In the Linux kernel, the following vulnerability has been resolved:

coresight: tmc-etr: Fix race condition between sysfs and perf mode

When trying to run perf and sysfs mode simultaneously, the WARN_ON()
in tmc_etr_enable_hw() is triggered sometimes:

 WARNING: CPU: 42 PID: 3911571 at drivers/hwtracing/coresight/coresight-tmc-etr.c:1060 tmc_etr_enable_hw+0xc0/0xd8 [coresight_tmc]
 [..snip..]
 Call trace:
  tmc_etr_enable_hw+0xc0/0xd8 [coresight_tmc] (P)
  tmc_enable_etr_sink+0x11c/0x250 [coresight_tmc] (L)
  tmc_enable_etr_sink+0x11c/0x250 [coresight_tmc]
  coresight_enable_path+0x1c8/0x218 [coresight]
  coresight_enable_sysfs+0xa4/0x228 [coresight]
  enable_source_store+0x58/0xa8 [coresight]
  dev_attr_store+0x20/0x40
  sysfs_kf_write+0x4c/0x68
  kernfs_fop_write_iter+0x120/0x1b8
  vfs_write+0x2c8/0x388
  ksys_write+0x74/0x108
  __arm64_sys_write+0x24/0x38
  el0_svc_common.constprop.0+0x64/0x148
  do_el0_svc+0x24/0x38
  el0_svc+0x3c/0x130
  el0t_64_sync_handler+0xc8/0xd0
  el0t_64_sync+0x1ac/0x1b0
 ---[ end trace 0000000000000000 ]---

Since the enablement of sysfs mode is separeted into two critical regions,
one for sysfs buffer allocation and another for hardware enablement, it's
possible to race with the perf mode. Fix this by double check whether
the perf mode's been used before enabling the hardware in sysfs mode.

 mode:
   [sysfs mode]                   [perf mode]
   tmc_etr_get_sysfs_buffer()
     spin_lock(&drvdata->spinlock)
     [sysfs buffer allocation]
     spin_unlock(&drvdata->spinlock)
                                  spin_lock(&drvdata->spinlock)
                                  tmc_etr_enable_hw()
                                    drvdata->etr_buf = etr_perf->etr_buf
                                  spin_unlock(&drvdata->spinlock)
   spin_lock(&drvdata->spinlock)
   tmc_etr_enable_hw()
     WARN_ON(drvdata->etr_buf) // WARN sicne etr_buf initialized at
                                  the perf side
   spin_unlock(&drvdata->spinlock)

With this fix, we retain the check for CS_MODE_PERF in get_etr_sysfs_buf.
This ensures we verify whether the perf mode's already running before we
actually allocate the buffer. Then we can save the time of
allocating/freeing the sysfs buffer if race with the perf mode.