We must properly handle an errors when we increase the rlimit counter
and the ucounts reference counter. We have to this with RCU protection
to prevent possible use-after-free that could occur due to concurrent
put_cred_rcu().

The following reproducer triggers the problem:

  $ cat testcase.sh
  case "${STEP:-0}" in
  0)
	ulimit -Si 1
	ulimit -Hi 1
	STEP=1 unshare -rU "$0"
	killall sleep
	;;
  1)
	for i in 1 2 3 4 5; do unshare -rU sleep 5 & done
	;;
  esac

with the KASAN report being along the lines of

  BUG: KASAN: use-after-free in put_ucounts+0x17/0xa0
  Write of size 4 at addr ffff8880045f031c by task swapper/2/0

  CPU: 2 PID: 0 Comm: swapper/2 Not tainted 5.13.0+ #19
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.14.0-alt4 04/01/2014
  Call Trace:
   <IRQ>
   put_ucounts+0x17/0xa0
   put_cred_rcu+0xd5/0x190
   rcu_core+0x3bf/0xcb0
   __do_softirq+0xe3/0x341
   irq_exit_rcu+0xbe/0xe0
   sysvec_apic_timer_interrupt+0x6a/0x90
   </IRQ>
   asm_sysvec_apic_timer_interrupt+0x12/0x20
   default_idle_call+0x53/0x130
   do_idle+0x311/0x3c0
   cpu_startup_entry+0x14/0x20
   secondary_startup_64_no_verify+0xc2/0xcb

  Allocated by task 127:
   kasan_save_stack+0x1b/0x40
   __kasan_kmalloc+0x7c/0x90
   alloc_ucounts+0x169/0x2b0
   set_cred_ucounts+0xbb/0x170
   ksys_unshare+0x24c/0x4e0
   __x64_sys_unshare+0x16/0x20
   do_syscall_64+0x37/0x70
   entry_SYSCALL_64_after_hwframe+0x44/0xae

  Freed by task 0:
   kasan_save_stack+0x1b/0x40
   kasan_set_track+0x1c/0x30
   kasan_set_free_info+0x20/0x30
   __kasan_slab_free+0xeb/0x120
   kfree+0xaa/0x460
   put_cred_rcu+0xd5/0x190
   rcu_core+0x3bf/0xcb0
   __do_softirq+0xe3/0x341

  The buggy address belongs to the object at ffff8880045f0300
   which belongs to the cache kmalloc-192 of size 192
  The buggy address is located 28 bytes inside of
   192-byte region [ffff8880045f0300, ffff8880045f03c0)
  The buggy address belongs to the page:
  page:000000008de0a388 refcount:1 mapcount:0 mapping:0000000000000000 index:0xffff8880045f0000 pfn:0x45f0
  flags: 0x100000000000200(slab|node=0|zone=1)
  raw: 0100000000000200 ffffea00000f4640 0000000a0000000a ffff888001042a00
  raw: ffff8880045f0000 000000008010000d 00000001ffffffff 0000000000000000
  page dumped because: kasan: bad access detected

  Memory state around the buggy address:
   ffff8880045f0200: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
   ffff8880045f0280: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
  >ffff8880045f0300: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                              ^
   ffff8880045f0380: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
   ffff8880045f0400: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
  ==================================================================
  Disabling lock debugging due to kernel taint

Fixes: d6469690 ("Reimplement RLIMIT_SIGPENDING on top of ucounts")
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Alexey Gladkov <legion@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>