From: Michal Hocko <mhocko@kernel.org>
To: Kees Cook <keescook@chromium.org>
Cc: Laura Abbott <labbott@redhat.com>,
"kernel-hardening@lists.openwall.com"
<kernel-hardening@lists.openwall.com>,
David Windsor <dave@nullcore.net>, Linux-MM <linux-mm@kvack.org>,
LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 22/23] usercopy: split user-controlled slabs to separate caches
Date: Tue, 27 Jun 2017 09:31:32 +0200 [thread overview]
Message-ID: <20170627073132.GC28078@dhcp22.suse.cz> (raw)
In-Reply-To: <CAGXu5jKBB8TF7e74QkuxOu0iy6TZe3Q_0Fs21tbyq23Js3v3Mw@mail.gmail.com>
On Tue 20-06-17 15:22:58, Kees Cook wrote:
> On Tue, Jun 20, 2017 at 1:24 PM, Laura Abbott <labbott@redhat.com> wrote:
> > On 06/19/2017 04:36 PM, Kees Cook wrote:
> >> From: David Windsor <dave@nullcore.net>
> >>
> >> Some userspace APIs (e.g. ipc, seq_file) provide precise control over
> >> the size of kernel kmallocs, which provides a trivial way to perform
> >> heap overflow attacks where the attacker must control neighboring
> >> allocations of a specific size. Instead, move these APIs into their own
> >> cache so they cannot interfere with standard kmallocs. This is enabled
> >> with CONFIG_HARDENED_USERCOPY_SPLIT_KMALLOC.
> >>
> >> This patch is modified from Brad Spengler/PaX Team's PAX_USERCOPY_SLABS
> >> 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: added SLAB_NO_MERGE flag to allow split of future no-merge Kconfig]
> >> Signed-off-by: Kees Cook <keescook@chromium.org>
> >
> > I just did a quick test of kspp/usercopy-whitelist/lateston my arm64 machine and got some spew:
> >
> > [ 21.818719] Unexpected gfp: 0x4000000 (0x4000000). Fixing up to gfp: 0x14000c0 (GFP_KERNEL). Fix your code!
> > [ 21.828427] CPU: 7 PID: 652 Comm: irqbalance Tainted: G W 4.12.0-rc5-whitelist+ #236
> > [ 21.837259] Hardware name: AppliedMicro X-Gene Mustang Board/X-Gene Mustang Board, BIOS 3.06.12 Aug 12 2016
> > [ 21.846955] Call trace:
> > [ 21.849396] [<ffff000008089b18>] dump_backtrace+0x0/0x210
> > [ 21.854770] [<ffff000008089d4c>] show_stack+0x24/0x30
> > [ 21.859798] [<ffff00000845b7bc>] dump_stack+0x90/0xb4
> > [ 21.864827] [<ffff00000826ff40>] new_slab+0x88/0x90
> > [ 21.869681] [<ffff000008272218>] ___slab_alloc+0x428/0x6b0
> > [ 21.875141] [<ffff0000082724f0>] __slab_alloc+0x50/0x68
> > [ 21.880341] [<ffff000008273208>] __kmalloc_node+0xd0/0x348
> > [ 21.885800] [<ffff000008223af0>] kvmalloc_node+0xa0/0xb8
> > [ 21.891088] [<ffff0000082bb400>] single_open_size+0x40/0xb0
> > [ 21.896636] [<ffff000008315a9c>] stat_open+0x54/0x60
> > [ 21.901576] [<ffff00000830adf8>] proc_reg_open+0x90/0x168
> > [ 21.906950] [<ffff00000828def4>] do_dentry_open+0x1c4/0x328
> > [ 21.912496] [<ffff00000828f470>] vfs_open+0x58/0x88
> > [ 21.917351] [<ffff0000082a1f14>] do_last+0x3d4/0x770
> > [ 21.922292] [<ffff0000082a233c>] path_openat+0x8c/0x2e8
> > [ 21.927492] [<ffff0000082a3888>] do_filp_open+0x70/0xe8
> > [ 21.932692] [<ffff00000828f940>] do_sys_open+0x178/0x208
> > [ 21.937977] [<ffff00000828fa54>] SyS_openat+0x3c/0x50
> > [ 21.943005] [<ffff0000080835f0>] el0_svc_naked+0x24/0x28
> >
> >
> > I don't think 7e7844226f10 ("lockdep: allow to disable reclaim lockup detection")
> > is correct after new flags are added because we will still need space
> > for another bit even if lockdep is disabled. That might need to
> > be fixed separately.
