linux-kernel - [PATCH] kcov: don't lose track of remote references during softirqs

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Message-ID: <20240611133229.527822-1-nogikh@google.com>
Date: Tue, 11 Jun 2024 15:32:29 +0200
From: Aleksandr Nogikh <nogikh@...gle.com>
To: dvyukov@...gle.com, andreyknvl@...il.com, arnd@...db.de, 
	akpm@...ux-foundation.org
Cc: elver@...gle.com, glider@...gle.com, syzkaller@...glegroups.com, 
	kasan-dev@...glegroups.com, linux-kernel@...r.kernel.org, 
	Aleksandr Nogikh <nogikh@...gle.com>
Subject: [PATCH] kcov: don't lose track of remote references during softirqs

In kcov_remote_start()/kcov_remote_stop(), we swap the previous KCOV
metadata of the current task into a per-CPU variable. However, the
kcov_mode_enabled(mode) check is not sufficient in the case of remote
KCOV coverage: current->kcov_mode always remains KCOV_MODE_DISABLED
for remote KCOV objects.

If the original task that has invoked the KCOV_REMOTE_ENABLE ioctl
happens to get interrupted and kcov_remote_start() is called, it
ultimately leads to kcov_remote_stop() NOT restoring the original
KCOV reference. So when the task exits, all registered remote KCOV
handles remain active forever.

Fix it by introducing a special kcov_mode that is assigned to the
task that owns a KCOV remote object. It makes kcov_mode_enabled()
return true and yet does not trigger coverage collection in
__sanitizer_cov_trace_pc() and write_comp_data().

Signed-off-by: Aleksandr Nogikh <nogikh@...gle.com>
Fixes: 5ff3b30ab57d ("kcov: collect coverage from interrupts")
---
 include/linux/kcov.h | 2 ++
 kernel/kcov.c        | 1 +
 2 files changed, 3 insertions(+)

diff --git a/include/linux/kcov.h b/include/linux/kcov.h
index b851ba415e03..3b479a3d235a 100644
--- a/include/linux/kcov.h
+++ b/include/linux/kcov.h
@@ -21,6 +21,8 @@ enum kcov_mode {
 	KCOV_MODE_TRACE_PC = 2,
 	/* Collecting comparison operands mode. */
 	KCOV_MODE_TRACE_CMP = 3,
+	/* The process owns a KCOV remote reference. */
+	KCOV_MODE_REMOTE = 4,
 };

 #define KCOV_IN_CTXSW	(1 << 30)
diff --git a/kernel/kcov.c b/kernel/kcov.c
index c3124f6d5536..5371d3f7b5c3 100644
--- a/kernel/kcov.c
+++ b/kernel/kcov.c
@@ -632,6 +632,7 @@ static int kcov_ioctl_locked(struct kcov *kcov, unsigned int cmd,
 			return -EINVAL;
 		kcov->mode = mode;
 		t->kcov = kcov;
+		WRITE_ONCE(t->kcov_mode, KCOV_MODE_REMOTE);
 		kcov->t = t;
 		kcov->remote = true;
 		kcov->remote_size = remote_arg->area_size;
-- 
2.45.2.505.gda0bf45e8d-goog