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Subject[PATCHv2 2/2] x86/ldt: Unmap PTEs for the slot before freeing LDT pages
Date
modify_ldt(2) leaves old LDT mapped after we switch over to the new one.
Memory for 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 userspace copy of page tables and Meltdown-like attack can
read these freed and possibly reused pages.

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

We can now avoid flushing TLB on map_ldt_struct() as the slot is
unmapped and flushed by unmap_ldt_struct() (or never mapped in
the first place). The overhead of the change should be negligible.
It shouldn't be a particularly hot path anyway.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Fixes: f55f0501cbf6 ("x86/pti: Put the LDT in its own PGD if PTI is on")
---
arch/x86/kernel/ldt.c | 59 ++++++++++++++++++++++++++++---------------
1 file changed, 38 insertions(+), 21 deletions(-)

diff --git a/arch/x86/kernel/ldt.c b/arch/x86/kernel/ldt.c
index ab18e0884dc6..5dc8ed202fa8 100644
--- 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 +206,7 @@ map_ldt_struct(struct mm_struct *mm, struct ldt_struct *ldt, int slot)
unsigned long va;
bool is_vmalloc;
spinlock_t *ptl;
- pgd_t *pgd;
- int i;
+ int i, nr_pages;

if (!static_cpu_has(X86_FEATURE_PTI))
return 0;
@@ -229,16 +220,10 @@ map_ldt_struct(struct mm_struct *mm, struct ldt_struct *ldt, int slot)
/* Check if the current mappings are sane */
sanity_check_ldt_mapping(mm);

- /*
- * Did we already have the top level entry allocated? We can't
- * use pgd_none() for this because it doens't do anything on
- * 4-level page table kernels.
- */
- pgd = pgd_offset(mm, LDT_BASE_ADDR);
-
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 +257,39 @@ 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;
+ pte_t *ptep;
+ spinlock_t *ptl;
+
+ 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, 0, false);
+}
+
#else /* !CONFIG_PAGE_TABLE_ISOLATION */

static int
@@ -286,6 +297,11 @@ 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 +540,7 @@ 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;

--
2.19.1
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