>
> Err... that commit has "___GFP_NOLOCKDEP 0x4000000u", but my
> tree shows it as 0x2000000u? Hmm, looks like 1bde33e05123
> ("include/linux/gfp.h: fix ___GFP_NOLOCKDEP value") fixed that?
yes
> Oh, or
> I have misread it. It looks like new GFP flags need to be added
> _above_ GFP_NOLOCKDEP and have to bump GFP_NOLOCKDEP's value too? Like
> this:
>
> diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> index ff4f4a698ad0..deb8ac39fba5 100644
> --- a/include/linux/gfp.h
> +++ b/include/linux/gfp.h
> @@ -40,12 +40,12 @@ struct vm_area_struct;
> #define ___GFP_DIRECT_RECLAIM 0x400000u
> #define ___GFP_WRITE 0x800000u
> #define ___GFP_KSWAPD_RECLAIM 0x1000000u
> +#define ___GFP_USERCOPY 0x2000000u
> #ifdef CONFIG_LOCKDEP
> -#define ___GFP_NOLOCKDEP 0x2000000u
> +#define ___GFP_NOLOCKDEP 0x4000000u
> #else
> #define ___GFP_NOLOCKDEP 0
> #endif
> -#define ___GFP_USERCOPY 0x4000000u
> /* If the above are modified, __GFP_BITS_SHIFT may need updating */
yes, this would work for your case. But...
> > I'm really not a fan the GFP approach though since the flags tend
> > to be a little bit fragile to manage. If we're going to have to
> > add something to callsites anyway, maybe we could just have an
> > alternate function (kmalloc_user?) instead of a GFP flag.
I agree. One additional concern I would have is that people tend to
propagate gfp maks down different layers. What would happen if such a
flag gets to a caller which doesn't expect it. Couldn't it reduce the
whole security point of this excercise?
But I am not really sure I understand consequences of this patch. So how
do those attacks look like. Do you have an example of a CVE which would
be prevented by this measure?
> This would mean building out *_user() versions for all the various
> *alloc() functions, though. That gets kind of long/ugly.
Only prepare those which are really needed. It seems only handful of
them in your patch.
> The other reason to use a GFP flag is to be able to interrogate a
> cache later, which will be handy for doing things like %p and kernel
> symbol censorship (this is what grsecurity does with their HIDESYM
> logic). "If this would write to a usercopy-whitelisted object, censor
> it" etc. Though now that I go double-check, it looks like grsecurity
> uses cache->usersize as an indicator of censorship-need on slab
> caches, not the GFP flag, which is only used to use the split kmalloc
> cache.
which sounds like a separate kmalloc API would do the trick without a new
gfp mask.
> (Though as far as flags go, there is also VM_USERCOPY for what
> are now the kvmalloc*() cases.)
OK, I was about to ask about vmalloc fallbacks. So this is not
implemented in your patch. Does it metter from the security point of
view?
> Perhaps this should be named GFP_USERSIZED or so?
