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

[Yicong Yang <yangyicong@huawei.com>]

On 2024/11/14 18:30, James Clark wrote:
> 
> 
> 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.
> 

alloc_etr_buf() on the perf path will read drvdata->size, not sure it matters if user
change it through sysfs in the meanwhile. Will test and have a check if there are any
other places using size on the perf path.

>> +        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:
> 

ok, I thought to also serialize the use of drvdata->size. But as you mentioned
use coresight_mutex is enough and will be consistenct with other places.

> 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>

Thanks.