linux-kernel - Re: [PATCH v3 1/4] kexec: simplify compat_sys_kexec

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Date:   Tue, 18 May 2021 11:01:57 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     linux-arch <linux-arch@...r.kernel.org>,
        Christoph Hellwig <hch@...radead.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Borislav Petkov <bp@...en8.de>,
        Brian Gerst <brgerst@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>, kexec@...ts.infradead.org
Subject: Re: [PATCH v3 1/4] kexec: simplify compat_sys_kexec_load

Arnd Bergmann <arnd@...nel.org> writes:

> On Tue, May 18, 2021 at 4:05 PM Arnd Bergmann <arnd@...nel.org> wrote:
>>
>> On Tue, May 18, 2021 at 3:41 PM Eric W. Biederman <ebiederm@...ssion.com> wrote:
>> >
>> > Arnd Bergmann <arnd@...nel.org> writes:
>> >
>> > > From: Arnd Bergmann <arnd@...db.de>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@...ssion.com>
>> >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.

Yes.  That is pretty much all there is.

I personally can't stand the sight of in_compat_syscall() doubly so when
you have to lie to the type system with casts.  The cognitive dissonance
I experience is extreme.

I will be happy to help you find another way to get rid of
compat_alloc_user, but not that way.


There is a whole mess in there that was introduced when someone added
do_kexec_load while I was napping in 2017 that makes the system calls an
absolute mess.  It all needs to be cleaned up.

Eric