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Date:   Fri, 17 Apr 2020 17:22:46 -0700 (PDT)
Subject:     Re: [PATCH v5 2/9] riscv: introduce interfaces to patch kernel code
In-Reply-To: <6f145470ffdf83b303ecd83db4f6e06477b61220.1586332296.git.zong.li@sifive.com>
CC:     Paul Walmsley <paul.walmsley@sifive.com>, aou@eecs.berkeley.edu,
        mhiramat@kernel.org, linux-riscv@lists.infradead.org,
        linux-kernel@vger.kernel.org, zong.li@sifive.com
From:   Palmer Dabbelt <palmer@dabbelt.com>
To:     zong.li@sifive.com
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On Wed, 08 Apr 2020 00:56:57 PDT (-0700), 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       | 128 ++++++++++++++++++++++++++++++++
>  4 files changed, 145 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..f16466123947
> --- /dev/null
> +++ b/arch/riscv/kernel/patch.c
> @@ -0,0 +1,128 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * Copyright (C) 2020 SiFive
> + */
> +
> +#include <linux/mm.h>
> +#include <linux/memory.h>
> +#include <linux/uaccess.h>
> +#include <linux/spinlock.h>
> +#include <linux/stop_machine.h>
> +#include <asm/fixmap.h>
> +#include <asm/kprobes.h>
> +#include <asm/cacheflush.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);
> +
> +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;
> +
> +	/*
> +	 * Before reaching here, it was expected to lock the text_mutex
> +	 * already, so we don't need to give another lock here and could
> +	 * ensure that it was safe between each cores.
> +	 */
> +	lockdep_assert_held(&text_mutex);
> +
> +	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);

Reviewed-by: Palmer Dabbelt <palmerdabbelt@google.com>