linux-kernel - Re: [PATCH] lib/test_hmm.c: fix harmless shift wrapping bug

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Date:   Tue, 16 Mar 2021 09:43:30 -0700
From:   Ralph Campbell <rcampbell@...dia.com>
To:     Dan Carpenter <dan.carpenter@...cle.com>,
        Jérôme Glisse <jglisse@...hat.com>
CC:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        <kernel-janitors@...r.kernel.org>
Subject: Re: [PATCH] lib/test_hmm.c: fix harmless shift wrapping bug


On 3/16/21 4:43 AM, Dan Carpenter wrote:
> The "cmd.npages" variable is a u64 that comes from the user.  I noticed
> during review that it could have a shift wrapping bug when it is used
> in the integer overflow test on the next line.
> 
> It turns out this is harmless.  The users all do:
> 
> 	unsigned long size = cmd->npages << PAGE_SHIFT;
> 
> and after that "size" is used consistently and "cmd->npages" is never
> used again.  So even when there is an integer overflow, everything works
> fine.
> 
> Even though this is harmless, I believe syzbot will complain and fixing
> it makes the code easier to read.
> 
> Fixes: b2ef9f5a5cb3 ("mm/hmm/test: add selftest driver for HMM")
> Signed-off-by: Dan Carpenter <dan.carpenter@...cle.com>
> ---
>   lib/test_hmm.c | 2 ++
>   1 file changed, 2 insertions(+)
> 
> diff --git a/lib/test_hmm.c b/lib/test_hmm.c
> index 80a78877bd93..541466034a6b 100644
> --- a/lib/test_hmm.c
> +++ b/lib/test_hmm.c
> @@ -930,6 +930,8 @@ static long dmirror_fops_unlocked_ioctl(struct file *filp,
>   
>   	if (cmd.addr & ~PAGE_MASK)
>   		return -EINVAL;
> +	if (cmd.npages > ULONG_MAX >> PAGE_SHIFT)
> +		return -EINVAL;
>   	if (cmd.addr >= (cmd.addr + (cmd.npages << PAGE_SHIFT)))
>   		return -EINVAL;

Looks good to me too. Thanks for sending this.
Reviewed-by: Ralph Campbell <rcampbell@...dia.com>