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Diffstat (limited to 'na/arch/riscv/config.cc')
| -rw-r--r-- | na/arch/riscv/config.cc | 59 |
1 files changed, 59 insertions, 0 deletions
diff --git a/na/arch/riscv/config.cc b/na/arch/riscv/config.cc index 139d932..9b4cc6e 100644 --- a/na/arch/riscv/config.cc +++ b/na/arch/riscv/config.cc @@ -1,10 +1,69 @@ /* SPDX-License-Identifier: GPL-2.0-or-later */ /* Copyright (C) 2026 Richard Qin */ +/* + * RISC-V Architecture Configuration — runtime state and KASLR + * + * Provides the runtime (post-KASLR) values of the higher-half + * virtual address bases. kaslr_init() is called once during + * early boot, before vmm_init(), to slide these bases by a + * random, 1GB-aligned offset. + * + * KASLR slide budget: + * The default Direct Map starts at 0xFFFFFFC0_0000_0000 (VPN[2]=256). + * The default Vmemmap starts at 0xFFFFFFD0_0000_0000 (VPN[2]=320). + * A 1GB superpage occupies one VPN[2] slot. We allow a slide of + * 0..63 slots (0..63 GB) downward, which keeps both regions inside + * the upper canonical half and avoids colliding with the kernel + * text region (linked at VPN[2] = 258 + 2 = ~0xFFFFFFC0_8020_0000). + * + * In practice, with 128MB RAM the kernel+direct map only touches + * VPN[2] 258-259, so a slide of up to 63GB is safe. + */ + #include <arch/config.h> +#include <lume/random.h> +#include <lume/kprintf.h> namespace arch { +/* Runtime bases — default to the compile-time constants. */ uint64 g_direct_map_base = kDirectMapBaseDefault; +uint64 g_vmemmap_base = kVmemmapBaseDefault; + +void kaslr_init() +{ +#ifdef CONFIG_KASLR + /* + * Generate a random 1GB-aligned slide. + * + * Strategy (matching Linux's approach): + * 1. Pull 64 random bits from the early PRNG. + * 2. Reduce to a slot number in [0, kMaxSlots). + * 3. Convert to a byte offset (slot * 1GB). + * 4. Add to both bases (slide upward in VA space). + * + * Why 1GB alignment? + * SV39 L2 (root) PTEs map 1GB superpages. Keeping the slide + * 1GB-aligned means the direct-map can still use gigapages, + * avoiding TLB pressure. This matches Linux's KASLR on RISC-V. + */ + constexpr uint64 k1GB = 1ULL << 30; + constexpr uint64 kMaxSlots = 64; /* 0..63 GB slide range */ + + uint64 raw = get_random_u64(); + uint64 slot = raw % kMaxSlots; + uint64 slide = slot * k1GB; + + g_direct_map_base = kDirectMapBaseDefault + slide; + g_vmemmap_base = kVmemmapBaseDefault + slide; + + kprintf("[kaslr] slide = %llu GB (slot %llu/64)\n", slide >> 30, slot); + kprintf("[kaslr] direct_map_base = 0x%llx\n", g_direct_map_base); + kprintf("[kaslr] vmemmap_base = 0x%llx\n", g_vmemmap_base); +#else + kprintf("[kaslr] CONFIG_KASLR disabled, using fixed layout\n"); +#endif +} } // namespace arch |