Messages in this thread | | | From | Kees Cook <> | Date | Fri, 10 Nov 2017 10:46:49 -0800 | Subject | Re: Does CONFIG_HARDENED_USERCOPY break /dev/mem? |
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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? Normally I'd expect any system built with secure configurations to want strict devmem.
-Kees
-- Kees Cook Pixel Security
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