lkml.org 
[lkml]   [2017]   [Nov]   [10]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectDoes 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

\
 
 \ /
  Last update: 2017-11-10 16:46    [W:0.045 / U:48.680 seconds]
©2003-2017 Jasper Spaans. hosted at Digital OceanAdvertise on this site