lguest: example launcher to use guard pages, drop PROT_EXEC, fix limit logic

PROT_EXEC seems to be completely unnecessary (as the lguest binary
never executes there), and will allow it to work with SELinux (and
more importantly, PaX :-) as they can/do forbid writable and
executable mappings.

Also, map PROT_NONE guard pages at start and end of guest memory for extra
paranoia.

I changed the length check to addr + size > guest_limit because >= is wrong
(addr of 0, size of getpagesize() with a guest_limit of getpagesize() would
false positive).

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

1 files changed, 15 insertions, 8 deletions

diff --git a/Documentation/lguest/lguest.c b/Documentation/lguest/lguest.c
index f64b85bcd6d4..d9da7e148538 100644
--- a/Documentation/lguest/lguest.c
+++ b/Documentation/lguest/lguest.c
@@ -301,20 +301,27 @@ static void *map_zeroed_pages(unsigned int num)
 
         /*
          * We use a private mapping (ie. if we write to the page, it will be
-         * copied).
+         * copied). We allocate an extra two pages PROT_NONE to act as guard
+         * pages against read/write attempts that exceed allocated space.
          */
-        addr = mmap(NULL, getpagesize() * num,
-                    PROT_READ|PROT_WRITE|PROT_EXEC, MAP_PRIVATE, fd, 0);
+        addr = mmap(NULL, getpagesize() * (num+2),
+                    PROT_NONE, MAP_PRIVATE, fd, 0);
+
         if (addr == MAP_FAILED)
                 err(1, "Mmapping %u pages of /dev/zero", num);
 
+        if (mprotect(addr + getpagesize(), getpagesize() * num,
+                     PROT_READ|PROT_WRITE) == -1)
+                err(1, "mprotect rw %u pages failed", num);
+
         /*
          * One neat mmap feature is that you can close the fd, and it
          * stays mapped.
          */
         close(fd);
 
-        return addr;
+        /* Return address after PROT_NONE page */
+        return addr + getpagesize();
 }
 
 /* Get some more pages for a device. */
@@ -346,7 +353,7 @@ static void map_at(int fd, void *addr, unsigned long offset, unsigned long len)
          * done to it.  This allows us to share untouched memory between
          * Guests.
          */
-        if (mmap(addr, len, PROT_READ|PROT_WRITE|PROT_EXEC,
+        if (mmap(addr, len, PROT_READ|PROT_WRITE,
                  MAP_FIXED|MAP_PRIVATE, fd, offset) != MAP_FAILED)
                 return;
 
@@ -576,10 +583,10 @@ static void *_check_pointer(unsigned long addr, unsigned int size,
                             unsigned int line)
 {
         /*
-         * We have to separately check addr and addr+size, because size could
-         * be huge and addr + size might wrap around.
+         * Check if the requested address and size exceeds the allocated memory,
+         * or addr + size wraps around.
          */
-        if (addr >= guest_limit || addr + size >= guest_limit)
+        if ((addr + size) > guest_limit || (addr + size) < addr)
                 errx(1, "%s:%i: Invalid address %#lx", __FILE__, line, addr);
         /*
          * We return a pointer for the caller's convenience, now we know it's