linux-kernel - Re: [PATCH] nfsd: avoid out of bounds read on array nfsd4_layout

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Message-ID: <5d21cd12-c842-d0ce-34bb-f037d0fd52ba@canonical.com>
Date:   Tue, 9 May 2017 22:14:48 +0100
From:   Colin Ian King <colin.king@...onical.com>
To:     "J . Bruce Fields" <bfields@...ldses.org>,
        Dan Carpenter <dan.carpenter@...cle.com>
Cc:     Ari Kauppi <ari@...opsys.com>,
        Jeff Layton <jlayton@...chiereds.net>,
        linux-nfs@...r.kernel.org, kernel-janitors@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] nfsd: avoid out of bounds read on array nfsd4_layout_ops

On 09/05/17 22:03, J . Bruce Fields wrote:
> On Tue, May 09, 2017 at 05:04:14PM +0300, Dan Carpenter wrote:
>> On Tue, May 09, 2017 at 02:31:21PM +0100, Colin King wrote:
>>> diff --git a/fs/nfsd/nfs4proc.c b/fs/nfsd/nfs4proc.c
>>> index 1dbf62190bee..c453a1998e00 100644
>>> --- a/fs/nfsd/nfs4proc.c
>>> +++ b/fs/nfsd/nfs4proc.c
>>> @@ -1259,7 +1259,8 @@ nfsd4_layout_verify(struct svc_export *exp, unsigned int layout_type)
>>>  		return NULL;
>>>  	}
>>>  
>>> -	if (layout_type >= 32 || !(exp->ex_layout_types & (1 << layout_type))) {
>>> +	if (layout_type >= LAYOUT_TYPE_MAX ||
>>> +	    !(exp->ex_layout_types & (1 << layout_type))) {
>>
>> The 32 is there to prevent a shift wrapping bug.  The bit test prevents
>> a buffer overflow so this can't actually overflow.
> 
> Yes, looks like a false positive for coverity.
> 
>> But this change doesn't hurt and is probably cleaner.
> 
> Sure.  Hope it's OK if I just merge this into the previous commit:

Fine by me.  Colin

> 
> --b.
> 
> commit 16b6f81d8ed9
> Author: Ari Kauppi <ari@...opsys.com>
> Date:   Fri May 5 16:07:55 2017 -0400
> 
>     nfsd: fix undefined behavior in nfsd4_layout_verify
>     
>       UBSAN: Undefined behaviour in fs/nfsd/nfs4proc.c:1262:34
>       shift exponent 128 is too large for 32-bit type 'int'
>     
>     Depending on compiler+architecture, this may cause the check for
>     layout_type to succeed for overly large values (which seems to be the
>     case with amd64). The large value will be later used in de-referencing
>     nfsd4_layout_ops for function pointers.
>     
>     Reported-by: Jani Tuovila <tuovila@...opsys.com>
>     Signed-off-by: Ari Kauppi <ari@...opsys.com>
>     [colin.king@...onical.com: use LAYOUT_TYPE_MAX instead of 32]
>     Reviewed-by: Dan Carpenter <dan.carpenter@...cle.com>
>     Signed-off-by: J. Bruce Fields <bfields@...hat.com>
> 
> diff --git a/fs/nfsd/nfs4proc.c b/fs/nfsd/nfs4proc.c
> index d86031b6ad79..c453a1998e00 100644
> --- a/fs/nfsd/nfs4proc.c
> +++ b/fs/nfsd/nfs4proc.c
> @@ -1259,7 +1259,8 @@ nfsd4_layout_verify(struct svc_export *exp, unsigned int layout_type)
>  		return NULL;
>  	}
>  
> -	if (!(exp->ex_layout_types & (1 << layout_type))) {
> +	if (layout_type >= LAYOUT_TYPE_MAX ||
> +	    !(exp->ex_layout_types & (1 << layout_type))) {
>  		dprintk("%s: layout type %d not supported\n",
>  			__func__, layout_type);
>  		return NULL;
> --
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