Re: [PATCH v2 15/30] xfs: Define usercopy region in xfs_inode slab cache

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Mon, Aug 28, 2017 at 02:34:56PM -0700, Kees Cook wrote:
> From: David Windsor <dave@nullcore.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?

--D

>             ...
> 
>     fs/xfs/xfs_symlink.c:
>         xfs_symlink(...):
>             ...
>             xfs_init_local_fork(ip, XFS_DATA_FORK, target_path, pathlen);
> 
> example usage trace:
>     readlink_copy+0x43/0x70
>     vfs_readlink+0x62/0x110
>     SyS_readlinkat+0x100/0x130
> 
>     fs/xfs/xfs_iops.c:
>         (via inode->i_op->get_link)
>         xfs_vn_get_link_inline(...):
>             ...
>             return XFS_I(inode)->i_df.if_u1.if_data;
> 
>     fs/namei.c:
>         readlink_copy(..., link):
>             ...
>             copy_to_user(..., link, len);
> 
>         generic_readlink(dentry, ...):
>             struct inode *inode = d_inode(dentry);
>             const char *link = inode->i_link;
>             ...
>             if (!link) {
>                     link = inode->i_op->get_link(dentry, inode, &done);
>             ...
>             readlink_copy(..., link);
> 
> In support of usercopy hardening, this patch defines a region in the
> xfs_inode slab cache in which userspace copy operations are allowed.
> 
> This region is known as the slab cache's usercopy region. Slab caches can
> now check that each copy operation involving cache-managed memory falls
> entirely within the slab's usercopy region.
> 
> This patch is modified from Brad Spengler/PaX Team's PAX_USERCOPY
> whitelisting code in the last public patch of grsecurity/PaX based on my
> understanding of the code. Changes or omissions from the original code are
> mine and don't reflect the original grsecurity/PaX code.
> 
> Signed-off-by: David Windsor <dave@nullcore.net>
> [kees: adjust commit log, provide usage trace]
> Cc: "Darrick J. Wong" <darrick.wong@oracle.com>
> Cc: linux-xfs@vger.kernel.org
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
>  fs/xfs/kmem.h      | 10 ++++++++++
>  fs/xfs/xfs_super.c |  7 +++++--
>  2 files changed, 15 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/xfs/kmem.h b/fs/xfs/kmem.h
> index 4d85992d75b2..08358f38dee6 100644
> --- a/fs/xfs/kmem.h
> +++ b/fs/xfs/kmem.h
> @@ -110,6 +110,16 @@ kmem_zone_init_flags(int size, char *zone_name, unsigned long flags,
>  	return kmem_cache_create(zone_name, size, 0, flags, construct);
>  }
>  
> +static inline kmem_zone_t *
> +kmem_zone_init_flags_usercopy(int size, char *zone_name, unsigned long flags,
> +				size_t useroffset, size_t usersize,
> +				void (*construct)(void *))
> +{
> +	return kmem_cache_create_usercopy(zone_name, size, 0, flags,
> +				useroffset, usersize, construct);
> +}
> +
> +
>  static inline void
>  kmem_zone_free(kmem_zone_t *zone, void *ptr)
>  {
> diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
> index 38aaacdbb8b3..6ca428c6f943 100644
> --- a/fs/xfs/xfs_super.c
> +++ b/fs/xfs/xfs_super.c
> @@ -1829,9 +1829,12 @@ xfs_init_zones(void)
>  		goto out_destroy_efd_zone;
>  
>  	xfs_inode_zone =
> -		kmem_zone_init_flags(sizeof(xfs_inode_t), "xfs_inode",
> +		kmem_zone_init_flags_usercopy(sizeof(xfs_inode_t), "xfs_inode",
>  			KM_ZONE_HWALIGN | KM_ZONE_RECLAIM | KM_ZONE_SPREAD |
> -			KM_ZONE_ACCOUNT, xfs_fs_inode_init_once);
> +				KM_ZONE_ACCOUNT,
> +			offsetof(xfs_inode_t, i_df.if_u2.if_inline_data),
> +			sizeof_field(xfs_inode_t, i_df.if_u2.if_inline_data),
> +			xfs_fs_inode_init_once);
>  	if (!xfs_inode_zone)
>  		goto out_destroy_efi_zone;
>  
> -- 
> 2.7.4
> 
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