Re: Compat 32-bit syscall entry from 64-bit task!?

[Andrew Lutomirski]

On Thu, Jan 19, 2012 at 8:01 AM, Jamie Lokier <jamie@xxxxxxxxxxxxx> wrote:
> Andrew Lutomirski wrote:
>> It's reasonable, obvious, and even more wrong than it appears.  On
>> Xen, there's an extra 64-bit GDT entry, and it gets used by default.
>> (I got bitten by this in some iteration of the vsyscall emulation
>> patches -- see user_64bit_mode for the correct and
>> unusable-from-user-mode way to do this.)
>
> Here it is:
>
>        static inline bool user_64bit_mode(struct pt_regs *regs)
>        {
>        #ifndef CONFIG_PARAVIRT
>                /*
>                 * On non-paravirt systems, this is the only long mode CPL 3
>                 * selector.  We do not allow long mode selectors in the LDT.
>                 */
>                return regs->cs == __USER_CS;
>        #else
>                /* Headers are too twisted for this to go in paravirt.h. */
>                return regs->cs == __USER_CS || regs->cs == pv_info.extra_user_64bit_cs;
>        #endif
>        }
>
> Perhaps userspace can do that.
> Would it be right for a ptracer to say:
>
>   CS == 0x23 -> 32-bit
>   (CS & 4)   -> 32-bit (LDT, "we do not allow long mode selectors in the LDT")
>   else       -> 64-bit (__USER_CS or some other GDT entry which must be pv_info's)
>
> I.e. assume that no other *GDT* CS values are available to userspace?
> There are other 32-bit GDT entries, but are they not all for data or kernel use only?

I suspect not.  asm/xen/interface_64.h has:

#define FLAT_RING3_CS32 0xe023  /* GDT index 260 */
#define FLAT_RING3_CS64 0xe033  /* GDT index 261 */
#define FLAT_RING3_DS32 0xe02b  /* GDT index 262 */
#define FLAT_RING3_DS64 0x0000  /* NULL selector */
#define FLAT_RING3_SS32 0xe02b  /* GDT index 262 */
#define FLAT_RING3_SS64 0xe02b  /* GDT index 262 */

which sounds like there's an extra 32-bit selector as well.  I haven't
checked, though.

--Andy
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