KASAN: slab-out-of-bounds Read in strcmp
Dmitry Vyukov
dvyukov at google.com
Fri Dec 8 12:22:45 UTC 2017
On Tue, Dec 5, 2017 at 11:00 AM, Dmitry Vyukov <dvyukov at google.com> wrote:
>>> > > > > > ===========================================================
>>> > > > > > =======
>>> > > > > > BUG: KASAN: slab-out-of-bounds in strcmp+0x96/0xb0
>>> > > > > > lib/string.c:328
>>> > > > > > Read of size 1 at addr ffff8801cd99d2c1 by task
>>> > > > > > syzkaller242593/3087
>>> > > > > >
>>> > > > > > CPU: 0 PID: 3087 Comm: syzkaller242593 Not tainted 4.15.0-
>>> > > > > > rc1-next-
>>> > > > > > 20171201+ #57
>>> > > > > > Hardware name: Google Google Compute Engine/Google Compute
>>> > > > > > Engine,
>>> > > > > > BIOS Google 01/01/2011
>>> > > > > > Call Trace:
>>> > > > > > __dump_stack lib/dump_stack.c:17 [inline]
>>> > > > > > dump_stack+0x194/0x257 lib/dump_stack.c:53
>>> > > > > > print_address_description+0x73/0x250 mm/kasan/report.c:252
>>> > > > > > kasan_report_error mm/kasan/report.c:351 [inline]
>>> > > > > > kasan_report+0x25b/0x340 mm/kasan/report.c:409
>>> > > > > > __asan_report_load1_noabort+0x14/0x20
>>> > > > > > mm/kasan/report.c:427
>>> > > > > > strcmp+0x96/0xb0 lib/string.c:328
>>> > > > >
>>> > > > > This seems to be out of bound read for "scontext" at
>>> > > > >
>>> > > > > for (i = 1; i < SECINITSID_NUM; i++) {
>>> > > > > if (!strcmp(initial_sid_to_string[i],
>>> > > > > scontext)) {
>>> > > > > *sid = i;
>>> > > > > return 0;
>>> > > > > }
>>> > > > > }
>>> > > > >
>>> > > > > because "initial_sid_to_string[i]" is "const char *".
>>> > > > >
>>> > > > > > security_context_to_sid_core+0x437/0x620
>>> > > > > > security/selinux/ss/services.c:1420
>>> > > > > > security_context_to_sid+0x32/0x40
>>> > > > > > security/selinux/ss/services.c:1479
>>> > > > > > selinux_setprocattr+0x51c/0xb50
>>> > > > > > security/selinux/hooks.c:5986
>>> > > > > > security_setprocattr+0x85/0xc0 security/security.c:1264
>>> > > > >
>>> > > > > If "value" does not terminate with '\0' or '\n', "value" and
>>> > > > > "size" are as-is passed to "scontext" and "scontext_len"
>>> > > > > above
>>> > > > >
>>> > > > > /* Obtain a SID for the context, if one was specified.
>>> > > > > */
>>> > > > > if (size && str[0] && str[0] != '\n') {
>>> > > > > if (str[size-1] == '\n') {
>>> > > > > str[size-1] = 0;
>>> > > > > size--;
>>> > > > > }
>>> > > > > error = security_context_to_sid(value, size,
>>> > > > > &sid,
>>> > > > > GFP_KERNEL);
>>> > > > >
>>> > > > > which will allow strcmp() to trigger out of bound read when
>>> > > > > "size" is
>>> > > > > larger than strlen(initial_sid_to_string[i]).
>>> > > > >
>>> > > > > Thus, I guess the simplest fix is to use strncmp() instead of
>>> > > > > strcmp().
>>> > > >
>>> > > > Already fixed by
>>> > > > https://www.spinics.net/lists/selinux/msg23589.html
>>> > >
>>> > >
>>> > > Paul, please also follow this part:
>>> > >
>>> > > > syzbot will keep track of this bug report.
>>> > > > Once a fix for this bug is committed, please reply to this
>>> > > > email with:
>>> > > > #syz fix: exact-commit-title
>>> > > > Note: all commands must start from beginning of the line in the
>>> > > > email body.
>>> > >
>>> > > This will greatly help to keep overall process running. Thanks.
>>> >
>>> > When is the right time to do this? The text say to reply when a
>>> > patch
>>> > has been committed, but where? My selinux/next branch? Linus'
>>> > master? Your docs and the end of the email needs to be more clear
>>> > on
>>> > this.
