Re: [PATCH v2] HID: uhid: forbid UHID_CREATE under KERNEL_DS or elevated privileges

[Andy Lutomirski]

> On Nov 15, 2018, at 4:09 AM, David Herrmann <dh.herrmann@xxxxxxxxx> wrote:
> 
> Hi
> 
>> On Wed, Nov 14, 2018 at 10:55 PM Eric Biggers <ebiggers@xxxxxxxxxx> wrote:
>> From: Eric Biggers <ebiggers@xxxxxxxxxx>
>> 
>> When a UHID_CREATE command is written to the uhid char device, a
>> copy_from_user() is done from a user pointer embedded in the command.
>> When the address limit is KERNEL_DS, e.g. as is the case during
>> sys_sendfile(), this can read from kernel memory.  Alternatively,
>> information can be leaked from a setuid binary that is tricked to write
>> to the file descriptor.  Therefore, forbid UHID_CREATE in these cases.
>> 
>> No other commands in uhid_char_write() are affected by this bug and
>> UHID_CREATE is marked as "obsolete", so apply the restriction to
>> UHID_CREATE only rather than to uhid_char_write() entirely.
>> 
>> Thanks to Dmitry Vyukov for adding uhid definitions to syzkaller and to
>> Jann Horn for commit 9da3f2b740544 ("x86/fault: BUG() when uaccess
>> helpers fault on kernel addresses"), allowing this bug to be found.
>> 
>> Reported-by: syzbot+72473edc9bf4eb1c6556@xxxxxxxxxxxxxxxxxxxxxxxxx
>> Fixes: d365c6cfd337 ("HID: uhid: add UHID_CREATE and UHID_DESTROY events")
>> Cc: <stable@xxxxxxxxxxxxxxx> # v3.6+
>> Cc: Jann Horn <jannh@xxxxxxxxxx>
>> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
>> Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>
>> ---
>> drivers/hid/uhid.c | 12 ++++++++++++
>> 1 file changed, 12 insertions(+)
>> 
>> diff --git a/drivers/hid/uhid.c b/drivers/hid/uhid.c
>> index 3c55073136064..051639c09f728 100644
>> --- a/drivers/hid/uhid.c
>> +++ b/drivers/hid/uhid.c
>> @@ -12,6 +12,7 @@
>> 
>> #include <linux/atomic.h>
>> #include <linux/compat.h>
>> +#include <linux/cred.h>
>> #include <linux/device.h>
>> #include <linux/fs.h>
>> #include <linux/hid.h>
>> @@ -722,6 +723,17 @@ static ssize_t uhid_char_write(struct file *file, const char __user *buffer,
>> 
>>        switch (uhid->input_buf.type) {
>>        case UHID_CREATE:
>> +               /*
>> +                * 'struct uhid_create_req' contains a __user pointer which is
>> +                * copied from, so it's unsafe to allow this with elevated
>> +                * privileges (e.g. from a setuid binary) or via kernel_write().
>> +                */
>> +               if (file->f_cred != current_cred() || uaccess_kernel()) {
> 
> I think `uaccess_kernel()` would be enough. UHID does not check any
> credentials. If you believe this should be there nevertheless, feel
> free to keep it.

The free check is needed.  Without it, consider what sudo >uhid_fd does.  It doesnât use sudoâs credentials, but it does read its address space.

Can this patch get a comment added?


> Either way:
> 
> Acked-by: David Herrmann <dh.herrmann@xxxxxxxxx>
> 
> Thanks
> David