Re: [Xen-devel] [PATCH v3 2/3] x86/ldt: Make modify_ldt optional

[Andy Lutomirski <luto@xxxxxxxxxxxxxx>]

On Thu, Jul 23, 2015 at 4:36 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> On Thu, Jul 23, 2015 at 3:24 AM, Willy Tarreau <w@xxxxxx> wrote:
>>  #ifdef CONFIG_SMP
>>  static void flush_ldt(void *current_mm)
>>  {
>> @@ -254,6 +260,9 @@ asmlinkage int sys_modify_ldt(int func, void __user *ptr,
>>  {
>>         int ret = -ENOSYS;
>>
>> +       if (!sysctl_modify_ldt)
>> +               return ret;
>> +
>>         switch (func) {
>>         case 0:
>>                 ret = read_ldt(ptr, bytecount);
>> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
>> index 2082b1a..60270c6 100644
>> --- a/kernel/sysctl.c
>> +++ b/kernel/sysctl.c
>> @@ -111,6 +111,9 @@ extern int sysctl_nr_open_min, sysctl_nr_open_max;
>>  #ifndef CONFIG_MMU
>>  extern int sysctl_nr_trim_pages;
>>  #endif
>> +#ifdef CONFIG_X86
>> +extern int sysctl_modify_ldt;
>> +#endif
>>
>>  /* Constants used for minimum and  maximum */
>>  #ifdef CONFIG_LOCKUP_DETECTOR
>> @@ -962,6 +965,13 @@ static struct ctl_table kern_table[] = {
>>                 .mode           = 0644,
>>                 .proc_handler   = proc_dointvec,
>>         },
>> +       {
>> +               .procname       = "modify_ldt",
>> +               .data           = &sysctl_modify_ldt,
>> +               .maxlen         = sizeof(int),
>> +               .mode           = 0644,
>> +               .proc_handler   = proc_dointvec,
>> +       },
>>  #endif
>>  #if defined(CONFIG_MMU)
>>         {
>
> I've been pondering something like this that is even MORE generic, for
> any syscall. Something like a "syscalls" directory under
> /proc/sys/kernel, with 1 entry per syscall. "0" is "available", "1" is
> disabled, and "-1" disabled until next boot.
>

It might want to be /proc/sys/kernel/syscalls/[abi]/[name], possibly
with more than just those options.  We might want "disabled, returns
ENOSYS", "disabled, returns EPERM", and a lock bit.

On x86 at least, the implementation's easy -- we can just poke the
syscall table.

--Andy

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