Subject: Re: [PATCH] powerpc/uprobes: Don't allow probe on suffix of prefixed instruction

From: Oleg Nesterov <[email-masked]>
Date: Tue, 19 Jan 2021 18:26:03 +0100

On 01/19, Ravi Bangoria wrote:
>
> Probe on 2nd word of a prefixed instruction is invalid scenario and
> should be restricted.

I don't understand this ppc-specific problem, but...

> +#ifdef CONFIG_PPC64
> +int arch_uprobe_verify_opcode(struct page *page, unsigned long vaddr,
> +			      uprobe_opcode_t opcode)
> +{
> +	uprobe_opcode_t prefix;
> +	void *kaddr;
> +	struct ppc_inst inst;
> +
> +	/* Don't check if vaddr is pointing to the beginning of page */
> +	if (!(vaddr & ~PAGE_MASK))
> +		return 0;

So the fix is incomplete? Or insn at the start of page can't be prefixed?

> +int __weak arch_uprobe_verify_opcode(struct page *page, unsigned long vaddr,
> +				     uprobe_opcode_t opcode)
> +{
> +	return 0;
> +}
> +
>  static int verify_opcode(struct page *page, unsigned long vaddr, uprobe_opcode_t *new_opcode)
>  {
>  	uprobe_opcode_t old_opcode;
> @@ -275,6 +281,8 @@ static int verify_opcode(struct page *page, unsigned long vaddr, uprobe_opcode_t
>  	if (is_swbp_insn(new_opcode)) {
>  		if (is_swbp)		/* register: already installed? */
>  			return 0;
> +		if (arch_uprobe_verify_opcode(page, vaddr, old_opcode))
> +			return -EINVAL;

Well, this doesn't look good...

To me it would be better to change the prepare_uprobe() path to copy
the potential prefix into uprobe->arch and check ppc_inst_prefixed()
in arch_uprobe_analyze_insn(). What do you think?

Oleg.