linux-kernel - Re: [PATCH v2 15/30] xfs: Define usercopy region in xfs

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Date:   Mon, 28 Aug 2017 14:57:14 -0700
From:   Kees Cook <keescook@...omium.org>
To:     "Darrick J. Wong" <darrick.wong@...cle.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        David Windsor <dave@...lcore.net>, linux-xfs@...r.kernel.org,
        Linux-MM <linux-mm@...ck.org>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v2 15/30] xfs: Define usercopy region in xfs_inode slab cache

On Mon, Aug 28, 2017 at 2:49 PM, Darrick J. Wong
<darrick.wong@...cle.com> wrote:
> On Mon, Aug 28, 2017 at 02:34:56PM -0700, Kees Cook wrote:
>> From: David Windsor <dave@...lcore.net>
>>
>> The XFS inline inode data, stored in struct xfs_inode_t field
>> i_df.if_u2.if_inline_data and therefore contained in the xfs_inode slab
>> cache, needs to be copied to/from userspace.
>>
>> cache object allocation:
>>     fs/xfs/xfs_icache.c:
>>         xfs_inode_alloc(...):
>>             ...
>>             ip = kmem_zone_alloc(xfs_inode_zone, KM_SLEEP);
>>
>>     fs/xfs/libxfs/xfs_inode_fork.c:
>>         xfs_init_local_fork(...):
>>             ...
>>             if (mem_size <= sizeof(ifp->if_u2.if_inline_data))
>>                     ifp->if_u1.if_data = ifp->if_u2.if_inline_data;
>
> Hmm, what happens when mem_size > sizeof(if_inline_data)?  A slab object
> will be allocated for ifp->if_u1.if_data which can then be used for
> readlink in the same manner as the example usage trace below.  Does
> that allocated object have a need for a usercopy annotation like
> the one we're adding for if_inline_data?  Or is that already covered
> elsewhere?

Yeah, the xfs helper kmem_alloc() is used in the other case, which
ultimately boils down to a call to kmalloc(), which is entirely
whitelisted by an earlier patch in the series:

https://lkml.org/lkml/2017/8/28/1026

(It's possible that at some future time we can start segregating
kernel-only kmallocs from usercopy-able kmallocs, but for now, there
are no plans for this.)

-Kees

-- 
Kees Cook
Pixel Security