Re: KASAN: slab-out-of-bounds Read in strcmp

From: Dmitry Vyukov
Date: Tue Dec 05 2017 - 05:01:46 EST

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On Mon, Dec 4, 2017 at 6:33 PM, Stephen Smalley <sds@xxxxxxxxxxxxx> wrote:
> On Mon, 2017-12-04 at 17:39 +0100, Dmitry Vyukov wrote:
>> On Mon, Dec 4, 2017 at 2:59 PM, Paul Moore <pmoore@xxxxxxxxxx> wrote:
>> > > > > On 2017/12/02 3:52, syzbot 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.

Next message: Gary Lin: "[RFC v3 PATCH 0/2] Introduce Security Version to EFI Stub"
Previous message: Paul Menzel: "net/wireless/shipped-certs.c:2:1: error: expected expression at end of input"
In reply to: Stephen Smalley: "Re: KASAN: slab-out-of-bounds Read in strcmp"
Next in thread: Tetsuo Handa: "Re: KASAN: slab-out-of-bounds Read in strcmp"
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