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

[Andy Lutomirski]

On Tue, Oct 17, 2017 at 12:22 PM, Dave Hansen
<dave.hansen@xxxxxxxxxxxxxxx> wrote:
>
> We do not have tracepoints for sys_modify_ldt() because we define
> it directly instead of using the normal SYSCALL_DEFINEx() macros.
>
> However, there is a reason sys_modify_ldt() does not use the macros:
> it has an 'int' return type instead of 'unsigned long'.  This is
> a bug, but it's a bug cemented in the ABI.
>
> What does this mean?  If we return -EINVAL from a function that
> returns 'int', we have 0x00000000ffffffea in %rax.  But, if we
> return -EINVAL from a function returning 'unsigned long', we end
> up with 0xffffffffffffffea in %rax, which is wrong.
>
> To work around this and maintain the 'int' behavior while using
> the SYSCALL_DEFINEx() macros, so we add a cast to 'unsigned int'
> in both implementations of sys_modify_ldt().

Reviewed-by: Andy Lutomirski <luto@xxxxxxxxxx>

>
> Cc: x86@xxxxxxxxxx
> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> Cc: Brian Gerst <brgerst@xxxxxxxxx>
>
> ---
>
>  b/arch/x86/include/asm/syscalls.h |    2 +-
>  b/arch/x86/kernel/ldt.c           |   16 +++++++++++++---
>  b/arch/x86/um/ldt.c               |    6 ++++--
>  3 files changed, 18 insertions(+), 6 deletions(-)
>
> diff -puN arch/x86/include/asm/syscalls.h~x86-syscall-macros-modify_ldt arch/x86/include/asm/syscalls.h
> --- a/arch/x86/include/asm/syscalls.h~x86-syscall-macros-modify_ldt     2017-10-17 10:30:53.032718307 -0700
> +++ b/arch/x86/include/asm/syscalls.h   2017-10-17 10:30:53.039718307 -0700
> @@ -21,7 +21,7 @@ asmlinkage long sys_ioperm(unsigned long
>  asmlinkage long sys_iopl(unsigned int);
>
>  /* kernel/ldt.c */
> -asmlinkage int sys_modify_ldt(int, void __user *, unsigned long);
> +asmlinkage long sys_modify_ldt(int, void __user *, unsigned long);
>
>  /* kernel/signal.c */
>  asmlinkage long sys_rt_sigreturn(void);
> 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       2017-10-17 10:30:53.034718307 -0700
> +++ b/arch/x86/kernel/ldt.c     2017-10-17 12:02:51.137704545 -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)
>  {
>         int ret = -ENOSYS;
>
> @@ -313,5 +314,14 @@ asmlinkage int sys_modify_ldt(int func,
>                 ret = write_ldt(ptr, bytecount, 0);
>                 break;
>         }
> -       return ret;
> +       /*
> +        * The SYSCALL_DEFINE() macros give us an 'unsigned long'
> +        * return type, but tht ABI for sys_modify_ldt() expects
> +        * 'int'.  This cast gives us an int-sized value in %rax
> +        * for the return code.  The 'unsigned' is necessary so
> +        * the compiler does not try to sign-extend the negative
> +        * return codes into the high half of the register when
> +        * taking the value from int->long.
> +        */
> +       return (unsigned int)ret;
>  }
> diff -puN arch/x86/um/ldt.c~x86-syscall-macros-modify_ldt arch/x86/um/ldt.c
> --- a/arch/x86/um/ldt.c~x86-syscall-macros-modify_ldt   2017-10-17 10:30:53.036718307 -0700
> +++ b/arch/x86/um/ldt.c 2017-10-17 10:58:58.711714103 -0700
> @@ -369,7 +369,9 @@ void free_ldt(struct mm_context *mm)
>         mm->arch.ldt.entry_count = 0;
>  }
>
> -int sys_modify_ldt(int func, void __user *ptr, unsigned long bytecount)
> +SYSCALL_DEFINE3(modify_ldt, int , func , void __user * , ptr ,
> +               unsigned long , bytecount)
>  {
> -       return do_modify_ldt_skas(func, ptr, bytecount);
> +       /* See non-um modify_ldt() for why we do this cast */
> +       return (unsigned int)do_modify_ldt_skas(func, ptr, bytecount);
>  }
> _