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

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Date:   Tue, 7 Apr 2020 11:19:08 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     Miles Chen <miles.chen@...iatek.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-mediatek@...ts.infradead.org,
        wsd_upstream@...iatek.com
Subject: Re: [PATCH] mm/gup: fix null pointer dereference detected by coverity

On Tue, Apr 07, 2020 at 05:51:07PM +0800, Miles Chen 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);
> ...
> }
> 
> Fixes: 4a9e1cda2748 ("mm: bring in additional flag for fixup_user_fault to signal unlock")
> Signed-off-by: Miles Chen <miles.chen@...iatek.com>

Reviewed-by: Ira Weiny <ira.weiny@...el.com>

> ---
>  mm/gup.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index da3e03185144..a68d11dc232d 100644
> --- 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;
>  		}
> -- 
> 2.18.0