linux-kernel - Re: [PATCH 3/6] x86/entry/64/compat: Fix Xen PV SYSENTER frame setup

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Date:   Wed, 1 Jul 2020 11:42:37 -0400
From:   Brian Gerst <brgerst@...il.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     "the arch/x86 maintainers" <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Juergen Gross <jgross@...e.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        xen-devel <xen-devel@...ts.xenproject.org>
Subject: Re: [PATCH 3/6] x86/entry/64/compat: Fix Xen PV SYSENTER frame setup

On Fri, Jun 26, 2020 at 1:30 PM Andy Lutomirski <luto@...nel.org> wrote:
>
> The SYSENTER frame setup was nonsense.  It worked by accident
> because the normal code into which the Xen asm jumped
> (entry_SYSENTER_32/compat) threw away SP without touching the stack.
> entry_SYSENTER_compat was recently modified such that it relied on
> having a valid stack pointer, so now the Xen asm needs to invoke it
> with a valid stack.
>
> Fix it up like SYSCALL: use the Xen-provided frame and skip the bare
> metal prologue.
>
> Cc: Boris Ostrovsky <boris.ostrovsky@...cle.com>
> Cc: Juergen Gross <jgross@...e.com>
> Cc: Stefano Stabellini <sstabellini@...nel.org>
> Cc: xen-devel@...ts.xenproject.org
> Fixes: 1c3e5d3f60e2 ("x86/entry: Make entry_64_compat.S objtool clean")
> Signed-off-by: Andy Lutomirski <luto@...nel.org>
> ---
>  arch/x86/entry/entry_64_compat.S |  1 +
>  arch/x86/xen/xen-asm_64.S        | 20 ++++++++++++++++----
>  2 files changed, 17 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S
> index 7b9d8150f652..381a6de7de9c 100644
> --- a/arch/x86/entry/entry_64_compat.S
> +++ b/arch/x86/entry/entry_64_compat.S
> @@ -79,6 +79,7 @@ SYM_CODE_START(entry_SYSENTER_compat)
>         pushfq                          /* pt_regs->flags (except IF = 0) */
>         pushq   $__USER32_CS            /* pt_regs->cs */
>         pushq   $0                      /* pt_regs->ip = 0 (placeholder) */
> +SYM_INNER_LABEL(entry_SYSENTER_compat_after_hwframe, SYM_L_GLOBAL)

This skips over the section that truncates the syscall number to
32-bits.  The comments present some doubt that it is actually
necessary, but the Xen path shouldn't differ from native.  That code
should be moved after this new label.

--
Brian Gerst