Re: Does CONFIG_HARDENED_USERCOPY break /dev/mem?

[Kees Cook]

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? Normally I'd expect any system built with secure
configurations to want strict devmem.

-Kees

-- 
Kees Cook
Pixel Security