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SubjectRe: Does CONFIG_HARDENED_USERCOPY break /dev/mem?
Am Wed, 22 Nov 2017 09:43:19 -0800
schrieb Kees Cook <keescook@chromium.org>:

> On Wed, Nov 22, 2017 at 1:28 AM, Michael Holzheu
> <holzheu@linux.vnet.ibm.com> wrote:
> > Am Mon, 13 Nov 2017 11:19:38 +0100
> > schrieb Michael Holzheu <holzheu@linux.vnet.ibm.com>:
> >
> >> Am Fri, 10 Nov 2017 10:46:49 -0800
> >> schrieb Kees Cook <keescook@chromium.org>:
> >>
> >> > On Fri, Nov 10, 2017 at 7:45 AM, Michael Holzheu
> >> > <holzheu@linux.vnet.ibm.com> wrote:
> >> > > Hello Kees,
> >> > >
> >> > > When I try to run the crash tool on my s390 live system I get a kernel panic
> >> > > when reading memory within the kernel image:
> >> > >
> >> > > # uname -a
> >> > > Linux r3545011 4.14.0-rc8-00066-g1c9dbd4615fd #45 SMP PREEMPT Fri Nov 10 16:16:22 CET 2017 s390x s390x s390x GNU/Linux
> >> > > # crash /boot/vmlinux-devel /dev/mem
> >> > > # crash> rd 0x100000
> >> > >
> >> > > usercopy: kernel memory exposure attempt detected from 0000000000100000 (<kernel text>) (8 bytes)
> >> > > ------------[ cut here ]------------
> >> > > kernel BUG at mm/usercopy.c:72!
> >> > > illegal operation: 0001 ilc:1 [#1] PREEMPT SMP.
> >> > > Modules linked in:
> >> > > CPU: 0 PID: 1461 Comm: crash Not tainted 4.14.0-rc8-00066-g1c9dbd4615fd-dirty #46
> >> > > Hardware name: IBM 2827 H66 706 (z/VM 6.3.0)
> >> > > task: 000000001ad10100 task.stack: 000000001df78000
> >> > > Krnl PSW : 0704d00180000000 000000000038165c (__check_object_size+0x164/0x1d0)
> >> > > R:0 T:1 IO:1 EX:1 Key:0 M:1 W:0 P:0 AS:3 CC:1 PM:0 RI:0 EA:3
> >> > > Krnl GPRS: 0000000012440e1d 0000000080000000 0000000000000061 00000000001cabc0
> >> > > 00000000001cc6d6 0000000000000000 0000000000cc4ed2 0000000000001000
> >> > > 000003ffc22fdd20 0000000000000008 0000000000100008 0000000000000001
> >> > > 0000000000000008 0000000000100000 0000000000381658 000000001df7bc90
> >> > > Krnl Code: 000000000038164c: c020004a1c4a larl %r2,cc4ee0
> >> > > 0000000000381652: c0e5fff2581b brasl %r14,1cc688
> >> > > #0000000000381658: a7f40001 brc 15,38165a
> >> > > >000000000038165c: eb42000c000c srlg %r4,%r2,12
> >> > > 0000000000381662: eb32001c000c srlg %r3,%r2,28
> >> > > 0000000000381668: c0110003ffff lgfi %r1,262143
> >> > > 000000000038166e: ec31ff752065 clgrj %r3,%r1,2,381558
> >> > > 0000000000381674: a7f4ff67 brc 15,381542
> >> > > Call Trace:
> >> > > ([<0000000000381658>] __check_object_size+0x160/0x1d0)
> >> > > [<000000000082263a>] read_mem+0xaa/0x130.
> >> > > [<0000000000386182>] __vfs_read+0x42/0x168.
> >> > > [<000000000038632e>] vfs_read+0x86/0x140.
> >> > > [<0000000000386a26>] SyS_read+0x66/0xc0.
> >> > > [<0000000000ace6a4>] system_call+0xc4/0x2b0.
> >> > > INFO: lockdep is turned off.
> >> > > Last Breaking-Event-Address:
> >> > > [<0000000000381658>] __check_object_size+0x160/0x1d0
> >> > >
> >> > > Kernel panic - not syncing: Fatal exception: panic_on_oops
> >> > >
> >> > > With CONFIG_HARDENED_USERCOPY copy_to_user() checks in __check_object_size()
> >> > > if the source address is within the kernel image:
> >> > >
> >> > > - __check_object_size() -> check_kernel_text_object():
> >> > >
> >> > > /* Is this address range in the kernel text area? */
> >> > > static inline const char *check_kernel_text_object(const void *ptr,
> >> > > unsigned long n)
> >> > > {
