Re: [Xen-devel] [PATCH v2] tboot: Avoid recursive fault in early boot panic with tboot

[Jason Andryuk <jandryuk@xxxxxxxxx>]

On Thu, Jul 19, 2018 at 6:39 AM Jason Andryuk <jandryuk@xxxxxxxxx> wrote:
>
> If panic is called before init_idle_domain on a tboot-launched system,
> then Xen recursively faults in write_ptbase as seen below.
>
> (XEN)    [<ffff82d080286690>] write_ptbase+0/0x10
> (XEN)    [<ffff82d0802c4c3b>] tboot_shutdown+0x6b/0x260
> (XEN)    [<ffff82d08029ddac>] machine_restart+0xac/0x2d0
> (XEN)    [<ffff82d080286690>] write_ptbase+0/0x10
> (XEN)    [<ffff82d0802446c1>] panic+0x111/0x120
> (XEN)    [<ffff82d0802a51c1>] do_general_protection+0x171/0x1f0
> (XEN)    [<ffff82d080287a82>] mm.c#virt_to_xen_l2e+0x12/0x1c0
> (XEN)    [<ffff82d080354720>] x86_64/entry.S#handle_exception_saved+0x66/0xa4
> (XEN)    [<ffff82d080286690>] write_ptbase+0/0x10
> (XEN)    [<ffff82d0802c4c3b>] tboot_shutdown+0x6b/0x260
> (XEN)    [<ffff82d08029ddac>] machine_restart+0xac/0x2d0
> (XEN)    [<ffff82d0802446c1>] panic+0x111/0x120
> (XEN)    [<ffff82d0803c11a0>] setup.c#bootstrap_map+0/0x11a
> (XEN)    [<ffff82d0803b82a0>] flask_op.c#parse_flask_param+0/0xb0
> (XEN)    [<ffff82d0803c11a0>] setup.c#bootstrap_map+0/0x11a
> (XEN)    [<ffff82d0803b6f6c>] xsm_multiboot_init+0x7c/0xb0
> (XEN)    [<ffff82d0803c34bb>] __start_xen+0x1d2b/0x2da0
> (XEN)    [<ffff82d0802000f3>] __high_start+0x53/0x60
>
> idle_vcpu[0] is still poisoned with INVALID_VCPU, so write_ptbase faults
> dereferencing the pointer.  This fault calls panic and recurses through
> the same code path.
>
> If tboot_shutdown is called while idle_vcpu[0] == INVALID_VCPU, then we
> are still operating with the initial page tables.  Therefore changing
> page tables with write_ptbase is unnecessary.
>
> An easy way to reproduce this is to use tboot to launch an XSM-enabled
> Xen without an XSM policy.
>
> Signed-off-by: Jason Andryuk <jandryuk@xxxxxxxxx>

Ping, Shane and Gang?

> ---
> v2: Correct comment and brace style
> ---
>  xen/arch/x86/tboot.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/xen/arch/x86/tboot.c b/xen/arch/x86/tboot.c
> index fb4616ae83..d5a5292d7e 100644
> --- a/xen/arch/x86/tboot.c
> +++ b/xen/arch/x86/tboot.c
> @@ -391,7 +391,12 @@ void tboot_shutdown(uint32_t shutdown_type)
>          tboot_gen_xenheap_integrity(g_tboot_shared->s3_key, &xenheap_mac);
>      }
>
> -    write_ptbase(idle_vcpu[0]);
> +    /*
> +     * During early boot, we can be called by panic before idle_vcpu[0] is
> +     * setup, but in that case we don't need to change page tables.
> +     */
> +    if ( idle_vcpu[0] != INVALID_VCPU )
> +        write_ptbase(idle_vcpu[0]);
>
>      ((void(*)(void))(unsigned long)g_tboot_shared->shutdown_entry)();
>
> --
> 2.17.1
>

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