[PATCH 1/2] coresight: tmc: Don't change the buffer size if it's in use
Yicong Yang
yangyicong at huawei.com
Thu Nov 14 06:51:41 PST 2024
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.
>> + 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.
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