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Message-Id: <20210708103301.2109330-1-legion@kernel.org>
Date: Thu, 8 Jul 2021 12:33:01 +0200
From: Alexey Gladkov <legion@...nel.org>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
Linux Containers <containers@...ts.linux.dev>,
Alexey Gladkov <legion@...nel.org>,
"Eric W . Biederman" <ebiederm@...ssion.com>,
Oleg Nesterov <oleg@...hat.com>
Subject: [PATCH v2] 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
[ 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@...ssion.com>
Cc: Oleg Nesterov <oleg@...hat.com>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Alexey Gladkov <legion@...nel.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
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