linux-kernel - Does CONFIG_HARDENED_USERCOPY break /dev/mem?

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Date:   Fri, 10 Nov 2017 16:45:29 +0100
From:   Michael Holzheu <holzheu@...ux.vnet.ibm.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Vasily Gorbik <gor@...ux.vnet.ibm.com>
Subject: Does CONFIG_HARDENED_USERCOPY break /dev/mem?

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?

Michael