>>> >
>>> > For the record, I did see that part of the syzbot mail but I was
>>> > waiting until I merged that patch; v2 was posted late in the week
>>> > and
>>> > I was giving it a few days in case someone saw something
>>> > objectionable.
>>> >
>>> > Also, while we are on the topic of syzbot, what SELinux policy (if
>>> > any) do you load on the system that is undergoing testing? Based
>>> > on
>>> > some of the recent reports it would appear that you are running a
>>> > SELinux enabled kernel but might not be loading a SELinux policy,
>>> > is
>>> > that correct?
>>>
>>>
>>> This is good question. The problem is that we are testing almost all
>>> kernel subsystems, but are not experts in most of them. So frequently
>>> we doing some non-sense. And that's where we need you help.
>>> That's what we have for grep SECURITY .config:
>>>
>>> CONFIG_EXT4_FS_SECURITY=y
>>> # CONFIG_9P_FS_SECURITY is not set
>>> # CONFIG_SECURITY_DMESG_RESTRICT is not set
>>> CONFIG_SECURITY=y
>>> CONFIG_SECURITY_WRITABLE_HOOKS=y
>>> # CONFIG_SECURITYFS is not set
>>> CONFIG_SECURITY_NETWORK=y
>>> CONFIG_SECURITY_NETWORK_XFRM=y
>>> CONFIG_SECURITY_PATH=y
>>> CONFIG_SECURITY_SELINUX=y
>>> CONFIG_SECURITY_SELINUX_BOOTPARAM=y
>>> CONFIG_SECURITY_SELINUX_BOOTPARAM_VALUE=1
>>> CONFIG_SECURITY_SELINUX_DISABLE=y
>>> CONFIG_SECURITY_SELINUX_DEVELOP=y
>>> CONFIG_SECURITY_SELINUX_AVC_STATS=y
>>> CONFIG_SECURITY_SELINUX_CHECKREQPROT_VALUE=0
>>> # CONFIG_SECURITY_SMACK is not set
>>> # CONFIG_SECURITY_TOMOYO is not set
>>> # CONFIG_SECURITY_APPARMOR is not set
>>> # CONFIG_SECURITY_LOADPIN is not set
>>> # CONFIG_SECURITY_YAMA is not set
>>> CONFIG_DEFAULT_SECURITY_SELINUX=y
>>> # CONFIG_DEFAULT_SECURITY_DAC is not set
>>> CONFIG_DEFAULT_SECURITY="selinux"
>>>
>>>
>>> I don't think we do anything else besides that.
>>> To be fair I don't know what is SELinux policy nor how to load it.
>>> Can
>>> you suggest a policy that would be good for testing of kernel in
>>> general and SELinux in particular? Obviously, we don't want to
>>> prohibit access to any major parts of kernel APIs entirely (because
>>> then we won't test them). syzkaller also creates a local temp dir per
>>> test process, and the process mostly accesses files there (except for
>>> opening /dev/* and /proc/self/*). Is it possible to selectively
>>> prohibit something there, so that we test both positive and negative
>>> cases?
>>
>> SELinux policy is loaded by userspace; the support is integrated into
>> the various init programs, but you need to have a policy installed.
>>
>> Best way would just be to run the test on Fedora (or CentOS) with
>> selinux-policy-targeted installed. Then you would be testing with a
>> real policy loaded. You could run syzkaller from the unconfined_t
>> domain and it should be largely unimpeded, and it could transition to a
>> different domain if you want to test denials. For reference, the
>> selinux testsuite itself is at
>> https://github.com/SELinuxProject/selinux-testsuite/
>> It includes a defconfig fragment that can be merged, although portions
>> of that are only required for particular test programs (see the
>> comments in it).
>>
>> If that's not viable, then another approach would be to generate a
>> minimal allow-all policy from the kernel source tree, see
>> Documentation/admin-guide/LSM/SELinux.rst and scripts/selinux/*.
>> The downside of that approach is that SELinux developers and users
>> don't use that policy themselves for anything, and it won't exercise
>> any permission denial code paths.
>
>
> Thanks, I will see what we can do here. Still lots to learn for me.
I figured out why selinux wasn't initialized in our setup. The image
missed selinux packages, so init could not initialize it. I've added
these packages:
https://github.com/google/syzkaller/commit/c0e5b8c81f054a301aeaaa16bf9c6220ba73d833
and now sestatus says that selinux is initialized.
I've also added some very basic description of selinux interface:
https://github.com/google/syzkaller/commit/fadd10ac0525a437fc32e412327bdf11587f4946#diff-517fae0da2ea50677a86c092cf96781e
Not enough to meaningfully test selinux, though.
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