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SubjectRe: [PATCH v4 2/9] riscv: introduce interfaces to patch kernel code
Hi Zong,

On Tue, 7 Apr 2020 22:46:47 +0800
Zong Li <zong.li@sifive.com> wrote:

> On strict kernel memory permission, we couldn't patch code without
> writable permission. Preserve two holes in fixmap area, so we can map
> the kernel code temporarily to fixmap area, then patch the instructions.
>
> We need two pages here because we support the compressed instruction, so
> the instruction might be align to 2 bytes. When patching the 32-bit
> length instruction which is 2 bytes alignment, it will across two pages.
>
> Introduce two interfaces to patch kernel code:
> riscv_patch_text_nosync:
> - patch code without synchronization, it's caller's responsibility to
> synchronize all CPUs if needed.
> riscv_patch_text:
> - patch code and always synchronize with stop_machine()
>
> Signed-off-by: Zong Li <zong.li@sifive.com>
> Suggested-by: Masami Hiramatsu <mhiramat@kernel.org>
> ---
> arch/riscv/include/asm/fixmap.h | 2 +
> arch/riscv/include/asm/patch.h | 12 ++++
> arch/riscv/kernel/Makefile | 4 +-
> arch/riscv/kernel/patch.c | 120 ++++++++++++++++++++++++++++++++
> 4 files changed, 137 insertions(+), 1 deletion(-)
> create mode 100644 arch/riscv/include/asm/patch.h
> create mode 100644 arch/riscv/kernel/patch.c
>
> diff --git a/arch/riscv/include/asm/fixmap.h b/arch/riscv/include/asm/fixmap.h
> index 42d2c42f3cc9..2368d49eb4ef 100644
> --- a/arch/riscv/include/asm/fixmap.h
> +++ b/arch/riscv/include/asm/fixmap.h
> @@ -27,6 +27,8 @@ enum fixed_addresses {
> FIX_FDT = FIX_FDT_END + FIX_FDT_SIZE / PAGE_SIZE - 1,
> FIX_PTE,
> FIX_PMD,
> + FIX_TEXT_POKE1,
> + FIX_TEXT_POKE0,
> FIX_EARLYCON_MEM_BASE,
> __end_of_fixed_addresses
> };
> diff --git a/arch/riscv/include/asm/patch.h b/arch/riscv/include/asm/patch.h
> new file mode 100644
> index 000000000000..9a7d7346001e
> --- /dev/null
> +++ b/arch/riscv/include/asm/patch.h
> @@ -0,0 +1,12 @@
> +/* SPDX-License-Identifier: GPL-2.0-only */
> +/*
> + * Copyright (C) 2020 SiFive
> + */
> +
> +#ifndef _ASM_RISCV_PATCH_H
> +#define _ASM_RISCV_PATCH_H
> +
> +int patch_text_nosync(void *addr, const void *insns, size_t len);
> +int patch_text(void *addr, u32 insn);
> +
> +#endif /* _ASM_RISCV_PATCH_H */
> diff --git a/arch/riscv/kernel/Makefile b/arch/riscv/kernel/Makefile
> index f40205cb9a22..d189bd3d8501 100644
> --- a/arch/riscv/kernel/Makefile
> +++ b/arch/riscv/kernel/Makefile
> @@ -4,7 +4,8 @@
> #
>
> ifdef CONFIG_FTRACE
> -CFLAGS_REMOVE_ftrace.o = -pg
> +CFLAGS_REMOVE_ftrace.o = -pg
> +CFLAGS_REMOVE_patch.o = -pg
> endif
>
> extra-y += head.o
> @@ -26,6 +27,7 @@ obj-y += traps.o
> obj-y += riscv_ksyms.o
> obj-y += stacktrace.o
> obj-y += cacheinfo.o
> +obj-y += patch.o
> obj-$(CONFIG_MMU) += vdso.o vdso/
>
> obj-$(CONFIG_RISCV_M_MODE) += clint.o
> diff --git a/arch/riscv/kernel/patch.c b/arch/riscv/kernel/patch.c
> new file mode 100644
> index 000000000000..5b4f0d37097f
> --- /dev/null
> +++ b/arch/riscv/kernel/patch.c
> @@ -0,0 +1,120 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * Copyright (C) 2020 SiFive
> + */
> +
> +#include <linux/spinlock.h>
> +#include <linux/mm.h>
> +#include <linux/uaccess.h>
> +#include <linux/stop_machine.h>
> +#include <asm/kprobes.h>
> +#include <asm/cacheflush.h>
> +#include <asm/fixmap.h>
> +
> +struct patch_insn_patch {
> + void *addr;
> + u32 insn;
> + atomic_t cpu_count;
> +};
> +
> +#ifdef CONFIG_MMU
> +static void *patch_map(void *addr, int fixmap)
> +{
> + uintptr_t uintaddr = (uintptr_t) addr;
> + struct page *page;
> +
> + if (core_kernel_text(uintaddr))
> + page = phys_to_page(__pa_symbol(addr));
> + else if (IS_ENABLED(CONFIG_STRICT_MODULE_RWX))
> + page = vmalloc_to_page(addr);
> + else
> + return addr;
> +
> + BUG_ON(!page);
> +
> + return (void *)set_fixmap_offset(fixmap, page_to_phys(page) +
> + (uintaddr & ~PAGE_MASK));
> +}
> +NOKPROBE_SYMBOL(patch_map);
> +
> +static void patch_unmap(int fixmap)
> +{
> + clear_fixmap(fixmap);
> +}
> +NOKPROBE_SYMBOL(patch_unmap);
> +

