Re: [PATCH v3 1/4] kexec: simplify compat_sys_kexec_load

[Arnd Bergmann]

On Tue, May 18, 2021 at 4:05 PM Arnd Bergmann <arnd@xxxxxxxxxx> wrote:
>
> On Tue, May 18, 2021 at 3:41 PM Eric W. Biederman <ebiederm@xxxxxxxxxxxx> wrote:
> >
> > Arnd Bergmann <arnd@xxxxxxxxxx> writes:
> >
> > > From: Arnd Bergmann <arnd@xxxxxxxx>KEXEC_ARCH_DEFAULT
> > >
> > > The compat version of sys_kexec_load() uses compat_alloc_user_space to
> > > convert the user-provided arguments into the native format.
> > >
> > > Move the conversion into the regular implementation with
> > > an in_compat_syscall() check to simplify it and avoid the
> > > compat_alloc_user_space() call.
> > >
> > > compat_sys_kexec_load() now behaves the same as sys_kexec_load().
> >
> > Nacked-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
> >KEXEC_ARCH_DEFAULT
> > The patch is wrong.
> >
> > The logic between the compat entry point and the ordinary entry point
> > are by necessity different.   This unifies the logic and breaks the compat
> > entry point.
> >
> > The fundamentally necessity is that the code being loaded needs to know
> > which mode the kernel is running in so it can safely transition to the
> > new kernel.
> >
> > Given that the two entry points fundamentally need different logic,
> > and that difference was not preserved and the goal of this patchset
> > was to unify that which fundamentally needs to be different.  I don't
> > think this patch series makes any sense for kexec.
>
> Sorry, I'm not following that explanation. Can you clarify what different
> modes of the kernel you are referring to here, and how my patch
> changes this?

I think I figured it out now myself after comparing the two functions:

--- a/kernel/kexec.c
+++ b/kernel/kexec.c
@@ -269,7 +269,8 @@ SYSCALL_DEFINE4(kexec_load, unsigned long, entry,
unsigned long, nr_segments,

        /* Verify we are on the appropriate architecture */
        if (((flags & KEXEC_ARCH_MASK) != KEXEC_ARCH) &&
-               ((flags & KEXEC_ARCH_MASK) != KEXEC_ARCH_DEFAULT))
+               (in_compat_syscall() ||
+               ((flags & KEXEC_ARCH_MASK) != KEXEC_ARCH_DEFAULT)))
                return -EINVAL;

        /* Because we write directly to the reserved memory

Not sure if that's the best way of doing it, but it looks like folding this
in restores the current behavior.

        Arnd