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Date:   Sat, 6 Feb 2021 19:06:05 +0100
From:   Oleg Nesterov <oleg@redhat.com>
To:     Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Cc:     mpe@ellerman.id.au, rostedt@goodmis.org, paulus@samba.org,
        jniethe5@gmail.com, naveen.n.rao@linux.ibm.com,
        sandipan@linux.ibm.com, linuxppc-dev@lists.ozlabs.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] powerpc/uprobes: Validation for prefixed instruction
Message-ID: <20210206180604.GA8897@redhat.com>
References: <20210204104703.273429-1-ravi.bangoria@linux.ibm.com>
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In-Reply-To: <20210204104703.273429-1-ravi.bangoria@linux.ibm.com>
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On 02/04, Ravi Bangoria wrote:
>
> +static int get_instr(struct mm_struct *mm, unsigned long addr, u32 *instr)
> +{
> +	struct page *page;
> +	struct vm_area_struct *vma;
> +	void *kaddr;
> +	unsigned int gup_flags = FOLL_FORCE | FOLL_SPLIT_PMD;
> +
> +	if (get_user_pages_remote(mm, addr, 1, gup_flags, &page, &vma, NULL) <= 0)
> +		return -EINVAL;

"vma" is not used, and I don't think you need FOLL_SPLIT_PMD.

Otherwise I can't really comment this ppc-specific change.

To be honest, I don't even understand why do we need this fix. Sure, the
breakpoint in the middle of 64-bit insn won't work, why do we care? The
user should know what does he do.

Not to mention we can't really trust get_user_pages() in that this page
can be modified by mm owner or debugger...

But I won't argue.

Oleg.