Re: [PATCH 1/2] coresight: tmc: Don't change the buffer size if it's in use

[James Clark <james.clark@linaro.org>]

On 14/11/2024 8:16 am, Yicong Yang wrote:
> From: Yicong Yang <yangyicong@hisilicon.com>
> 
> Enable the trace in below steps will crash the kernel by NULL pointer
> dereferencing:
> echo 1 > /sys/bus/coresight/devices/tmc_etr0/enable_sink
> echo 1 > /sys/bus/coresight/devices/etm0/enable_source
> echo 0x400000 > /sys/bus/coresight/devices/tmc_etr0/buffer_size
> echo 1 > /sys/bus/coresight/devices/etm2/enable_source
> dd if=/dev/tmc_etr0 of=test_etm_sysfs_etr_030.data
> 
> The call trace will be like:
>   WARNING: CPU: 39 PID: 8586 at drivers/hwtracing/coresight/coresight-tmc-etr.c:1123 __tmc_etr_disable_hw+0x108/0x140 [coresight_tmc]
>   [...]
>   Call trace:
>    __tmc_etr_disable_hw+0x108/0x140 [coresight_tmc]
>    tmc_read_prepare_etr+0xc0/0xd0 [coresight_tmc]
>    tmc_open+0x60/0xa0 [coresight_tmc]
>    misc_open+0x11c/0x170
>    chrdev_open+0xcc/0x2b0
>    do_dentry_open+0x140/0x4e0
>    vfs_open+0x34/0xf8
>    path_openat+0x2b0/0xf58
>    do_filp_open+0x8c/0x148
>    do_sys_openat2+0xb8/0xe8
>    __arm64_sys_openat+0x70/0xc0
>    el0_svc_common.constprop.0+0x64/0x148
>    do_el0_svc+0x24/0x38
>    el0_svc+0x40/0x140
>    el0t_64_sync_handler+0xc0/0xc8
>    el0t_64_sync+0x1a4/0x1a8
>   ---[ end trace 0000000000000000 ]---
>   Unable to handle kernel NULL pointer dereference at virtual address 0000000000000028
>   [...]
>   Call trace:
>    tmc_etr_get_sysfs_trace+0x10/0x80 [coresight_tmc]
>    vfs_read+0xcc/0x310
>    ksys_read+0x74/0x108
>    __arm64_sys_read+0x24/0x38
>    el0_svc_common.constprop.0+0x64/0x148
>    do_el0_svc+0x24/0x38
>    el0_svc+0x40/0x140
> 
> Due to the buffer size changed, the buffer will be reallocated in
> tmc_etr_get_sysfs_buffer() when the second source enabled. At trace
> end tmc_etr_sync_sysfs_buf() will reset the drvdata->sysfs_buf and
> trigger the later NULL pointer dereference when reading out the
> data.
> 
> But it doesn't make sense to change the buffer size when it's
> already in use. So block such behavior.
> 
> Signed-off-by: Yicong Yang <yangyicong@hisilicon.com>
> ---
>   drivers/hwtracing/coresight/coresight-tmc-core.c | 5 +++++
>   1 file changed, 5 insertions(+)
> 
> diff --git a/drivers/hwtracing/coresight/coresight-tmc-core.c b/drivers/hwtracing/coresight/coresight-tmc-core.c
> index 475fa4bb6813..9660af63e9bc 100644
> --- a/drivers/hwtracing/coresight/coresight-tmc-core.c
> +++ b/drivers/hwtracing/coresight/coresight-tmc-core.c
> @@ -319,6 +319,11 @@ static ssize_t buffer_size_store(struct device *dev,
>   	if (drvdata->config_type != TMC_CONFIG_TYPE_ETR)
>   		return -EPERM;
>   
> +	/* Don't change the buffer size if it's in use */
> +	guard(spinlock)(&drvdata->spinlock);
> +	if (coresight_get_mode(drvdata->csdev) != CS_MODE_DISABLED)

Size isn't used in perf mode is it? So it can be -EBUSY only when mode 
== CS_MODE_SYSFS.

> +		return -EBUSY;
> +
>   	ret = kstrtoul(buf, 0, &val);
>   	if (ret)
>   		return ret;

Looks ok to me. Although for consistency it might be worth changing to 
guard(mutex)(&coresight_mutex) because this is about sysfs mode only and 
other usages of mode and comments point to coresight_mutex. Using the 
device's spinlock will technically work but it did make me go and double 
check the code. And there are other cases of reading the mode like this:

static ssize_t enable_source_show(struct device *dev,
				  struct device_attribute *attr,
				  char *buf)
{
	struct coresight_device *csdev = to_coresight_device(dev);

	guard(mutex)(&coresight_mutex);
	return scnprintf(buf, PAGE_SIZE, "%u\n",
			 coresight_get_mode(csdev) == CS_MODE_SYSFS);
}

Mode can change to CS_MODE_PERF while inside coresight_mutex but the 
device would end up not being enabled for sysfs, so it's still ok to 
update the sysfs size value in that case.

With that change:

Reviewed-by: James Clark <james.clark@linaro.org>