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SubjectRe: kernel BUG at kernel/cred.c:434!
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On 2019/4/19 21:24, Paul Moore wrote:
> On Thu, Apr 18, 2019 at 10:42 PM Yang Yingliang
> <yangyingliang@huawei.com> wrote:
>> On 2019/4/19 10:04, Paul Moore wrote:
>>> On Wed, Apr 17, 2019 at 10:50 PM Yang Yingliang
>>> <yangyingliang@huawei.com> wrote:
>>>> On 2019/4/18 8:24, Casey Schaufler wrote:
>>>>> On 4/17/2019 4:39 PM, Paul Moore wrote:
>>>>>> Since it looks like all three LSMs which implement the setprocattr
>>>>>> hook are vulnerable I'm open to the idea that proc_pid_attr_write() is
>>>>>> a better choice for the cred != read_cred check, but I would want to
>>>>>> make sure John and Casey are okay with that.
>>>>>>
>>>>>> John?
>>>>>>
>>>>>> Casey?
>>>>> I'm fine with the change going into proc_pid_attr_write().
>>>> The cred != real_cred checking is not enough.
>>>>
>>>> Consider this situation, when doing override, cred, real_cred and
>>>> new_cred are all same:
>>>>
>>>> after override_creds() cred == real_cred == new1_cred
>>> I'm sorry, you've lost me. After override_creds() returns
>>> current->cred and current->real_cred are not going to be the same,
>>> yes?
>> It's possible the new cred is equal to current->real_cred and
>> current->cred,
>> so after overrides_creds(), they have the same value.
> Both task_struct.cred and task_struct.real_cred are pointer values,
> assuming that one uses prepare_creds() to allocate/initialize a new
> cred struct for use with override_creds() then the newly created cred
> should never be equal to task_struct.real_cred. Am I missing
> something, or are you thinking of something else?
In do_acct_process(), file->f_cred may equal to current->real_cred, I
confirm
it by adding some debug message in do_acct_process() like this:

--- a/kernel/acct.c
+++ b/kernel/acct.c
@@ -481,6 +481,7 @@ static void do_acct_process(struct bsd_acct_struct
*acct)
flim = current->signal->rlim[RLIMIT_FSIZE].rlim_cur;
current->signal->rlim[RLIMIT_FSIZE].rlim_cur = RLIM_INFINITY;
/* Perform file operations on behalf of whoever enabled
accounting */
+ pr_info("task:%px new cred:%px real cred:%px cred:%px\n",
current, file->f_cred, current->real_cred, current->cred);
orig_cred = override_creds(file->f_cred);


Messages:
[ 56.643298] task:ffff88841a9595c0 new cred:ffff88841ae450c0 real
cred:ffff88841ae450c0 cred:ffff88841ae450c0 //They are same.
[ 56.646609] Process accounting resumed
[ 56.649943] task:ffff88841a9595c0 new cred:ffff88841ae450c0 real
cred:ffff88841c96c300 cred:ffff88841ae450c0
[ 56.653565] ------------[ cut here ]------------
[ 56.655119] kernel BUG at kernel/cred.c:434!
[ 56.656590] invalid opcode: 0000 [#1] SMP PTI
[ 56.658033] CPU: 2 PID: 4169 Comm: syz-executor.15 Not tainted
5.1.0-rc4-00034-g869e3305f23d-dirty #143
[ 56.661077] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
BIOS rel-1.12.1-0-ga5cab58e9a3f-prebuilt.qemu.org 04/01/2014
[ 56.664895] RIP: 0010:commit_creds+0x1eb/0x230
[ 56.666344] Code: 43 1c 0f 85 08 ff ff ff e9 10 ff ff ff 8b 45 10 39
43 10 0f 85 18 ff ff ff 8b 43 20 39 45 20 0f 85 0c ff ff ff e9 14 ff ff
ff <0f> 0b 48 c7 c7 d0 d2 49 82 e8 17 3b 3e 00 0f 0b 48 c7 c7 c0 d2 49
[ 56.672410] RSP: 0018:ffffc90003a17b20 EFLAGS: 00010287
[ 56.674098] RAX: ffff88841a9595c0 RBX: ffff88841ae450c0 RCX:
0000000000000000
[ 56.676410] RDX: 0000000000000001 RSI: 0000000000000020 RDI:
ffff88841c96ce40
[ 56.678691] RBP: 0000000000000001 R08: 0000000000800000 R09:
0000000000000000
[ 56.680997] R10: ffff88841c9265a0 R11: ffffffff810d6940 R12:
ffff88841a9595c0
[ 56.681198] task:ffff88841a9195c0 new cred:ffff88841aeaa0c0 real
cred:ffff88841aeaa0c0 cred:ffff88841aeaa0c0
[ 56.683293] R13: 0000000000000040 R14: ffff88841c96ce40 R15:
0000000000000040
[ 56.683296] FS: 00007f5969a5c700(0000) GS:ffff88842fa80000(0000)
knlGS:0000000000000000
[ 56.683297] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 56.683299] CR2: 00007f82742214f0 CR3: 000000041cbc0005 CR4:
00000000000206e0
[ 56.683305] Call Trace:
[ 56.683340] selinux_setprocattr+0x17b/0x480
[ 56.686513] Process accounting resumed
[ 56.688849] proc_pid_attr_write+0xc0/0xf0
[ 56.688857] __kernel_write+0x4f/0xf0
[ 56.688866] do_acct_process+0x538/0x750
[ 56.703090] ? __schedule+0x290/0x960
[ 56.704311] ? __queue_work+0x160/0x5c0
[ 56.705571] acct_pin_kill+0x1e/0x70
[ 56.706743] pin_kill+0x81/0x150
[ 56.707813] ? finish_wait+0x80/0x80
[ 56.708985] mnt_pin_kill+0x1e/0x30
[ 56.710127] cleanup_mnt+0x6e/0x70
[ 56.711247] task_work_run+0x8a/0xb0
[ 56.712453] do_exit+0x2e0/0xc80
[ 56.713525] do_group_exit+0x33/0xb0
[ 56.714701] get_signal+0x143/0x810
[ 56.715865] do_signal+0x36/0x610
[ 56.716962] ? __x64_sys_futex+0x134/0x180
[ 56.718307] ? _copy_to_user+0x22/0x30
[ 56.719606] exit_to_usermode_loop+0x80/0xe0
[ 56.721003] do_syscall_64+0x16c/0x180
[ 56.722242] entry_SYSCALL_64_after_hwframe+0x44/0xa9


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