linux-kernel - Re: [PATCH 07/34] x86/entry/32: Restore segments before int registers

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Date:   Mon, 5 Mar 2018 12:21:36 -0500
From:   Brian Gerst <brgerst@...il.com>
To:     Joerg Roedel <joro@...tes.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, Peter Anvin <hpa@...or.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        Andrew Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jürgen Groß <jgross@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>, Jiri Kosina <jkosina@...e.cz>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        David Laight <David.Laight@...lab.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Eduardo Valentin <eduval@...zon.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Will Deacon <will.deacon@....com>,
        "Liguori, Anthony" <aliguori@...zon.com>,
        Daniel Gruss <daniel.gruss@...k.tugraz.at>,
        Hugh Dickins <hughd@...gle.com>,
        Kees Cook <keescook@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Waiman Long <llong@...hat.com>, Pavel Machek <pavel@....cz>,
        Joerg Roedel <jroedel@...e.de>
Subject: Re: [PATCH 07/34] x86/entry/32: Restore segments before int registers

On Mon, Mar 5, 2018 at 11:44 AM, Joerg Roedel <joro@...tes.org> wrote:
> On Mon, Mar 05, 2018 at 09:51:29AM -0500, Brian Gerst wrote:
>> For the IRET fault case you will still need to catch it in the
>> exception code.  See the 64-bit code (.Lerror_bad_iret) for example.
>> For 32-bit, you could just expand that check to cover the whole exit
>> prologue after the CR3 switch, including the data segment loads.
>
> I had a look at the 64 bit code and the exception-in-kernel case seems
> to be handled differently than on 32 bit. The 64 bit entry code has
> checks for certain kinds of errors like iret exceptions.
>
> On 32 bit this is implemented via the standard exception tables which
> get an entry for every EIP that might fault (usually segment loading
> operations, but also iret).
>
> So, unless I am missing something, all the exception entry code has to
> do is to remember the stack and the cr3 with which it was entered (if
> entered from kernel mode) and restore those before iret. And this is
> what I implemented in v3 of this patch-set.

I also noticed that 32-bit will raise SIGILL for all IRET faults,
while 64-bit will raise SIGBUS (#NP/#SS) or SIGSEGV (#GP).  The 64-bit
code is better since it doesn't lose the original fault type, whereas
SIGILL is wrong for this case (illegal opcode).

--
Brian Gerst