From 8b813adf6df7c4ec2c988888a5b6ef2e398aa103 Mon Sep 17 00:00:00 2001
From: Augustin Cavalier <waddlesplash@gmail.com>
Date: Sat, 12 Oct 2024 14:37:32 -0400
Subject: [PATCH] bootloader/bios_ia32: Allocate physical memory from the
 kernel separate from the bootloader.

Just start the kernel allocations after the end of the identity map
(i.e. the first 8 MB of RAM.) This way, we can avoid putting the
bootloader heap and page table memory into the kernel ranges
at all, which avoids leaking it.
---
 src/system/boot/platform/bios_ia32/mmu.cpp | 39 +++++++++++++---------
 1 file changed, 24 insertions(+), 15 deletions(-)

diff --git a/src/system/boot/platform/bios_ia32/mmu.cpp b/src/system/boot/platform/bios_ia32/mmu.cpp
index 1a580f44cc..b070665e97 100644
--- a/src/system/boot/platform/bios_ia32/mmu.cpp
+++ b/src/system/boot/platform/bios_ia32/mmu.cpp
@@ -73,7 +73,7 @@ segment_descriptor gBootGDT[BOOT_GDT_SEGMENT_COUNT];
 
 static const uint32 kDefaultPageTableFlags = 0x07;	// present, user, R/W
 static const size_t kMaxKernelSize = 0x1000000;		// 16 MB for the kernel
-static const size_t kIdentityMapEnd = (8 * 1024 * 1024);
+static const size_t kIdentityMapEnd = 0x0800000;	// 8 MB
 
 // working page directory and page table
 static uint32 *sPageDirectory = 0;
@@ -81,6 +81,7 @@ static uint32 *sPageDirectory = 0;
 #ifdef _PXE_ENV
 
 static addr_t sNextPhysicalAddress = 0x112000;
+static addr_t sNextPhysicalKernelAddress = kIdentityMapEnd;
 static addr_t sNextVirtualAddress = KERNEL_LOAD_BASE + kMaxKernelSize;
 
 static addr_t sNextPageTableAddress = 0x7d000;
@@ -90,6 +91,7 @@ static const uint32 kPageTableRegionEnd = 0x8b000;
 #else
 
 static addr_t sNextPhysicalAddress = 0x100000;
+static addr_t sNextPhysicalKernelAddress = kIdentityMapEnd;
 static addr_t sNextVirtualAddress = KERNEL_LOAD_BASE + kMaxKernelSize;
 
 static addr_t sNextPageTableAddress = 0x90000;
@@ -100,7 +102,7 @@ static const uint32 kPageTableRegionEnd = 0x9e000;
 
 
 static addr_t
-get_next_virtual_address(size_t size)
+allocate_virtual(size_t size)
 {
 	addr_t address = sNextVirtualAddress;
 	sNextVirtualAddress += size;
@@ -110,18 +112,29 @@ get_next_virtual_address(size_t size)
 
 
 static addr_t
-get_next_physical_address(size_t size)
+allocate_physical(size_t size, bool forKernel)
 {
 	uint64 base;
+	if (!forKernel) {
+		base = sNextPhysicalAddress;
+		if ((base + size) > kIdentityMapEnd) {
+			panic("Out of identity-map physical memory!");
+			return 0;
+		}
+
+		sNextPhysicalAddress += size;
+		return base;
+	}
+
 	if (!get_free_address_range(gKernelArgs.physical_allocated_range,
-			gKernelArgs.num_physical_allocated_ranges, sNextPhysicalAddress,
+			gKernelArgs.num_physical_allocated_ranges, sNextPhysicalKernelAddress,
 			size, &base)) {
 		panic("Out of physical memory!");
 		return 0;
 	}
 
 	insert_physical_allocated_range(base, size);
-	sNextPhysicalAddress = base + size;
+	sNextPhysicalKernelAddress = base + size;
 		// TODO: Can overflow theoretically.
 
 	return base;
@@ -131,14 +144,14 @@ get_next_physical_address(size_t size)
 static addr_t
 get_next_virtual_page()
 {
-	return get_next_virtual_address(B_PAGE_SIZE);
+	return allocate_virtual(B_PAGE_SIZE);
 }
 
 
 static addr_t
 get_next_physical_page()
 {
-	return get_next_physical_address(B_PAGE_SIZE);
+	return allocate_physical(B_PAGE_SIZE, true);
 }
 
 
@@ -151,7 +164,7 @@ get_next_page_table()
 
 	addr_t address = sNextPageTableAddress;
 	if (address >= kPageTableRegionEnd)
-		return (uint32 *)get_next_physical_page();
+		return (uint32 *)allocate_physical(B_PAGE_SIZE, false);
 
 	sNextPageTableAddress += B_PAGE_SIZE;
 	return (uint32 *)address;
@@ -321,7 +334,7 @@ init_page_directory(void)
 	TRACE("init_page_directory\n");
 
 	// allocate a new pgdir
-	sPageDirectory = (uint32 *)get_next_physical_page();
+	sPageDirectory = (uint32 *)allocate_physical(B_PAGE_SIZE, false);
 	gKernelArgs.arch_args.phys_pgdir = (uint32)sPageDirectory;
 
 	// clear out the pgdir
@@ -630,7 +643,7 @@ mmu_init(void)
 
 	gKernelArgs.arch_args.virtual_end = KERNEL_LOAD_BASE;
 
-	gKernelArgs.physical_allocated_range[0].start = sNextPhysicalAddress;
+	gKernelArgs.physical_allocated_range[0].start = sNextPhysicalKernelAddress;
 	gKernelArgs.physical_allocated_range[0].size = 0;
 	gKernelArgs.num_physical_allocated_ranges = 1;
 		// remember the start of the allocated physical pages
@@ -813,13 +826,10 @@ platform_free_region(void *address, size_t size)
 ssize_t
 platform_allocate_heap_region(size_t size, void **_base)
 {
-	addr_t base = get_next_physical_address(size);
+	addr_t base = allocate_physical(size, false);
 	if (base == 0)
 		return B_NO_MEMORY;
 
-	if ((base + size) > kIdentityMapEnd)
-		panic("platform_allocate_heap_region: region end is beyond identity map");
-
 	*_base = (void*)base;
 	return size;
 }
@@ -829,7 +839,6 @@ void
 platform_free_heap_region(void *_base, size_t size)
 {
 	addr_t base = (addr_t)_base;
-	remove_physical_allocated_range(base, size);
 	if (sNextPhysicalAddress == (base + size))
 		sNextPhysicalAddress -= size;