Date: Tue, 29 Aug 2017 11:48:49 -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 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. -Kees -- Kees Cook Pixel Security
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