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Kernel: Add a random offset to the base of the per-process VM allocator

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This is not ASLR, but it does de-trivialize exploiting the ELF loader
which would previously always parse executables at 0x01001000 in every
single exec(). I've taken advantage of this multiple times in my own
toy exploits and it's starting to feel cheesy. :^)
This commit is contained in:
Andreas Kling 2020-01-17 23:05:37 +01:00
parent 536c0ff3ee
commit a850a89c1b
3 changed files with 25 additions and 8 deletions
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16
Kernel/VM/PageDirectory.cpp
View file

@ -1,9 +1,11 @@
#include <Kernel/Process.h>
#include <Kernel/Random.h>
#include <Kernel/Thread.h>
#include <Kernel/VM/MemoryManager.h>
#include <Kernel/VM/PageDirectory.h>
static const u32 userspace_range_base = 0x01000000;
static const u32 userspace_range_base = 0x00800000;
static const u32 userspace_range_ceiling = 0xbe000000;
static const u32 kernelspace_range_base = 0xc0800000;
static HashMap<u32, PageDirectory*>& cr3_map()
@ -26,8 +28,9 @@ extern "C" PageDirectoryEntry boot_pd0[1024];
extern "C" PageDirectoryEntry boot_pd3[1024];
PageDirectory::PageDirectory()
: m_range_allocator(VirtualAddress(0xc0c00000), 0x3f000000)
{
m_range_allocator.initialize_with_range(VirtualAddress(0xc0800000), 0x3f000000);
// Adopt the page tables already set up by boot.S
PhysicalAddress boot_pdpt_paddr(virtual_to_low_physical((u32)boot_pdpt));
PhysicalAddress boot_pd0_paddr(virtual_to_low_physical((u32)boot_pd0));
@ -42,8 +45,15 @@ PageDirectory::PageDirectory()
PageDirectory::PageDirectory(Process& process, const RangeAllocator* parent_range_allocator)
: m_process(&process)
, m_range_allocator(parent_range_allocator ? RangeAllocator(*parent_range_allocator) : RangeAllocator(VirtualAddress(userspace_range_base), kernelspace_range_base - userspace_range_base))
{
if (parent_range_allocator) {
m_range_allocator.initialize_from_parent(*parent_range_allocator);
} else {
size_t random_offset = (get_good_random<u32>() % 32 * MB) & PAGE_MASK;
u32 base = userspace_range_base + random_offset;
m_range_allocator.initialize_with_range(VirtualAddress(base), userspace_range_ceiling - base);
}
// Set up a userspace page directory
m_directory_table = MM.allocate_user_physical_page();
m_directory_pages[0] = MM.allocate_user_physical_page();