[PATCH v2] Fix UCOUNT_RLIMIT_SIGPENDING counter leak

Alexey Gladkov
2021-07-08T10:34:12Z

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

[ 84.670919] ==================================================================
[ 84.673326] BUG: KASAN: use-after-free in put_ucounts+0x17/0xa0
[ 84.674983] Write of size 4 at addr ffff8880045f031c by task swapper/2/0
[ 84.676702]
[ 84.677119] CPU: 2 PID: 0 Comm: swapper/2 Not tainted 5.13.0+ #19
[ 84.678920] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.14.0-alt4 04/01/2014
[ 84.683128] Call Trace:
[ 84.683950] <IRQ>
[ 84.684635] dump_stack+0x8a/0xb5
[ 84.685751] print_address_description.constprop.0+0x18/0x130
[ 84.687633] ? put_ucounts+0x17/0xa0
[ 84.688831] ? put_ucounts+0x17/0xa0
[ 84.690050] kasan_report.cold+0x7f/0x111
[ 84.691413] ? put_ucounts+0x17/0xa0
[ 84.692618] kasan_check_range+0xf9/0x1e0
[ 84.694039] put_ucounts+0x17/0xa0
[ 84.695237] put_cred_rcu+0xd5/0x190
[ 84.696998] rcu_core+0x3bf/0xcb0
[ 84.699692] ? note_gp_changes+0x90/0x90
[ 84.700938] ? recalibrate_cpu_khz+0x10/0x10
[ 84.702575] ? lapic_timer_set_periodic+0x30/0x30
[ 84.704152] ? clockevents_program_event+0xd3/0x130
[ 84.705754] ? hrtimer_interrupt+0x418/0x440
[ 84.707198] __do_softirq+0xe3/0x341
[ 84.708412] irq_exit_rcu+0xbe/0xe0
[ 84.709572] sysvec_apic_timer_interrupt+0x6a/0x90
[ 84.711235] </IRQ>
[ 84.711972] asm_sysvec_apic_timer_interrupt+0x12/0x20
[ 84.713661] RIP: 0010:default_idle+0xb/0x10
[ 84.715070] Code: ff f0 80 63 02 df 5b 41 5c c3 0f ae f0 0f ae 3b 0f ae f0 eb 90 66 2e 0f 1f 84 00 00 00 00 00 eb 07 0f 00 2d 37 7d 5a 5
[ 84.721579] RSP: 0018:ffffc900000dfe80 EFLAGS: 00000202
[ 84.723330] RAX: ffffffffad279280 RBX: ffff8880013f3e00 RCX: ffffffffad269c56
[ 84.725724] RDX: 000000000001a8c6 RSI: 0000000000000004 RDI: ffff8880361322a0
[ 84.728085] RBP: 0000000000000002 R08: 0000000000000001 R09: ffff8880361322a3
[ 84.730485] R10: ffffed1006c26454 R11: 0000000000000001 R12: 0000000000000002
[ 84.732814] R13: 0000000000000000 R14: 0000000000000000 R15: 1ffff9200001bfd6
[ 84.736325] ? mwait_idle+0xc0/0xc0
[ 84.737488] ? rcu_eqs_enter.constprop.0+0x86/0xa0
[ 84.739085] default_idle_call+0x53/0x130
[ 84.740410] do_idle+0x311/0x3c0
[ 84.741512] ? _raw_spin_lock_irqsave+0x7b/0xd0
[ 84.743060] ? arch_cpu_idle_exit+0x30/0x30
[ 84.744442] ? swake_up_locked+0x6d/0x80
[ 84.745673] cpu_startup_entry+0x14/0x20
[ 84.746657] secondary_startup_64_no_verify+0xc2/0xcb
[ 84.747746]
[ 84.748073] Allocated by task 127:
[ 84.748742] kasan_save_stack+0x1b/0x40
[ 84.750483] __kasan_kmalloc+0x7c/0x90
[ 84.751359] alloc_ucounts+0x169/0x2b0
[ 84.752270] set_cred_ucounts+0xbb/0x170
[ 84.753266] ksys_unshare+0x24c/0x4e0
[ 84.754194] __x64_sys_unshare+0x16/0x20
[ 84.755332] do_syscall_64+0x37/0x70
[ 84.756503] entry_SYSCALL_64_after_hwframe+0x44/0xae
[ 84.758106]
[ 84.758625] Freed by task 0:
[ 84.759589] kasan_save_stack+0x1b/0x40
[ 84.760841] kasan_set_track+0x1c/0x30
[ 84.762082] kasan_set_free_info+0x20/0x30
[ 84.763421] __kasan_slab_free+0xeb/0x120
[ 84.764736] kfree+0xaa/0x460
[ 84.765722] put_cred_rcu+0xd5/0x190
[ 84.766925] rcu_core+0x3bf/0xcb0
[ 84.768074] __do_softirq+0xe3/0x341
[ 84.769262]
[ 84.769765] The buggy address belongs to the object at ffff8880045f0300
[ 84.769765] which belongs to the cache kmalloc-192 of size 192
[ 84.773675] The buggy address is located 28 bytes inside of
[ 84.773675] 192-byte region [ffff8880045f0300, ffff8880045f03c0)
[ 84.778876] The buggy address belongs to the page:
[ 84.780338] page:000000008de0a388 refcount:1 mapcount:0 mapping:0000000000000000 index:0xffff8880045f0000 pfn:0x45f0
[ 84.782844] flags: 0x100000000000200(slab|node=0|zone=1)
[ 84.784640] raw: 0100000000000200 ffffea00000f4640 0000000a0000000a ffff888001042a00
[ 84.787229] raw: ffff8880045f0000 000000008010000d 00000001ffffffff 0000000000000000
[ 84.789685] page dumped because: kasan: bad access detected
[ 84.791511]
[ 84.792037] Memory state around the buggy address:
[ 84.793590] ffff8880045f0200: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
[ 84.795920] ffff8880045f0280: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
[ 84.798252] >ffff8880045f0300: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
[ 84.800636] ^
[ 84.801953] ffff8880045f0380: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
[ 84.804261] ffff8880045f0400: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
[ 84.806622] ==================================================================
[ 84.808909] Disabling lock debugging due to kernel taint

Fixes: d64696905554 ("Reimplement RLIMIT_SIGPENDING on top of ucounts")
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Alexey Gladkov <legion@kernel.org>
---
kernel/signal.c | 20 ++++++++++++++++----
1 file changed, 16 insertions(+), 4 deletions(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index de0920353d30..e1e4d6d07f08 100644
--- a/kernel/signal.c
+++ b/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);
--
2.29.3