Kernel: Disallow mapping anonymous memory as executable

This adds another layer of defense against introducing new code into a
running process. The only permitted way of doing so is by mmapping an
open file with PROT_READ | PROT_EXEC.

This does make any future JIT implementations slightly more complicated
but I think it's a worthwhile trade-off at this point. :^)

Kernel/Syscalls/mmap.cpp

@@ -84,7 +84,7 @@ static bool should_make_executable_exception_for_dynamic_loader(bool make_readab
  return true;
 }
 
-static bool validate_mmap_prot(int prot, bool map_stack, const Region* region = nullptr, bool* is_making_executable_exception_for_dynamic_loader = nullptr)
+static bool validate_mmap_prot(int prot, bool map_stack, bool map_anonymous, const Region* region = nullptr, bool* is_making_executable_exception_for_dynamic_loader = nullptr)
 {
  if (is_making_executable_exception_for_dynamic_loader)
  *is_making_executable_exception_for_dynamic_loader = false;
@@ -93,6 +93,9 @@ static bool validate_mmap_prot(int prot, bool map_stack, const Region* region =
  bool make_writable = prot & PROT_WRITE;
  bool make_executable = prot & PROT_EXEC;
 
+ if (map_anonymous && make_executable)
+ return false;
+
  if (make_writable && make_executable)
  return false;
 
@@ -197,7 +200,7 @@ void* Process::sys$mmap(Userspace<const Syscall::SC_mmap_params*> user_params)
  if (map_fixed && map_randomized)
  return (void*)-EINVAL;
 
- if (!validate_mmap_prot(prot, map_stack))
+ if (!validate_mmap_prot(prot, map_anonymous, map_stack))
  return (void*)-EINVAL;
 
  if (map_stack && (!map_private || !map_anonymous))
@@ -283,7 +286,7 @@ int Process::sys$mprotect(void* addr, size_t size, int prot)
  if (!whole_region->is_mmap())
  return -EPERM;
  bool is_making_executable_exception_for_dynamic_loader = false;
- if (!validate_mmap_prot(prot, whole_region->is_stack(), whole_region, &is_making_executable_exception_for_dynamic_loader))
+ if (!validate_mmap_prot(prot, whole_region->is_stack(), whole_region->vmobject().is_anonymous(), whole_region, &is_making_executable_exception_for_dynamic_loader))
  return -EINVAL;
  if (whole_region->access() == prot_to_region_access_flags(prot))
  return 0;
@@ -307,7 +310,7 @@ int Process::sys$mprotect(void* addr, size_t size, int prot)
  if (!old_region->is_mmap())
  return -EPERM;
  bool is_making_executable_exception_for_dynamic_loader = false;
- if (!validate_mmap_prot(prot, old_region->is_stack(), old_region, &is_making_executable_exception_for_dynamic_loader))
+ if (!validate_mmap_prot(prot, old_region->is_stack(), old_region->vmobject().is_anonymous(), old_region, &is_making_executable_exception_for_dynamic_loader))
  return -EINVAL;
  if (old_region->access() == prot_to_region_access_flags(prot))
  return 0;