linux-kernel - Re: [PATCH] mm/gup: fix null pointer dereference detected by coverity

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Message-Id: <20200414170827.d32fc1fc12a33b140b740b94@linux-foundation.org>
Date:   Tue, 14 Apr 2020 17:08:27 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Miles Chen <miles.chen@...iatek.com>
Cc:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        <linux-mediatek@...ts.infradead.org>, <wsd_upstream@...iatek.com>,
        Peter Xu <peterx@...hat.com>
Subject: Re: [PATCH] mm/gup: fix null pointer dereference detected by
 coverity

On Tue, 7 Apr 2020 17:51:07 +0800 Miles Chen <miles.chen@...iatek.com> wrote:

> In fixup_user_fault(), it is possible that unlocked is NULL,
> so we should test unlocked before using it.
> 
> For example, in arch/arc/kernel/process.c, NULL is passed
> to fixup_user_fault().
> 
> SYSCALL_DEFINE3(arc_usr_cmpxchg, int *, uaddr, int, expected, int, new)
> {
> ...
> 	ret = fixup_user_fault(current, current->mm, (unsigned long) uaddr,
> 			       FAULT_FLAG_WRITE, NULL);
> ...
> }

(cc Peter)

> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1230,7 +1230,8 @@ int fixup_user_fault(struct task_struct *tsk, struct mm_struct *mm,
>  	if (ret & VM_FAULT_RETRY) {
>  		down_read(&mm->mmap_sem);
>  		if (!(fault_flags & FAULT_FLAG_TRIED)) {
> -			*unlocked = true;
> +			if (unlocked)
> +				*unlocked = true;
>  			fault_flags |= FAULT_FLAG_TRIED;
>  			goto retry;
>  		}

Not sure.  If the caller passes FAULT_FLAG_ALLOW_RETRY then they must
also pass in a valid non-NULL `unlocked'.  If the caller passed
FAULT_FLAG_ALLOW_RETRY and unlocked==NULL then the resulting oops is an
appropriate way of reporting this mistake.  I think?