[PATCH] tracing/timerlat: Check tlat_var for NULL in timerlat_fd_release

[tglozar@redhat.com]

From: Tomas Glozar <tglozar@redhat.com>

When running timerlat with a userspace workload (NO_OSNOISE_WORKLOAD),
NULL pointer dereference can be triggered by sending consequent SIGINT
and SIGTERM signals to the workload process. That then causes
timerlat_fd_release to be called twice in a row, and the second time,
hrtimer_cancel is called on a zeroed hrtimer struct, causing the NULL
dereference.

This can be reproduced using rtla:
```
$ while true; do rtla timerlat top -u -q & PID=$!; sleep 5; \
 kill -INT $PID; sleep 0.001; kill -TERM $PID; wait $PID; done
[1] 1675
[1]+  Aborted (SIGTERM)      rtla timerlat top -u -q
[1] 1688
client_loop: send disconnect: Broken pipe
```
triggering the bug:
```
BUG: kernel NULL pointer dereference, address: 0000000000000010
Oops: Oops: 0000 [#1] PREEMPT SMP NOPTI
CPU: 6 PID: 1679 Comm: timerlatu/6 Kdump: loaded Not tainted
6.10.0-rc2+ #1
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-1.fc39
04/01/2014
RIP: 0010:hrtimer_active+0xd/0x50
RSP: 0018:ffffa86641567cc0 EFLAGS: 00010286
RAX: 000000000002e2c0 RBX: ffff994c6bf2e2c8 RCX: ffff994b0911ac18
RDX: 0000000000000000 RSI: ffff994b02f10700 RDI: ffff994c6bf2e2c8
RBP: ffff994c6bf2e340 R08: ffff994b158f7400 R09: ffff994b0911ac18
R10: 0000000000000010 R11: ffff994b00d40f00 R12: ffff994c6bf2e2c8
R13: ffff994b02049b20 R14: ffff994b011806c0 R15: 0000000000000000
FS:  0000000000000000(0000) GS:ffff994c6bf00000(0000)
knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000010 CR3: 0000000139020006 CR4: 0000000000770ef0
PKRU: 55555554
Call Trace:
  <TASK>
  ? __die+0x24/0x70
  ? page_fault_oops+0x75/0x170
  ? mt_destroy_walk.isra.0+0x2b3/0x320
  ? exc_page_fault+0x70/0x160
  ? asm_exc_page_fault+0x26/0x30
  ? hrtimer_active+0xd/0x50
  hrtimer_cancel+0x15/0x40
  timerlat_fd_release+0x48/0xe0
  __fput+0xed/0x2c0
  task_work_run+0x59/0x90
  do_exit+0x275/0x4b0
  do_group_exit+0x30/0x80
  get_signal+0x917/0x960
  ? vfs_read+0xb7/0x340
  arch_do_signal_or_restart+0x29/0xf0
  ? syscall_exit_to_user_mode+0x70/0x1f0
  ? syscall_exit_work+0xf3/0x120
  syscall_exit_to_user_mode+0x1a0/0x1f0
  do_syscall_64+0x89/0x160
  ? clear_bhb_loop+0x25/0x80
  ? clear_bhb_loop+0x25/0x80
  ? clear_bhb_loop+0x25/0x80
  entry_SYSCALL_64_after_hwframe+0x76/0x7e
  RIP: 0033:0x7f75790fd9ec
...
  </TASK>
```

Fix the NULL pointer dereference by checking tlat_var->kthread for zero
first in timerlat_fd_release, before calling hrtimer_cancel.
tlat_var->kthread is always non-zero unless the entire tlat_var is zero,
since it is set to the TID of the userspace workload in timerlat_fd_open
under a mutex.

Cc: stable@vger.kernel.org
Fixes: e88ed227f639e ("tracing/timerlat: Add user-space interface")
Co-developed-by: Luis Claudio R. Goncalves <lgoncalv@redhat.com>
Signed-off-by: Luis Claudio R. Goncalves <lgoncalv@redhat.com>
Signed-off-by: Tomas Glozar <tglozar@redhat.com>
---
 kernel/trace/trace_osnoise.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/kernel/trace/trace_osnoise.c b/kernel/trace/trace_osnoise.c
index 66a871553d4a..6d2b39da4dce 100644
--- a/kernel/trace/trace_osnoise.c
+++ b/kernel/trace/trace_osnoise.c
@@ -2572,6 +2572,7 @@ static int timerlat_fd_release(struct inode *inode, struct file *file)
 	struct osnoise_variables *osn_var;
 	struct timerlat_variables *tlat_var;
 	long cpu = (long) file->private_data;
+	int ret = 0;
 
 	migrate_disable();
 	mutex_lock(&interface_lock);
@@ -2579,6 +2580,12 @@ static int timerlat_fd_release(struct inode *inode, struct file *file)
 	osn_var = per_cpu_ptr(&per_cpu_osnoise_var, cpu);
 	tlat_var = per_cpu_ptr(&per_cpu_timerlat_var, cpu);
 
+	if (!tlat_var->kthread) {
+		/* the fd has been closed already */
+		ret = -EBADF;
+		goto out;
+	}
+
 	hrtimer_cancel(&tlat_var->timer);
 	memset(tlat_var, 0, sizeof(*tlat_var));
 
@@ -2593,9 +2600,10 @@ static int timerlat_fd_release(struct inode *inode, struct file *file)
 		osn_var->kthread = NULL;
 	}
 
+out:
 	mutex_unlock(&interface_lock);
 	migrate_enable();
-	return 0;
+	return ret;
 }
 #endif
 
-- 
2.46.0