Please leave a comment here about text_mutex,

> +static int patch_insn_write(void *addr, const void *insn, size_t len)
> +{
> + void *waddr = addr;
> + bool across_pages = (((uintptr_t) addr & ~PAGE_MASK) + len) > PAGE_SIZE;
> + int ret;

Or use lockdep_assert_held(&text_mutex); here so that user can easily
understand they have to lock the text_mutex before calling this.

Thank you,

> +
> + if (across_pages)
> + patch_map(addr + len, FIX_TEXT_POKE1);
> +
> + waddr = patch_map(addr, FIX_TEXT_POKE0);
> +
> + ret = probe_kernel_write(waddr, insn, len);
> +
> + patch_unmap(FIX_TEXT_POKE0);
> +
> + if (across_pages)
> + patch_unmap(FIX_TEXT_POKE1);
> +
> + return ret;
> +}
> +NOKPROBE_SYMBOL(patch_insn_write);
> +#else
> +static int patch_insn_write(void *addr, const void *insn, size_t len)
> +{
> + return probe_kernel_write(addr, insn, len);
> +}
> +NOKPROBE_SYMBOL(patch_insn_write);
> +#endif /* CONFIG_MMU */
> +
> +int patch_text_nosync(void *addr, const void *insns, size_t len)
> +{
> + u32 *tp = addr;
> + int ret;
> +
> + ret = patch_insn_write(tp, insns, len);
> +
> + if (!ret)
> + flush_icache_range((uintptr_t) tp, (uintptr_t) tp + len);
> +
> + return ret;
> +}
> +NOKPROBE_SYMBOL(patch_text_nosync);
> +
> +static int patch_text_cb(void *data)
> +{
> + struct patch_insn_patch *patch = data;
> + int ret = 0;
> +
> + if (atomic_inc_return(&patch->cpu_count) == 1) {
> + ret =
> + patch_text_nosync(patch->addr, &patch->insn,
> + GET_INSN_LENGTH(patch->insn));
> + atomic_inc(&patch->cpu_count);
> + } else {
> + while (atomic_read(&patch->cpu_count) <= num_online_cpus())
> + cpu_relax();
> + smp_mb();
> + }
> +
> + return ret;
> +}
> +NOKPROBE_SYMBOL(patch_text_cb);
> +
> +int patch_text(void *addr, u32 insn)
> +{
> + struct patch_insn_patch patch = {
> + .addr = addr,
> + .insn = insn,
> + .cpu_count = ATOMIC_INIT(0),
> + };
> +
> + return stop_machine_cpuslocked(patch_text_cb,
> + &patch, cpu_online_mask);
> +}
> +NOKPROBE_SYMBOL(patch_text);
> --
> 2.26.0
>


--
Masami Hiramatsu <mhiramat@kernel.org>

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