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SubjectRe: [PATCH] debugfs: Add mount restriction option
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On 5/28/20 10:27 AM, Greg Kroah-Hartman wrote:
> On Thu, May 28, 2020 at 10:00:31AM +0200, Peter Enderborg wrote:
>> Since debugfs include sensitive information it need to be treated
>> carefully. But it also has many very useful debug functions for userspace.
>> With this option we can have same configuration for system with
>> need of debugfs and a way to turn it off. It is needed new
>> kernel command line parameter to be activated.
> By "configuration" do you mean "kernel configuration"? What is wrong
> with relying on the build option like we do today?
>
> You might want to reword all of this to make more sense about the
> "problem" you are trying to solve here, as I don't really understand it,
> sorry.
>
>
>> Signed-off-by: Peter Enderborg <peter.enderborg@sony.com>
>> ---
>> fs/debugfs/inode.c | 17 ++++++++++++++++-
>> lib/Kconfig.debug | 10 ++++++++++
>> 2 files changed, 26 insertions(+), 1 deletion(-)
> No documentation update? That's not good :(
>
>
>> diff --git a/fs/debugfs/inode.c b/fs/debugfs/inode.c
>> index b7f2e971ecbc..bde37dab77e0 100644
>> --- a/fs/debugfs/inode.c
>> +++ b/fs/debugfs/inode.c
>> @@ -786,10 +786,25 @@ bool debugfs_initialized(void)
>> }
>> EXPORT_SYMBOL_GPL(debugfs_initialized);
>>
>> +static int allow_debugfs;
>> +
>> +static int __init debugfs_kernel(char *str)
>> +{
>> + if (str && !strcmp(str, "true"))
>> + allow_debugfs = true;
>> +
>> + return 0;
>> +
>> +}
>> +early_param("debugfs", debugfs_kernel);
>> +
>> static int __init debugfs_init(void)
>> {
>> int retval;
>> -
>> +#ifdef CONFIG_DEBUG_FS_MOUNT_RESTRICTED
>> + if (!allow_debugfs)
>> + return -EPERM;
>> +#endif
> But you are not restricting the ability to mount it here, you are
> removing the ability for it to even start up at all. What does this
> break for code that thinks the filesystem is registered (i.e. the call
> to simple_pin_fs() in start_creating() in fs/debugfs/inode.c?
>
If it does, the lines below is also cause the same problem.


>
>> retval = sysfs_create_mount_point(kernel_kobj, "debug");
>> if (retval)
>> return retval;
>> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
>> index 21d9c5f6e7ec..d3a3338740d2 100644
>> --- a/lib/Kconfig.debug
>> +++ b/lib/Kconfig.debug
>> @@ -443,6 +443,16 @@ config DEBUG_FS
>>
>> If unsure, say N.
>>
>> +config DEBUG_FS_MOUNT_RESTRICTED
>> + bool "Debug Filesystem mount restricted"
>> + depends on DEBUG_FS
>> + help
>> + This is an additional restriction for mounting debugfs. It allows
>> + the kernel to have debugfs compiled, but requires that kernel command
>> + line has a debugfs parameter to register as a filesystem.
>> +
>> + If unsure, say N.
> No hint as to what the command line option is? And again, it's not a
> restriction for mounting it, you are preventing it from initializing
> entirely.
>
> Which, as I have found out over time, can had side affects for codepaths
> that try to check the return value of debugfs calls, and if they fail to
> succeed, abort the whole driver/subsystem/whatever initializaion path,
> which is not what I think you want to do here at all.
>
> thanks,
>
> greg k-h

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