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Date: Fri, 10 Nov 2017 10:46:49 -0800
From: Kees Cook <keescook@...omium.org>
To: Michael Holzheu <holzheu@...ux.vnet.ibm.com>,
Tycho Andersen <tycho@...ker.com>
Cc: Arnd Bergmann <arnd@...db.de>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
LKML <linux-kernel@...r.kernel.org>,
Heiko Carstens <heiko.carstens@...ibm.com>,
Martin Schwidefsky <schwidefsky@...ibm.com>,
Vasily Gorbik <gor@...ux.vnet.ibm.com>
Subject: Re: Does CONFIG_HARDENED_USERCOPY break /dev/mem?
On Fri, Nov 10, 2017 at 7:45 AM, Michael Holzheu
<holzheu@...ux.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? Normally I'd expect any system built with secure
configurations to want strict devmem.
-Kees
--
Kees Cook
Pixel Security
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