We need to increment the ucounts reference counter befor security_prepare_creds()
because this function may fail and abort_creds() will try to decrement
this reference.

[   96.465056][ T8641] FAULT_INJECTION: forcing a failure.
[   96.465056][ T8641] name fail_page_alloc, interval 1, probability 0, space 0, times 0
[   96.478453][ T8641] CPU: 1 PID: 8641 Comm: syz-executor668 Not tainted 5.14.0-rc6-syzkaller #0
[   96.487215][ T8641] Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
[   96.497254][ T8641] Call Trace:
[   96.500517][ T8641]  dump_stack_lvl+0x1d3/0x29f
[   96.505758][ T8641]  ? show_regs_print_info+0x12/0x12
[   96.510944][ T8641]  ? log_buf_vmcoreinfo_setup+0x498/0x498
[   96.516652][ T8641]  should_fail+0x384/0x4b0
[   96.521141][ T8641]  prepare_alloc_pages+0x1d1/0x5a0
[   96.526236][ T8641]  __alloc_pages+0x14d/0x5f0
[   96.530808][ T8641]  ? __rmqueue_pcplist+0x2030/0x2030
[   96.536073][ T8641]  ? lockdep_hardirqs_on_prepare+0x3e2/0x750
[   96.542056][ T8641]  ? alloc_pages+0x3f3/0x500
[   96.546635][ T8641]  allocate_slab+0xf1/0x540
[   96.551120][ T8641]  ___slab_alloc+0x1cf/0x350
[   96.555689][ T8641]  ? kzalloc+0x1d/0x30
[   96.559740][ T8641]  __kmalloc+0x2e7/0x390
[   96.563980][ T8641]  ? kzalloc+0x1d/0x30
[   96.568029][ T8641]  kzalloc+0x1d/0x30
[   96.571903][ T8641]  security_prepare_creds+0x46/0x220
[   96.577174][ T8641]  prepare_creds+0x411/0x640
[   96.581747][ T8641]  __sys_setfsuid+0xe2/0x3a0
[   96.586333][ T8641]  do_syscall_64+0x3d/0xb0
[   96.590739][ T8641]  entry_SYSCALL_64_after_hwframe+0x44/0xae
[   96.596611][ T8641] RIP: 0033:0x445a69
[   96.600483][ T8641] Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 11 15 00 00 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 b8 ff ff ff f7 d8 64 89 01 48
[   96.620152][ T8641] RSP: 002b:00007f1054173318 EFLAGS: 00000246 ORIG_RAX: 000000000000007a
[   96.628543][ T8641] RAX: ffffffffffffffda RBX: 00000000004ca4c8 RCX: 0000000000445a69
[   96.636600][ T8641] RDX: 0000000000000010 RSI: 00007f10541732f0 RDI: 0000000000000000
[   96.644550][ T8641] RBP: 00000000004ca4c0 R08: 0000000000000001 R09: 0000000000000000
[   96.652500][ T8641] R10: 0000000000000000 R11: 0000000000000246 R12: 00000000004ca4cc
[   96.660631][ T8641] R13: 00007fffffe0b62f R14: 00007f1054173400 R15: 0000000000022000

Fixes: 905ae01c

 ("Add a reference to ucounts for each cred")
Reported-by:  <syzbot+01985d7909f9468f013c@syzkaller.appspotmail.com>
Signed-off-by: Alexey Gladkov <legion@kernel.org>
Link: https://lkml.kernel.org/r/97433b1742c3331f02ad92de5a4f07d673c90613.1629735352.git.legion@kernel.org


Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>