[PATCH 7/7] uprobes: deny mremap(xol_vma)

[Oleg Nesterov]

kernel/events/uprobes.c assumes that xol_area->vaddr is always correct but
a malicious application can remap its "[uprobes]" vma to another adress to
confuse the kernel. Introduce xol_mremap() to make this impossible.

With this change utask->xol_vaddr in xol_free_insn_slot() can't be invalid,
we can turn the offset check into WARN_ON_ONCE(offset >= PAGE_SIZE).

Signed-off-by: Oleg Nesterov <o...@redhat.com>
---
 kernel/events/uprobes.c | 30 +++++++++++++++++-------------
 1 file changed, 17 insertions(+), 13 deletions(-)

diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index da45d0e5bcf4..20c58b6ee1ad 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -1475,9 +1475,15 @@ static vm_fault_t xol_fault(const struct 
vm_special_mapping *sm,
        return 0;
 }
 
+static int xol_mremap(const struct vm_special_mapping *sm, struct 
vm_area_struct *new_vma)
+{
+       return -EPERM;
+}
+
 static const struct vm_special_mapping xol_mapping = {
        .name = "[uprobes]",
        .fault = xol_fault,
+       .mremap = xol_mremap,
 };
 
 /* Slot allocation for XOL */
@@ -1670,21 +1676,19 @@ static void xol_free_insn_slot(struct uprobe_task 
*utask)
 {
        struct xol_area *area = current->mm->uprobes_state.xol_area;
        unsigned long offset = utask->xol_vaddr - area->vaddr;
+       unsigned int slot_nr;
 
        utask->xol_vaddr = 0;
-       /*
-        * xol_vaddr must fit into [area->vaddr, area->vaddr + PAGE_SIZE).
-        * This check can only fail if the "[uprobes]" vma was mremap'ed.
-        */
-       if (offset < PAGE_SIZE) {
-               int slot_nr = offset / UPROBE_XOL_SLOT_BYTES;
-
-               clear_bit(slot_nr, area->bitmap);
-               atomic_dec(&area->slot_count);
-               smp_mb__after_atomic(); /* pairs with prepare_to_wait() */
-               if (waitqueue_active(&area->wq))
-                       wake_up(&area->wq);
-       }
+       /* xol_vaddr must fit into [area->vaddr, area->vaddr + PAGE_SIZE) */
+       if (WARN_ON_ONCE(offset >= PAGE_SIZE))
+               return;
+
+       slot_nr = offset / UPROBE_XOL_SLOT_BYTES;
+       clear_bit(slot_nr, area->bitmap);
+       atomic_dec(&area->slot_count);
+       smp_mb__after_atomic(); /* pairs with prepare_to_wait() */
+       if (waitqueue_active(&area->wq))
+               wake_up(&area->wq);
 }
 
 void __weak arch_uprobe_copy_ixol(struct page *page, unsigned long vaddr,
-- 
2.25.1.362.g51ebf55