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SubjectRe: Does CONFIG_HARDENED_USERCOPY break /dev/mem?
Am Fri, 10 Nov 2017 10:46:49 -0800
schrieb Kees Cook <keescook@chromium.org>:

> On Fri, Nov 10, 2017 at 7:45 AM, Michael Holzheu
> <holzheu@linux.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?

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.

Michael

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