[PATCH] perf: Improve missing SIGTRAP checking

[Marco Elver]

To catch missing SIGTRAP we employ a WARN in __perf_event_overflow(),
which fires if pending_sigtrap was already set: returning to user space
without consuming pending_sigtrap, and then having the event fire again
would re-enter the kernel and trigger the WARN.

This, however, seemed to miss the case where some events not associated
with progress in the user space task can fire and the interrupt handler
runs before the IRQ work meant to consume pending_sigtrap (and generate
the SIGTRAP).

syzbot gifted us this stack trace:

 | WARNING: CPU: 0 PID: 3607 at kernel/events/core.c:9313 __perf_event_overflow
 | Modules linked in:
 | CPU: 0 PID: 3607 Comm: syz-executor100 Not tainted 6.1.0-rc2-syzkaller-00073-g88619e77b33d #0
 | Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/11/2022
 | RIP: 0010:__perf_event_overflow+0x498/0x540 kernel/events/core.c:9313
 | <...>
 | Call Trace:
 |  <TASK>
 |  perf_swevent_hrtimer+0x34f/0x3c0 kernel/events/core.c:10729
 |  __run_hrtimer kernel/time/hrtimer.c:1685 [inline]
 |  __hrtimer_run_queues+0x1c6/0xfb0 kernel/time/hrtimer.c:1749
 |  hrtimer_interrupt+0x31c/0x790 kernel/time/hrtimer.c:1811
 |  local_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1096 [inline]
 |  __sysvec_apic_timer_interrupt+0x17c/0x640 arch/x86/kernel/apic/apic.c:1113
 |  sysvec_apic_timer_interrupt+0x40/0xc0 arch/x86/kernel/apic/apic.c:1107
 |  asm_sysvec_apic_timer_interrupt+0x16/0x20 arch/x86/include/asm/idtentry.h:649
 | <...>
 |  </TASK>

In this case, syzbot produced a program with event type
PERF_TYPE_SOFTWARE and config PERF_COUNT_SW_CPU_CLOCK. The hrtimer
manages to fire again before the IRQ work got a chance to run, all while
never having returned to user space.

Improve the WARN to check for real progress in user space: approximate
this by storing a 32-bit hash of the current IP into pending_sigtrap,
and if an event fires while pending_sigtrap still matches the previous
IP, we assume no progress (false negatives are possible given we could
return to user space and trigger again on the same IP).

Fixes: ca6c21327c6a ("perf: Fix missing SIGTRAPs")
Reported-by: syzbot+b8ded3e2e2c6adde4990@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
---
 kernel/events/core.c | 25 +++++++++++++++++++------
 1 file changed, 19 insertions(+), 6 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 068412fe8dff..f87030487d9b 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -9312,14 +9312,27 @@ static int __perf_event_overflow(struct perf_event *event,
 	}
 
 	if (event->attr.sigtrap) {
-		/*
-		 * Should not be able to return to user space without processing
-		 * pending_sigtrap (kernel events can overflow multiple times).
-		 */
-		WARN_ON_ONCE(event->pending_sigtrap && event->attr.exclude_kernel);
+		unsigned int pending_id = 1;
+
+		if (regs)
+			pending_id = hash32_ptr((void *)instruction_pointer(regs)) ?: 1;
 		if (!event->pending_sigtrap) {
-			event->pending_sigtrap = 1;
+			event->pending_sigtrap = pending_id;
 			local_inc(&event->ctx->nr_pending);
+		} else if (event->attr.exclude_kernel) {
+			/*
+			 * Should not be able to return to user space without
+			 * consuming pending_sigtrap; with exceptions:
+			 *
+			 *  1. Where !exclude_kernel, events can overflow again
+			 *     in the kernel without returning to user space.
+			 *
+			 *  2. Events that can overflow again before the IRQ-
+			 *     work without user space progress (e.g. hrtimer).
+			 *     To approximate progress (with false negatives),
+			 *     check 32-bit hash of the current IP.
+			 */
+			WARN_ON_ONCE(event->pending_sigtrap != pending_id);
 		}
 		event->pending_addr = data->addr;
 		irq_work_queue(&event->pending_irq);
-- 
2.38.1.273.g43a17bfeac-goog