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SubjectRe: [PATCH] riscv/kprobes: allocate detour buffer from module area
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On Tue, 28 Jun 2022 18:13:17 PDT (-0700), liaochang1@huawei.com wrote:
> To address the limitation of PC-relative branch instruction
> on riscv architecture, detour buffer slot is allocated from
> a area, the distance of which from kernel should be less than 4GB.
>
> For the time being, Modules region always live before the kernel.
> But Vmalloc region reside far away from kernel, the distance is
> half of the kernel address space.
>
> Signed-off-by: Liao Chang <liaochang1@huawei.com>
> ---
> arch/riscv/kernel/probes/kprobes.c | 20 ++++++++++++++++++++
> 1 file changed, 20 insertions(+)
>
> diff --git a/arch/riscv/kernel/probes/kprobes.c b/arch/riscv/kernel/probes/kprobes.c
> index e6e950b7cf32..bc027a663b17 100644
> --- a/arch/riscv/kernel/probes/kprobes.c
> +++ b/arch/riscv/kernel/probes/kprobes.c
> @@ -6,12 +6,14 @@
> #include <linux/extable.h>
> #include <linux/slab.h>
> #include <linux/stop_machine.h>
> +#include <linux/moduleloader.h>
> #include <asm/ptrace.h>
> #include <linux/uaccess.h>
> #include <asm/sections.h>
> #include <asm/cacheflush.h>
> #include <asm/bug.h>
> #include <asm/patch.h>
> +#include <asm/set_memory.h>
>
> #include "decode-insn.h"
>
> @@ -86,10 +88,28 @@ int __kprobes arch_prepare_kprobe(struct kprobe *p)
> #ifdef CONFIG_MMU
> void *alloc_insn_page(void)
> {
> +#if defined(CONFIG_MODULES) && defined(CONFIG_64BIT)
> + void *page;
> +
> + page = module_alloc(PAGE_SIZE);
> + if (!page)
> + return NULL;
> +
> + set_vm_flush_reset_perms(page);
> + /*
> + * First make the page read-only, and only then make it executable to
> + * prevent it from being W+X in between.
> + */
> + set_memory_ro((unsigned long)page, 1);
> + set_memory_x((unsigned long)page, 1);
> +
> + return page;
> +#else
> return __vmalloc_node_range(PAGE_SIZE, 1, VMALLOC_START, VMALLOC_END,
> GFP_KERNEL, PAGE_KERNEL_READ_EXEC,
> VM_FLUSH_RESET_PERMS, NUMA_NO_NODE,
> __builtin_return_address(0));
> +#endif
> }
> #endif

This probably shouldn't depend on CONFIG_MODULES, it's just as broken to
assume we can jump to the vmalloc region regardless of whether modules
are enabled or disabled. We should be able to just allocate from the
jump-safe region either way.

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