[PATCH -tip] perf, hw_breakpoint: Fix use-after-free if perf_event_open() fails

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Marco Elver

Sep 27, 2022, 1:20:50 PMSep 27
to el...@google.com, Peter Zijlstra, Ingo Molnar, Arnaldo Carvalho de Melo, Mark Rutland, Alexander Shishkin, Jiri Olsa, Namhyung Kim, linux-pe...@vger.kernel.org, linux-...@vger.kernel.org, Dmitry Vyukov, kasa...@googlegroups.com, syzkaller
Local testing revealed that we can trigger a use-after-free during
rhashtable lookup as follows:

| BUG: KASAN: use-after-free in memcmp lib/string.c:757
| Read of size 8 at addr ffff888107544dc0 by task perf-rhltable-n/1293
| CPU: 0 PID: 1293 Comm: perf-rhltable-n Not tainted 6.0.0-rc3-00014-g85260862789c #46
| Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.0-debian-1.16.0-4 04/01/2014
| Call Trace:
| <TASK>
| memcmp lib/string.c:757
| rhashtable_compare include/linux/rhashtable.h:577 [inline]
| __rhashtable_lookup include/linux/rhashtable.h:602 [inline]
| rhltable_lookup include/linux/rhashtable.h:688 [inline]
| task_bp_pinned kernel/events/hw_breakpoint.c:324
| toggle_bp_slot kernel/events/hw_breakpoint.c:462
| __release_bp_slot kernel/events/hw_breakpoint.c:631 [inline]
| release_bp_slot kernel/events/hw_breakpoint.c:639
| register_perf_hw_breakpoint kernel/events/hw_breakpoint.c:742
| hw_breakpoint_event_init kernel/events/hw_breakpoint.c:976
| perf_try_init_event kernel/events/core.c:11261
| perf_init_event kernel/events/core.c:11325 [inline]
| perf_event_alloc kernel/events/core.c:11619
| __do_sys_perf_event_open kernel/events/core.c:12157
| do_syscall_x64 arch/x86/entry/common.c:50 [inline]
| do_syscall_64 arch/x86/entry/common.c:80
| entry_SYSCALL_64_after_hwframe
| </TASK>
| Allocated by task 1292:
| perf_event_alloc kernel/events/core.c:11505
| __do_sys_perf_event_open kernel/events/core.c:12157
| do_syscall_x64 arch/x86/entry/common.c:50 [inline]
| do_syscall_64 arch/x86/entry/common.c:80
| entry_SYSCALL_64_after_hwframe
| Freed by task 1292:
| perf_event_alloc kernel/events/core.c:11716
| __do_sys_perf_event_open kernel/events/core.c:12157
| do_syscall_x64 arch/x86/entry/common.c:50 [inline]
| do_syscall_64 arch/x86/entry/common.c:80
| entry_SYSCALL_64_after_hwframe
| The buggy address belongs to the object at ffff888107544c00
| which belongs to the cache perf_event of size 1352
| The buggy address is located 448 bytes inside of
| 1352-byte region [ffff888107544c00, ffff888107545148)

This happens because the first perf_event_open() managed to reserve a HW
breakpoint slot, however, later fails for other reasons and returns. The
second perf_event_open() runs concurrently, and during rhltable_lookup()
looks up an entry which is being freed: since rhltable_lookup() may run
concurrently (under the RCU read lock) with rhltable_remove(), we may
end up with a stale entry, for which memory may also have already been
freed when being accessed.

To fix, only free the failed perf_event after an RCU grace period. This
allows subsystems that store references to an event to always access it
concurrently under the RCU read lock, even if initialization will fail.

Given failure is unlikely and a slow-path, turning the immediate free
into a call_rcu()-wrapped free does not affect performance elsewhere.

Fixes: 0370dc314df3 ("perf/hw_breakpoint: Optimize list of per-task breakpoints")
Reported-by: syzkaller <syzk...@googlegroups.com>
Signed-off-by: Marco Elver <el...@google.com>
kernel/events/core.c | 4 +---
1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index df90777262bf..007a87c1599c 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -11776,11 +11776,9 @@ perf_event_alloc(struct perf_event_attr *attr, int cpu,
- if (event->ns)
- put_pid_ns(event->ns);
if (event->hw.target)
- kmem_cache_free(perf_event_cache, event);
+ call_rcu(&event->rcu_head, free_event_rcu);

return ERR_PTR(err);

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