[PATCH v2] tboot: Avoid recursive fault in early boot panic with tboot

[PATCH v2] tboot: Avoid recursive fault in early boot panic with tboot

[Jason Andryuk]

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@gmail.com>

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

[Jan Beulich]

>>> On 19.07.18 at 12:39, <jandryuk@gmail.com> 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@gmail.com>

Reviewed-by: Jan Beulich <jbeulich@suse.com>



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

[Jason Andryuk]

On Thu, Jul 19, 2018 at 6:39 AM Jason Andryuk <jandryuk@gmail.com> 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@gmail.com>

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

[Wei, Gang]

> On Aug 28, 2018, at 12:44 AM, Jason Andryuk <jandryuk@gmail.com> wrote:
> 
>> On Thu, Jul 19, 2018 at 6:39 AM Jason Andryuk <jandryuk@gmail.com> 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@gmail.com>
> 
> Ping, Shane and Gang?
Acked-by: Gang Wei <gang.wei@intel.com>
> 
>> ---
>> 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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