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Date:   Sun, 2 Aug 2020 13:56:01 +0200
From:   Pavel Machek <pavel@ucw.cz>
To:     David Laight <David.Laight@ACULAB.COM>
Cc:     'Andy Lutomirski' <luto@kernel.org>,
        "madvenka@linux.microsoft.com" <madvenka@linux.microsoft.com>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        Linux API <linux-api@vger.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
        Linux FS Devel <linux-fsdevel@vger.kernel.org>,
        linux-integrity <linux-integrity@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        LSM List <linux-security-module@vger.kernel.org>,
        Oleg Nesterov <oleg@redhat.com>, X86 ML <x86@kernel.org>
Subject: Re: [PATCH v1 0/4] [RFC] Implement Trampoline File Descriptor
Message-ID: <20200802115600.GB1162@bug>
References: <20200728131050.24443-1-madvenka@linux.microsoft.com>
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Hi!

> > This is quite clever, but now I???m wondering just how much kernel help
> > is really needed. In your series, the trampoline is an non-executable
> > page.  I can think of at least two alternative approaches, and I'd
> > like to know the pros and cons.
> > 
> > 1. Entirely userspace: a return trampoline would be something like:
> > 
> > 1:
> > pushq %rax
> > pushq %rbc
> > pushq %rcx
> > ...
> > pushq %r15
> > movq %rsp, %rdi # pointer to saved regs
> > leaq 1b(%rip), %rsi # pointer to the trampoline itself
> > callq trampoline_handler # see below
> 
> For nested calls (where the trampoline needs to pass the
> original stack frame to the nested function) I think you
> just need a page full of:
> 	mov	$0, scratch_reg; jmp trampoline_handler

I believe you could do with mov %pc, scratch_reg; jmp ...

That has advantage of being able to share single physical page across multiple virtual pages...


									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html