Re: [RFC][PATCH] x86, syscalls: use SYSCALL_DEFINE() macros for sys_modify_ldt()

From: Brian Gerst
Date: Fri Oct 13 2017 - 19:49:51 EST


On Fri, Oct 13, 2017 at 5:03 PM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> On Fri, Oct 13, 2017 at 1:39 PM, Dave Hansen
> <dave.hansen@xxxxxxxxxxxxxxx> wrote:
>>
>> I noticed that we don't have tracepoints for sys_modify_ldt(). I
>> think that's because we define it directly instead of using the
>> normal SYSCALL_DEFINEx() macros.
>>
>> Is there a reason for that, or were they just missed when the
>> macros were created?
>
> No, and it's a longstanding fsckup that I think you can't fix like
> this because...
>
>>
>> Cc: x86@xxxxxxxxxx
>> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
>>
>> ---
>>
>> b/arch/x86/include/asm/syscalls.h | 2 +-
>> b/arch/x86/kernel/ldt.c | 5 +++--
>> b/arch/x86/um/ldt.c | 3 ++-
>> 3 files changed, 6 insertions(+), 4 deletions(-)
>>
>> diff -puN arch/x86/kernel/ldt.c~x86-syscall-macros-modify_ldt arch/x86/kernel/ldt.c
>> --- a/arch/x86/kernel/ldt.c~x86-syscall-macros-modify_ldt 2017-10-13 13:30:12.802553391 -0700
>> +++ b/arch/x86/kernel/ldt.c 2017-10-13 13:30:12.817553391 -0700
>> @@ -12,6 +12,7 @@
>> #include <linux/string.h>
>> #include <linux/mm.h>
>> #include <linux/smp.h>
>> +#include <linux/syscalls.h>
>> #include <linux/slab.h>
>> #include <linux/vmalloc.h>
>> #include <linux/uaccess.h>
>> @@ -294,8 +295,8 @@ out:
>> return error;
>> }
>>
>> -asmlinkage int sys_modify_ldt(int func, void __user *ptr,
>> - unsigned long bytecount)
>> +SYSCALL_DEFINE3(modify_ldt, int , func , void __user * , ptr ,
>> + unsigned long , bytecount)
>
> sys_modify_ldt() returns int, which is wrong, and it's visibly wrong
> to 64-bit user code. So I think you need to make sure that the return
> value is cast to int in all cases.

I don't think there will be a problem here. If 64-bit userspace
treats it as an int, it will truncate to 32-bit signed and all is
well. If it is treating it as a long, then it's currently broken for
errors anyways.

--
Brian Gerst