Fix UCOUNT_RLIMIT_SIGPENDING counter leak

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>

kernel/signal.c

@@ -426,18 +426,30 @@ __sigqueue_alloc(int sig, struct task_struct *t, gfp_t gfp_flags,
 	rcu_read_lock();
 	ucounts = task_ucounts(t);
 	sigpending = inc_rlimit_ucounts(ucounts, UCOUNT_RLIMIT_SIGPENDING, 1);
-	if (sigpending == 1)
-		ucounts = get_ucounts(ucounts);
+	switch (sigpending) {
+	case 1:
+		if (likely(get_ucounts(ucounts)))
+			break;
+		fallthrough;
+	case LONG_MAX:
+		/*
+		 * we need to decrease the ucount in the userns tree on any
+		 * failure to avoid counts leaking.
+		 */
+		dec_rlimit_ucounts(ucounts, UCOUNT_RLIMIT_SIGPENDING, 1);
+		rcu_read_unlock();
+		return NULL;
+	}
 	rcu_read_unlock();
 
-	if (override_rlimit || (sigpending < LONG_MAX && sigpending <= task_rlimit(t, RLIMIT_SIGPENDING))) {
+	if (override_rlimit || likely(sigpending <= task_rlimit(t, RLIMIT_SIGPENDING))) {
 		q = kmem_cache_alloc(sigqueue_cachep, gfp_flags);
 	} else {
 		print_dropped_signal(sig);
 	}
 
 	if (unlikely(q == NULL)) {
-		if (ucounts && dec_rlimit_ucounts(ucounts, UCOUNT_RLIMIT_SIGPENDING, 1))
+		if (dec_rlimit_ucounts(ucounts, UCOUNT_RLIMIT_SIGPENDING, 1))
 			put_ucounts(ucounts);
 	} else {
 		INIT_LIST_HEAD(&q->list);