linux-kernel - Re: [PATCH] mm: Add check for NULL for unlocked in fixup_user

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Date:   Thu, 15 Dec 2022 13:22:36 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Anastasia Belova <abelova@...ralinux.ru>
Cc:     Max Filippov <jcmvbkbc@...il.com>, Ingo Molnar <mingo@...nel.org>,
        Vineet Gupta <vgupta@...nel.org>,
        "Russell King (Oracle)" <rmk+kernel@...linux.org.uk>,
        Geert Uytterhoeven <geert@...ux-m68k.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, lvc-project@...uxtesting.org
Subject: Re: [PATCH] mm: Add check for NULL for unlocked in fixup_user_fault

On Thu, 15 Dec 2022 12:39:30 +0300 Anastasia Belova <abelova@...ralinux.ru> wrote:

> Check unlocked for NULL before dereference.
> 
> Found by Linux Verification Center (linuxtesting.org) with SVACE.
> 
> ...
>
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1282,7 +1282,8 @@ int fixup_user_fault(struct mm_struct *mm,
>  		 * could tell the callers so they do not need to unlock.
>  		 */
>  		mmap_read_lock(mm);
> -		*unlocked = true;
> +		if (unlocked)
> +			*unlocked = true;
>  		return 0;
>  	}

I don't believe this is necessary unless the caller passed
FAULT_FLAG_KILLABLE or FAULT_FLAG_ALLOW_RETRY in fault_flags.

It's clear as mud and some code comments would help.