[kernel-hardening] 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 02:57:14PM -0700, Kees Cook wrote:
> On Mon, Aug 28, 2017 at 2:49 PM, Darrick J. Wong
> <darrick.wong@oracle.com> wrote:
> > 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?
> 
> 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

Ah.  It would've been helpful to have the first three patches cc'd to
the xfs list.  So basically this series establishes the ability to set
regions within a slab object into which copy_to_user can copy memory
contents, and vice versa.  Have you seen any runtime performance impact?
The overhead looks like it ought to be minimal.

> (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.)

A pity.  It would be interesting to create no-usercopy versions of the
kmalloc-* slabs and see how much of XFS' memory consumption never
touches userspace buffers. :)

--D

> 
> -Kees
> 
> -- 
> Kees Cook
> Pixel Security
> --
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