> >> > > unsigned long textlow = (unsigned long)_stext;
> >> > > unsigned long texthigh = (unsigned long)_etext;
> >> > > unsigned long textlow_linear, texthigh_linear;
> >> > >
> >> > > if (overlaps(ptr, n, textlow, texthigh))
> >> > > return "<kernel text>";
> >> > >
> >> > > When the crash tool reads from 0x100000, this check leads to the kernel BUG()
> >> > > in drivers/char/mem.c:
> >> > >
> >> > > 144 } else {
> >> > > 145 /*
> >> > > 146 * On ia64 if a page has been mapped somewhere as
> >> > > 147 * uncached, then it must also be accessed uncached
> >> > > 148 * by the kernel or data corruption may occur.
> >> > > 149 */
> >> > > 150 ptr = xlate_dev_mem_ptr(p);
> >> > > 151 if (!ptr)
> >> > > 152 return -EFAULT;
> >> > > 153
> >> > > 154 remaining = copy_to_user(buf, ptr, sz); <<<---- BUG
> >> > > 155
> >> > > 156 unxlate_dev_mem_ptr(p, ptr);
> >> > > 157 }
> >> > >
> >> > > Here the reporting function in mm/usercopy.c:
> >> > >
> >> > > 61 static void report_usercopy(const void *ptr, unsigned long len,
> >> > > 62 bool to_user, const char *type)
> >> > > 63 {
> >> > > 64 pr_emerg("kernel memory %s attempt detected %s %p (%s) (%lu bytes)\n",
> >> > > 65 to_user ? "exposure" : "overwrite",
> >> > > 66 to_user ? "from" : "to", ptr, type ? : "unknown", len);
> >> > > 67 /*
> >> > > 68 * For greater effect, it would be nice to do do_group_exit(),
> >> > > 69 * but BUG() actually hooks all the lock-breaking and per-arch
> >> > > 70 * Oops code, so that is used here instead.
> >> > > 71 */
> >> > > 72 BUG();
> >> > > 73 }
> >> > >
> >> > > Shouldn't we skip the kernel address check for /dev/mem - at least when
> >> > > CONFIG_STRICT_DEVMEM is not enabled?
> >> >
> >> > Some kind of better interaction is needed here, I agree. The prior
> >> > discussions around this basically resulted in declaring that
> >> > HARDENED_USERCOPY without STRICT_DEVMEM didn't make a lot of sense.
> >> > i.e. HARDENED_USERCOPY should maybe require STRICT_DEVMEM, etc. Tycho
> >> > wrote this up after some back-and-forth:
> >> >
> >> > https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git/commit/?h=kspp/kconfig&id=ae98b44ceb338ae165a7f18f29f6244893712da3
> >> >
> >> > In the end, I was still uncomfortable with it because the usercopy
> >> > protection is wider than just the kernel text, so it seemed strange to
> >> > force it off when not using STRICT_DEVMEM.
> >> >
> >> > What's the use-case here where you've got hardened usercopy without
> >> > strict devmem?
> >>
> >> We use that configuration for development and test. We disabled STRICT_DEVMEM
> >> for debugging the live system with crash. We enabled HARDENED_USERCOPY for
> >> finding user-copy bugs.
> >
> > So what's your plan now? How will you fix this issue?
>
> I think the best plan here would be to use the Kconfig "imply
> STRICT_DEVMEM" in HARDENED_USERCOPY. That would make STRICT_DEVMEM
> enabled by default but still configurable. Then the kernel-text check
> in hardened usercopy could be skip when !STRICT_DEVMEM.
>
> My primary concern is with failing closed. If someone is only paying
> attention to choosing HARDENED_USERCOPY, it should not be possible to
> read out kernel memory unless they specifically try to unset something
> else (in this case, STRICT_DEVMEM).
>
> How does that sound?

Looks ok to me.

Michael

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