_struct *mm, struct ldt_struct *ldt, int slot)
 
        is_vmalloc = is_vmalloc_addr(ldt->entries);
 
-       for (i = 0; i * PAGE_SIZE < ldt->nr_entries * LDT_ENTRY_SIZE; i++) {
+       nr_pages = DIV_ROUND_UP(ldt->nr_entries * LDT_ENTRY_SIZE, PAGE_SIZE);
+
+       for (i = 0; i < nr_pages; i++) {
                unsigned long offset = i << PAGE_SHIFT;
                const void *src = (char *)ldt->entries + offset;
                unsigned long pfn;
@@ -272,13 +266,39 @@ map_ldt_struct(struct mm_struct *mm, struct ldt_struct *ldt, int slot)
        /* Propagate LDT mapping to the user page-table */
        map_ldt_struct_to_user(mm);
 
-       va = (unsigned long)ldt_slot_va(slot);
-       flush_tlb_mm_range(mm, va, va + LDT_SLOT_STRIDE, PAGE_SHIFT, false);
-
        ldt->slot = slot;
        return 0;
 }
 
+static void unmap_ldt_struct(struct mm_struct *mm, struct ldt_struct *ldt)
+{
+       unsigned long va;
+       int i, nr_pages;
+
+       if (!ldt)
+               return;
+
+       /* LDT map/unmap is only required for PTI */
+       if (!static_cpu_has(X86_FEATURE_PTI))
+               return;
+
+       nr_pages = DIV_ROUND_UP(ldt->nr_entries * LDT_ENTRY_SIZE, PAGE_SIZE);
+
+       for (i = 0; i < nr_pages; i++) {
+               unsigned long offset = i << PAGE_SHIFT;
+               spinlock_t *ptl;
+               pte_t *ptep;
+
+               va = (unsigned long)ldt_slot_va(ldt->slot) + offset;
+               ptep = get_locked_pte(mm, va, &ptl);
+               pte_clear(mm, va, ptep);
+               pte_unmap_unlock(ptep, ptl);
+       }
+
+       va = (unsigned long)ldt_slot_va(ldt->slot);
+       flush_tlb_mm_range(mm, va, va + nr_pages * PAGE_SIZE, PAGE_SHIFT, false);
+}
+
 #else /* !CONFIG_PAGE_TABLE_ISOLATION */
 
 static int
@@ -286,6 +306,10 @@ map_ldt_struct(struct mm_struct *mm, struct ldt_struct *ldt, int slot)
 {
        return 0;
 }
+
+static void unmap_ldt_struct(struct mm_struct *mm, struct ldt_struct *ldt)
+{
+}
 #endif /* CONFIG_PAGE_TABLE_ISOLATION */
 
 static void free_ldt_pgtables(struct mm_struct *mm)
@@ -524,6 +548,7 @@ static int write_ldt(void __user *ptr, unsigned long bytecount, int oldmode)
        }
 
        install_ldt(mm, new_ldt);
+       unmap_ldt_struct(mm, old_ldt);
        free_ldt_struct(old_ldt);
        error = 0;