--
Michal Hocko
SUSE Labs
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next prev parent reply other threads:[~2017-06-27 7:31 UTC|newest]
Thread overview: 45+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-06-19 23:36 [PATCH 00/23] Hardened usercopy whitelisting Kees Cook
2017-06-19 23:36 ` [PATCH 01/23] usercopy: Prepare for " Kees Cook
2017-06-19 23:36 ` [PATCH 02/23] usercopy: Enforce slab cache usercopy region boundaries Kees Cook
2017-06-19 23:36 ` [PATCH 03/23] vfs: define usercopy region in names_cache slab caches Kees Cook
2017-06-19 23:36 ` [PATCH 04/23] vfs: copy struct mount.mnt_id to userspace using put_user() Kees Cook
2017-06-19 23:36 ` [PATCH 05/23] befs: define usercopy region in befs_inode_cache slab cache Kees Cook
2017-06-19 23:36 ` [PATCH 06/23] cifs: define usercopy region in cifs_request " Kees Cook
2017-06-19 23:36 ` [PATCH 07/23] exofs: define usercopy region in exofs_inode_cache " Kees Cook
2017-06-19 23:36 ` [PATCH 08/23] ext2: define usercopy region in ext2_inode_cache " Kees Cook
2017-06-19 23:36 ` [PATCH 09/23] ext4: define usercopy region in ext4_inode_cache " Kees Cook
2017-06-19 23:36 ` [PATCH 10/23] vxfs: define usercopy region in vxfs_inode " Kees Cook
2017-06-19 23:36 ` [PATCH 11/23] jfs: define usercopy region in jfs_ip " Kees Cook
2017-06-19 23:36 ` [PATCH 12/23] orangefs: define usercopy region in orangefs_inode_cache " Kees Cook
2017-06-19 23:36 ` [PATCH 13/23] ufs: define usercopy region in ufs_inode_cache " Kees Cook
2017-06-19 23:36 ` [PATCH 14/23] fork: define usercopy region in thread_stack, task_struct, mm_struct slab caches Kees Cook
2017-06-19 23:36 ` [PATCH 15/23] net: define usercopy region in struct proto slab cache Kees Cook
2017-06-19 23:36 ` [PATCH 16/23] net: copy struct sctp_sock.autoclose to userspace using put_user() Kees Cook
2017-06-19 23:36 ` [PATCH 17/23] dcache: define usercopy region in dentry_cache slab cache Kees Cook
2017-06-20 4:08 ` [kernel-hardening] " Eric Biggers
2017-06-28 16:44 ` Kees Cook
2017-06-28 16:55 ` Eric Biggers
2017-06-19 23:36 ` [PATCH 18/23] scsi: define usercopy region in scsi_sense_cache " Kees Cook
2017-06-19 23:36 ` [PATCH 19/23] xfs: define usercopy region in xfs_inode " Kees Cook
2017-06-19 23:36 ` [PATCH 20/23] usercopy: convert kmalloc caches to usercopy caches Kees Cook
2017-06-19 23:36 ` [PATCH 21/23] usercopy: Restrict non-usercopy caches to size 0 Kees Cook
2017-06-20 4:04 ` [kernel-hardening] " Eric Biggers
2017-06-28 17:03 ` Kees Cook
2017-06-19 23:36 ` [PATCH 22/23] usercopy: split user-controlled slabs to separate caches Kees Cook
2017-06-20 4:24 ` [kernel-hardening] " Eric Biggers
2017-06-20 4:47 ` Eric Biggers
2017-06-20 22:27 ` Kees Cook
2017-06-20 20:24 ` Laura Abbott
2017-06-20 22:22 ` Kees Cook
2017-06-27 7:31 ` Michal Hocko [this message]
2017-06-27 22:07 ` Kees Cook
2017-06-28 8:54 ` Michal Hocko
2017-06-19 23:36 ` [PATCH 23/23] mm: Allow slab_nomerge to be set at build time Kees Cook
2017-06-20 4:09 ` [kernel-hardening] " Daniel Micay
2017-06-20 22:51 ` Kees Cook
2017-06-20 4:29 ` Eric Biggers
2017-06-20 23:09 ` Kees Cook
2017-06-20 19:41 ` [kernel-hardening] [PATCH 00/23] Hardened usercopy whitelisting Rik van Riel
2017-10-20 22:40 ` Paolo Bonzini
2017-10-20 23:25 ` Paolo Bonzini
2017-10-21 3:04 ` Kees Cook
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