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

[James Clark]

On 14/11/2024 2:51 pm, Yicong Yang wrote:
> On 2024/11/14 18:30, James Clark wrote:
>>
>>
>> On 14/11/2024 8:16 am, Yicong Yang wrote:
>>> From: Yicong Yang <yangyicong at 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 at 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.
> 

Hmmm I assumed that Perf mode completely ignored anything from sysfs 
mode. I see that alloc_etr_buf() does sometimes use the sysfs value. I 
don't really see why that's necessary because that means it sometimes 
ignores the buffer size from the perf command line depending on what's 
in sysfs, but the modes should be mutually exclusive.

Unless we fix that then I think you do need to use the device spinlock. 
But I think we should tidy up alloc_etr_buf() to only try to allocate 
from the Perf size down to TMC_ETR_PERF_MIN_BUF_SIZE, ignoring 
drvdata->size. Then the behavior is less surprising to users and also 
anyone reading the code. And rename it to alloc_etr_buf_perf().

Unless Suzuki knows of a reason it was done that way to begin with? I 
checked the commit message but it just says that it was like that but 
not why.

>>> +        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 at linaro.org>
> 
> Thanks.
>