Re: [PATCH v2 2/6] x86/entry/64: Convert SYSRET validation tests to C

[Brian Gerst]

On Sun, Jul 23, 2023 at 5:53 AM Li, Xin3 <xin3.li@xxxxxxxxx> wrote:
>
>
> > @@ -84,6 +85,43 @@ __visible noinstr void do_syscall_64(struct pt_regs *regs, int
> > nr)
> >
> >       instrumentation_end();
> >       syscall_exit_to_user_mode(regs);
>
> Would it be better to make the following code a new function?
>
> And then the similar changes in patch 6 could be merged into the new
> function with #ifdef CONFIG_X86_64.
>
> > +
> > +     /*
> > +      * Check that the register state is valid for using SYSRET to exit
> > +      * to userspace.  Otherwise use the slower but fully capable IRET
> > +      * exit path.
> > +      */
> > +
> > +     /* XEN PV guests always use IRET path */
> > +     if (cpu_feature_enabled(X86_FEATURE_XENPV))
> > +             return false;
> > +
> > +     /* SYSRET requires RCX == RIP and R11 == EFLAGS */
> > +     if (unlikely(regs->cx != regs->ip || regs->r11 != regs->flags))
> > +             return false;
> > +
> > +     /* CS and SS must match the values set in MSR_STAR */
> > +     if (unlikely(regs->cs != __USER_CS || regs->ss != __USER_DS))
> > +             return false;
> > +
> > +     /*
> > +      * On Intel CPUs, SYSRET with non-canonical RCX/RIP will #GP
> > +      * in kernel space.  This essentially lets the user take over
> > +      * the kernel, since userspace controls RSP.
> > +      */
> > +     if (unlikely(!__is_canonical_address(regs->ip, __VIRTUAL_MASK_SHIFT +
> > 1)))
> > +             return false;
> > +
> > +     /*
> > +      * SYSRET cannot restore RF.  It can restore TF, but unlike IRET,
> > +      * restoring TF results in a trap from userspace immediately after
> > +      * SYSRET.
> > +      */
> > +     if (unlikely(regs->flags & (X86_EFLAGS_RF | X86_EFLAGS_TF)))
> > +             return false;
> > +
> > +     /* Use SYSRET to exit to userspace */
> > +     return true;
> >  }
> >  #endif
> >

The tests are similar but not enough to combine them.  If
IA32_EMULATION is enabled, both versions are needed so one copy of the
function with #ifdefs won't work..

Brian Gerst