Re: kernel BUG at kernel/cred.c:434!

[Yang Yingliang]

On 2019/4/19 21:24, Paul Moore wrote:
> On Thu, Apr 18, 2019 at 10:42 PM Yang Yingliang
> <yangyingliang@xxxxxxxxxx> wrote:
> > On 2019/4/19 10:04, Paul Moore wrote:
> > > On Wed, Apr 17, 2019 at 10:50 PM Yang Yingliang
> > > <yangyingliang@xxxxxxxxxx> 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