linux-kernel - Ping: [PATCH] x86/stackframe/32: repair 32-bit Xen PV

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Date:   Thu, 24 Oct 2019 09:40:16 +0200
From:   Jan Beulich <jbeulich@...e.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     lkml <linux-kernel@...r.kernel.org>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>
Subject: Ping: [PATCH] x86/stackframe/32: repair 32-bit Xen PV

On 07.10.2019 12:41, Jan Beulich wrote:
> Once again RPL checks have been introduced which don't account for a
> 32-bit kernel living in ring 1 when running in a PV Xen domain. The
> case in FIXUP_FRAME has been preventing boot; adjust BUG_IF_WRONG_CR3
> as well just in case.
> 
> Fixes: 3c88c692c287 ("x86/stackframe/32: Provide consistent pt_regs")
> Signed-off-by: Jan Beulich <jbeulich@...e.com>

Ping?

I'd like to further note that there appears to a likely related
2nd problem - I'm seeing seemingly random attempts to enter VM86
mode when running PV under Xen. I suspect a never written eflags
value to get inspected. While the issue here kills the kernel
reliably during boot, that other issue sometimes allows the
system to at least come up in a partly functional way (depending
on which user processes get killed because of there not being
any VM86 mode available when running PV under [64-bit] Xen).

Jan

> --- a/arch/x86/entry/entry_32.S
> +++ b/arch/x86/entry/entry_32.S
> @@ -48,6 +48,17 @@
>  
>  #include "calling.h"
>  
> +#ifndef CONFIG_XEN_PV
> +# define USER_SEGMENT_RPL_MASK SEGMENT_RPL_MASK
> +#else
> +/*
> + * When running paravirtualized on Xen the kernel runs in ring 1, and hence
> + * simple mask based tests (i.e. ones not comparing against USER_RPL) have to
> + * ignore bit 0. See also the C-level get_kernel_rpl().
> + */
> +# define USER_SEGMENT_RPL_MASK (SEGMENT_RPL_MASK & ~1)
> +#endif
> +
>  	.section .entry.text, "ax"
>  
>  /*
> @@ -172,7 +183,7 @@
>  	ALTERNATIVE "jmp .Lend_\@", "", X86_FEATURE_PTI
>  	.if \no_user_check == 0
>  	/* coming from usermode? */
> -	testl	$SEGMENT_RPL_MASK, PT_CS(%esp)
> +	testl	$USER_SEGMENT_RPL_MASK, PT_CS(%esp)
>  	jz	.Lend_\@
>  	.endif
>  	/* On user-cr3? */
> @@ -217,7 +228,7 @@
>  	testl	$X86_EFLAGS_VM, 4*4(%esp)
>  	jnz	.Lfrom_usermode_no_fixup_\@
>  #endif
> -	testl	$SEGMENT_RPL_MASK, 3*4(%esp)
> +	testl	$USER_SEGMENT_RPL_MASK, 3*4(%esp)
>  	jnz	.Lfrom_usermode_no_fixup_\@
>  
>  	orl	$CS_FROM_KERNEL, 3*4(%esp)
>