process: Ensure that ELF doesn't map important kernel memory

2 files changed, 12 insertions, 13 deletions

diff --git a/src/lib/paging.zig b/src/lib/paging.zig
index 7915b9b..dfe44cc 100644
--- a/src/lib/paging.zig
+++ b/src/lib/paging.zig
@@ -30,10 +30,11 @@ inline fn heapSize() usize {
 }
 
 pub const page_size: usize = 0x1000; // 4096 bytes
-pub const log2_page_size: u8 = @intCast(std.math.log2(page_size));
 
-pub var next_mmio_vaddr: usize = 0xff000000;
+var num_pages: usize = undefined;
+var next_mmio_vaddr: usize = 0xff000000;
 
+pub var alloc_start: usize = undefined;
 pub var kmem: *Table = undefined;
 
 pub const Error = error{
@@ -480,13 +481,17 @@ pub const Table = struct {
 };
 
 pub fn init() !void {
-    const num_pages = heapSize() / page_size;
+    num_pages = heapSize() / page_size;
     const pages: [*]Page = @ptrCast(heap_start);
 
     for (0..num_pages) |i| {
         pages[i].flags = Page.Flags.clear;
     }
 
+    // Start allocating beyond page descriptors.
+    const descriptors_end = @intFromPtr(heap_start) + num_pages * @sizeOf(Page);
+    alloc_start = std.mem.alignForward(usize, descriptors_end, page_size);
+
     kmem = @ptrCast(try zeroedAlloc(1));
     try kmem.mapKernel();
 }
@@ -495,11 +500,6 @@ pub fn init() !void {
 pub fn alloc(n: usize) ![]align(page_size) u8 {
     if (n <= 0) return Error.ZeroSize;
 
-    const num_pages = heapSize() / page_size;
-    // Start allocating beyond page descriptors.
-    const pages = @intFromPtr(heap_start) + num_pages * @sizeOf(Page);
-    const alloc_start = std.mem.alignForwardLog2(pages, log2_page_size);
-
     const descriptors: [*]Page = @ptrCast(heap_start);
 
     // Iterate over potential starting points.
@@ -545,11 +545,6 @@ pub fn free(memory: anytype) void {
     const bytes_len = bytes.len + if (Slice.sentinel != null) @sizeOf(Slice.child) else 0;
     if (bytes_len == 0) return;
 
-    const num_pages = heapSize() / page_size;
-    // Start allocating beyond page descriptors.
-    const pages = @intFromPtr(heap_start) + num_pages * @sizeOf(Page);
-    const alloc_start = std.mem.alignForwardLog2(pages, log2_page_size);
-
     // Restore the address to the page descriptor flags from the address of its contents
     // by restoring the descriptor number and indexing the descriptor table
     // at the start of the heap using it.
diff --git a/src/lib/process.zig b/src/lib/process.zig
index 265d9b3..b59d255 100644
--- a/src/lib/process.zig
+++ b/src/lib/process.zig
@@ -31,6 +31,7 @@ pub const ExeError = error{
     BadBitLen,
     NotStaticExe,
     LengthOutOfBounds,
+    ReservedMemMapping,
     BranchPerms,
     WritableCode,
 };
@@ -275,6 +276,9 @@ pub fn create(allocator: Allocator, elf_buf: []align(@alignOf(elf.Elf64_Ehdr)) c
                 .dirty = @bitCast(phdr.p_flags & elf.PF_W != 0),
             };
 
+            if (vaddr >= @intFromPtr(paging.text_start) and vaddr < paging.alloc_start) {
+                return ExeError.ReservedMemMapping;
+            }
             if (!@bitCast(flags.read) and !@bitCast(flags.write) and !@bitCast(flags.exec)) {
                 return ExeError.BranchPerms;
             }