linux-kernel - Re: [PATCH] x86/ioremap: tighten integer overflow checking

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Date:   Mon, 29 Oct 2018 19:34:58 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Dan Carpenter <dan.carpenter@...cle.com>
cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org
Subject: Re: [PATCH] x86/ioremap: tighten integer overflow checking

Dan,

On Thu, 25 Oct 2018, Dan Carpenter wrote:

> The current check is a bit off in the case where "phys_addr + size"
> wraps to zero because then "last_addr" is set to ULONG_MAX which is >=
> phys_addr.
> 
> Signed-off-by: Dan Carpenter <dan.carpenter@...cle.com>
> ---
>  arch/x86/mm/ioremap.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/mm/ioremap.c b/arch/x86/mm/ioremap.c
> index 5378d10f1d31..ee43df3ebe66 100644
> --- a/arch/x86/mm/ioremap.c
> +++ b/arch/x86/mm/ioremap.c
> @@ -146,9 +146,9 @@ static void __iomem *__ioremap_caller(resource_size_t phys_addr,
>  	void __iomem *ret_addr;
>  
>  	/* Don't allow wraparound or zero size */
> -	last_addr = phys_addr + size - 1;
> -	if (!size || last_addr < phys_addr)
> +	if (!size || phys_addr + size < phys_addr)

Assume the following (resource_size_t == u32, which is the case when
CONFIG_PHYS_ADDR_T_64BIT=n):

       phys_addr = 0xFFFF0000
       size	 = 0x00010000

       sum	 = 0x00000000  which is < phys_addr

But the existing code does:

       last_addr = phys_addr + size - 1

       		 = 0xFFFFFFFF

which is correct. last_addr is the last valid address in the to be remapped
range starting @phys_addr.

Thanks,

	tglx