lguest: Map switcher text R/O

Pavel noted that lguest maps the switcher code executable and
read-write.  This is a bad idea for any kernel text, but
particularly for text mapped at a fixed address.

Create two vmas, one for the text (PAGE_KERNEL_RX) and another
for the stacks (PAGE_KERNEL).  Use VM_NO_GUARD to map them
adjacent (as expected by the rest of the code).

Reported-by: Pavel Machek <pavel@ucw.cz>
Tested-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/arch/x86/include/asm/lguest.h b/arch/x86/include/asm/lguest.h
index 3bbc07a57a31e9d68f232fd56651be6788491f95..73d0c9b92087c4eb64f834c21df64889529f72fd 100644 (file)
--- a/arch/x86/include/asm/lguest.h
+++ b/arch/x86/include/asm/lguest.h
@@ -12,7 +12,9 @@
 #define GUEST_PL 1
 
 /* Page for Switcher text itself, then two pages per cpu */
-#define TOTAL_SWITCHER_PAGES (1 + 2 * nr_cpu_ids)
+#define SWITCHER_TEXT_PAGES (1)
+#define SWITCHER_STACK_PAGES (2 * nr_cpu_ids)
+#define TOTAL_SWITCHER_PAGES (SWITCHER_TEXT_PAGES + SWITCHER_STACK_PAGES)
 
 /* Where we map the Switcher, in both Host and Guest. */
 extern unsigned long switcher_addr;

diff --git a/drivers/lguest/core.c b/drivers/lguest/core.c
index 312ffd3d00177ca5a5e21393377c760e9bdec91e..9e385b38debfd1c9d6a0b374ee8a44098f577e0d 100644 (file)
--- a/drivers/lguest/core.c
+++ b/drivers/lguest/core.c
@@ -22,7 +22,8 @@
 
 unsigned long switcher_addr;
 struct page **lg_switcher_pages;
-static struct vm_struct *switcher_vma;
+static struct vm_struct *switcher_text_vma;
+static struct vm_struct *switcher_stacks_vma;
 
 /* This One Big lock protects all inter-guest data structures. */
 DEFINE_MUTEX(lguest_lock);
@@ -82,55 +83,81 @@ static __init int map_switcher(void)
                }
        }
 
+       /*
+        * Copy in the compiled-in Switcher code (from x86/switcher_32.S).
+        * It goes in the first page, which we map in momentarily.
+        */
+       memcpy(kmap(lg_switcher_pages[0]), start_switcher_text,
+              end_switcher_text - start_switcher_text);
+       kunmap(lg_switcher_pages[0]);
+
        /*
         * We place the Switcher underneath the fixmap area, which is the
         * highest virtual address we can get.  This is important, since we
         * tell the Guest it can't access this memory, so we want its ceiling
         * as high as possible.
         */
-       switcher_addr = FIXADDR_START - (TOTAL_SWITCHER_PAGES+1)*PAGE_SIZE;
+       switcher_addr = FIXADDR_START - TOTAL_SWITCHER_PAGES*PAGE_SIZE;
 
        /*
-        * Now we reserve the "virtual memory area" we want.  We might
-        * not get it in theory, but in practice it's worked so far.
-        * The end address needs +1 because __get_vm_area allocates an
-        * extra guard page, so we need space for that.
+        * Now we reserve the "virtual memory area"s we want.  We might
+        * not get them in theory, but in practice it's worked so far.
+        *
+        * We want the switcher text to be read-only and executable, and
+        * the stacks to be read-write and non-executable.
         */
-       switcher_vma = __get_vm_area(TOTAL_SWITCHER_PAGES * PAGE_SIZE,
-                                    VM_ALLOC, switcher_addr, switcher_addr
-                                    + (TOTAL_SWITCHER_PAGES+1) * PAGE_SIZE);
-       if (!switcher_vma) {
+       switcher_text_vma = __get_vm_area(PAGE_SIZE, VM_ALLOC|VM_NO_GUARD,
+                                         switcher_addr,
+                                         switcher_addr + PAGE_SIZE);
+
+       if (!switcher_text_vma) {
                err = -ENOMEM;
                printk("lguest: could not map switcher pages high\n");
                goto free_pages;
        }
 
+       switcher_stacks_vma = __get_vm_area(SWITCHER_STACK_PAGES * PAGE_SIZE,
+                                           VM_ALLOC|VM_NO_GUARD,
+                                           switcher_addr + PAGE_SIZE,
+                                           switcher_addr + TOTAL_SWITCHER_PAGES * PAGE_SIZE);
+       if (!switcher_stacks_vma) {
+               err = -ENOMEM;
+               printk("lguest: could not map switcher pages high\n");
+               goto free_text_vma;
+       }
+
        /*
         * This code actually sets up the pages we've allocated to appear at
         * switcher_addr.  map_vm_area() takes the vma we allocated above, the
-        * kind of pages we're mapping (kernel pages), and a pointer to our
-        * array of struct pages.
+        * kind of pages we're mapping (kernel text pages and kernel writable
+        * pages respectively), and a pointer to our array of struct pages.
         */
-       err = map_vm_area(switcher_vma, PAGE_KERNEL_EXEC, lg_switcher_pages);
+       err = map_vm_area(switcher_text_vma, PAGE_KERNEL_RX, lg_switcher_pages);
+       if (err) {
+               printk("lguest: text map_vm_area failed: %i\n", err);
+               goto free_vmas;
+       }
+
+       err = map_vm_area(switcher_stacks_vma, PAGE_KERNEL,
+                         lg_switcher_pages + SWITCHER_TEXT_PAGES);
        if (err) {
-               printk("lguest: map_vm_area failed: %i\n", err);
-               goto free_vma;
+               printk("lguest: stacks map_vm_area failed: %i\n", err);
+               goto free_vmas;
        }
 
        /*
         * Now the Switcher is mapped at the right address, we can't fail!
-        * Copy in the compiled-in Switcher code (from x86/switcher_32.S).
         */
-       memcpy(switcher_vma->addr, start_switcher_text,
-              end_switcher_text - start_switcher_text);
-
        printk(KERN_INFO "lguest: mapped switcher at %p\n",
-              switcher_vma->addr);
+              switcher_text_vma->addr);
        /* And we succeeded... */
        return 0;
 
-free_vma:
-       vunmap(switcher_vma->addr);
+free_vmas:
+       /* Undoes map_vm_area and __get_vm_area */
+       vunmap(switcher_stacks_vma->addr);
+free_text_vma:
+       vunmap(switcher_text_vma->addr);
 free_pages:
        i = TOTAL_SWITCHER_PAGES;
 free_some_pages:
@@ -148,7 +175,8 @@ static void unmap_switcher(void)
        unsigned int i;
 
        /* vunmap() undoes *both* map_vm_area() and __get_vm_area(). */
-       vunmap(switcher_vma->addr);
+       vunmap(switcher_text_vma->addr);
+       vunmap(switcher_stacks_vma->addr);
        /* Now we just need to free the pages we copied the switcher into */
        for (i = 0; i < TOTAL_SWITCHER_PAGES; i++)
                __free_pages(lg_switcher_pages[i], 0);