Re: Does CONFIG_HARDENED_USERCOPY break /dev/mem?

[Michael Holzheu]

Am Wed, 22 Nov 2017 09:43:19 -0800
schrieb Kees Cook <keescook@xxxxxxxxxxxx>:

> On Wed, Nov 22, 2017 at 1:28 AM, Michael Holzheu
> <holzheu@xxxxxxxxxxxxxxxxxx> wrote:
> > Am Mon, 13 Nov 2017 11:19:38 +0100
> > schrieb Michael Holzheu <holzheu@xxxxxxxxxxxxxxxxxx>:
> >
> >> Am Fri, 10 Nov 2017 10:46:49 -0800
> >> schrieb Kees Cook <keescook@xxxxxxxxxxxx>:
> >>
> >> > On Fri, Nov 10, 2017 at 7:45 AM, Michael Holzheu
> >> > <holzheu@xxxxxxxxxxxxxxxxxx> 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