Re: [PATCH v0] RISC-V: Use Zkr to seed KASLR base address

Zong Li
2024-06-05T04:51:38Z

On Sat, Jun 1, 2024 at 12:23 AM Jesse Taube <jesse@rivosinc.com> wrote:
>
> Dectect the Zkr extension and use it to seed the kernel base address.
>
> Detection of the extension can not be done in the typical fashion, as
> this is very early in the boot process. Instead, add a trap handler
> and run it to see if the extension is present.
>
> Signed-off-by: Jesse Taube <jesse@rivosinc.com>
> ---
> arch/riscv/kernel/pi/Makefile | 2 +-
> arch/riscv/kernel/pi/archrandom_early.c | 71 +++++++++++++++++++++++++
> arch/riscv/mm/init.c | 3 ++
> 3 files changed, 75 insertions(+), 1 deletion(-)
> create mode 100644 arch/riscv/kernel/pi/archrandom_early.c
>
> diff --git a/arch/riscv/kernel/pi/Makefile b/arch/riscv/kernel/pi/Makefile
> index 50bc5ef7dd2f..9025eb52945a 100644
> --- a/arch/riscv/kernel/pi/Makefile
> +++ b/arch/riscv/kernel/pi/Makefile
> @@ -32,5 +32,5 @@ $(obj)/string.o: $(srctree)/lib/string.c FORCE
> $(obj)/ctype.o: $(srctree)/lib/ctype.c FORCE
> $(call if_changed_rule,cc_o_c)
>
> -obj-y := cmdline_early.pi.o fdt_early.pi.o string.pi.o ctype.pi.o lib-fdt.pi.o lib-fdt_ro.pi.o
> +obj-y := cmdline_early.pi.o fdt_early.pi.o string.pi.o ctype.pi.o lib-fdt.pi.o lib-fdt_ro.pi.o archrandom_early.pi.o
> extra-y := $(patsubst %.pi.o,%.o,$(obj-y))
> diff --git a/arch/riscv/kernel/pi/archrandom_early.c b/arch/riscv/kernel/pi/archrandom_early.c
> new file mode 100644
> index 000000000000..311be9388b5c
> --- /dev/null
> +++ b/arch/riscv/kernel/pi/archrandom_early.c
> @@ -0,0 +1,71 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +
> +/*
> + * To avoid rewriteing code include asm/archrandom.h and create macros
> + * for the functions that won't be included.
> + */
> +
> +#define riscv_has_extension_likely(...) false
> +#define pr_err_once(...)
> +
> +#include <linux/types.h>
> +#include <asm/hwcap.h>
> +#include <asm/archrandom.h>
> +
> +/*
> + * Asm goto is needed so that the compiler does not remove the label.
> + */
> +
> +#define csr_goto_swap(csr, val) \
> +({ \
> + unsigned long __v; \
> + __asm__ __volatile__ goto("csrrw %0, " __ASM_STR(csr) ", %1" \
> + : "=r" (__v) : "rK" (&&val) \
> + : "memory" : val); \
> + __v; \
> +})
> +
> +/*
> + * Declare the functions that are exported (but prefixed) here so that LLVM
> + * does not complain it lacks the 'static' keyword (which, if added, makes
> + * LLVM complain because the function is actually unused in this file).
> + */
> +
> +u64 get_kaslr_seed_zkr(void);
> +
> +/*
> + * This function is called by setup_vm to check if the kernel has the ZKR.
> + * Traps haven't been set up yet, but save and restore the TVEC to avoid
> + * any side effects.
> + */
> +
> +static inline bool __must_check riscv_has_zkr(void)
> +{
> + unsigned long tvec;
> +
> + tvec = csr_goto_swap(CSR_TVEC, not_zkr);
> + csr_swap(CSR_SEED, 0);
> + csr_write(CSR_TVEC, tvec);
> + return true;
> +not_zkr:
> + csr_write(CSR_TVEC, tvec);
> + return false;
> +}
> +
> +u64 get_kaslr_seed_zkr(void)
> +{
> + const int needed_seeds = sizeof(u64) / sizeof(long);
> + int i = 0;
> + u64 seed = 0;
> + long *entropy = (long *)(&seed);
> +
> + if (!riscv_has_zkr())
> + return 0;
> +
> + for (i = 0; i < needed_seeds; i++) {
> + if (!csr_seed_long(&entropy[i]))
> + return 0;
> + }
> +
> + return seed;
> +}
> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> index 9940171c79f0..8ef1edd2cddd 100644
> --- a/arch/riscv/mm/init.c
> +++ b/arch/riscv/mm/init.c
> @@ -1025,6 +1025,7 @@ static void __init pt_ops_set_late(void)
> #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);
> +extern u64 __init __pi_get_kaslr_seed_zkr(void);
>
> static int __init print_nokaslr(char *p)
> {
> @@ -1049,6 +1050,8 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa)
> u32 kernel_size = (uintptr_t)(&_end) - (uintptr_t)(&_start);
> u32 nr_pos;
>
> + if (kaslr_seed == 0)
> + kaslr_seed = __pi_get_kaslr_seed_zkr();

Could you elaborate on why we try to get the seed from DT first,
rather than from ZKR? Also, I was wondering if, by any chance, it can
leverage arch_get_random_seed_longs() to get the seed instead of
__pi_get_kasler_seed_zkr()? Thanks

> /*
> * Compute the number of positions available: we are limited
> * by the early page table that only has one PUD and we must
> --
> 2.43.0
>
>
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