Merge patch series "riscv: Introduce KASLR"

Alexandre Ghiti <alexghiti@rivosinc.com> says:

The following KASLR implementation allows to randomize the kernel mapping:

- virtually: we expect the bootloader to provide a seed in the device-tree
- physically: only implemented in the EFI stub, it relies on the firmware to
  provide a seed using EFI_RNG_PROTOCOL. arm64 has a similar implementation
  hence the patch 3 factorizes KASLR related functions for riscv to take
  advantage.

The new virtual kernel location is limited by the early page table that only
has one PUD and with the PMD alignment constraint, the kernel can only take
< 512 positions.

* b4-shazam-merge:
  riscv: libstub: Implement KASLR by using generic functions
  libstub: Fix compilation warning for rv32
  arm64: libstub: Move KASLR handling functions to kaslr.c
  riscv: Dump out kernel offset information on panic
  riscv: Introduce virtual kernel mapping KASLR

Link: https://lore.kernel.org/r/20230722123850.634544-1-alexghiti@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

1 files changed, 35 insertions, 1 deletions

diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
index 943c18d6ef4d..0798bd861dcb 100644
--- a/arch/riscv/mm/init.c
+++ b/arch/riscv/mm/init.c
@@ -1014,11 +1014,45 @@ static void __init pt_ops_set_late(void)
 #endif
 }
 
+#ifdef CONFIG_RANDOMIZE_BASE
+extern bool __init __pi_set_nokaslr_from_cmdline(uintptr_t dtb_pa);
+extern u64 __init __pi_get_kaslr_seed(uintptr_t dtb_pa);
+
+static int __init print_nokaslr(char *p)
+{
+	pr_info("Disabled KASLR");
+	return 0;
+}
+early_param("nokaslr", print_nokaslr);
+
+unsigned long kaslr_offset(void)
+{
+	return kernel_map.virt_offset;
+}
+#endif
+
 asmlinkage void __init setup_vm(uintptr_t dtb_pa)
 {
 	pmd_t __maybe_unused fix_bmap_spmd, fix_bmap_epmd;
 
-	kernel_map.virt_addr = KERNEL_LINK_ADDR;
+#ifdef CONFIG_RANDOMIZE_BASE
+	if (!__pi_set_nokaslr_from_cmdline(dtb_pa)) {
+		u64 kaslr_seed = __pi_get_kaslr_seed(dtb_pa);
+		u32 kernel_size = (uintptr_t)(&_end) - (uintptr_t)(&_start);
+		u32 nr_pos;
+
+		/*
+		 * Compute the number of positions available: we are limited
+		 * by the early page table that only has one PUD and we must
+		 * be aligned on PMD_SIZE.
+		 */
+		nr_pos = (PUD_SIZE - kernel_size) / PMD_SIZE;
+
+		kernel_map.virt_offset = (kaslr_seed % nr_pos) * PMD_SIZE;
+	}
+#endif
+
+	kernel_map.virt_addr = KERNEL_LINK_ADDR + kernel_map.virt_offset;
 	kernel_map.page_offset = _AC(CONFIG_PAGE_OFFSET, UL);
 
 #ifdef CONFIG_XIP_KERNEL