Date: Tue, 29 Aug 2017 12:00:36 -0700 From: "Darrick J. Wong" <darrick.wong@...cle.com> To: Kees Cook <keescook@...omium.org> 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 Tue, Aug 29, 2017 at 11:48:49AM -0700, Kees Cook wrote: > On Mon, Aug 28, 2017 at 9:47 PM, Darrick J. Wong > <darrick.wong@...cle.com> wrote: > > 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@...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 > > > > 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 > > I went back and forth on that, and given all the things it touched, it > seemed like too large a CC list. :) I can explicitly add the xfs list > to the first three for any future versions. > > > 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. > > Under CONFIG_HARDENED_USERCOPY, there's no difference in performance > between the earlier bounds checking (of the whole slab object) vs the > new bounds checking (of the useroffset/usersize portion of the slab > object). Perf difference of CONFIG_HARDENED_USERCOPY itself has proven > hard to measure, which likely means it's very 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. :) > > There are plans for building either a new helper (kmalloc_usercopy()) > or adding a new flag (GFP_USERCOPY), but I haven't had time yet to > come back around to it. I wanted to land this step first, and we could > then move forward on the rest in future. Heh, fair enough. For the XFS bits, Reviewed-by: Darrick J. Wong <darrick.wong@...cle.com> --D > > -Kees > > -- > Kees Cook > Pixel Security > -- > To unsubscribe from this list: send the line "unsubscribe linux-xfs" in > the body of a message to majordomo@...r.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html
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