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authorAlexey Gladkov <legion@kernel.org>2021-07-08 12:33:01 +0200
committerLinus Torvalds <torvalds@linux-foundation.org>2021-07-08 11:43:24 -0700
commitf3791f4df569eadb3b1f5f2a247068d031fb91f5 (patch)
treed126e8148da3fbf1d7a593246394b8772c3ca12f /kernel/signal.c
parente9f1cbc0c4114880090c7a578117d3b9cf184ad4 (diff)
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: d64696905554 ("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>
Diffstat (limited to 'kernel/signal.c')
-rw-r--r--kernel/signal.c20
1 files changed, 16 insertions, 4 deletions
diff --git a/kernel/signal.c b/kernel/signal.c
index f6371dfa1f89..a3229add4455 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);

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