SEPT_VE_DISABLE check is required to keep the TD protected from VMM
attacks, but it makes harder to debug guest kernel bugs. If guest
touches unaccepted memory the TD will get terminated without any
traces on what has happened.
Relax the SEPT_VE_DISABLE check to warning on debug TD and panic() in
the #VE handler on EPT-violation on private memory. It will produce
useful backtrace.
Signed-off-by: Kirill A. Shutemov <[email protected]>
---
arch/x86/coco/tdx/tdx.c | 14 ++++++++++++--
1 file changed, 12 insertions(+), 2 deletions(-)
diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
index 8ad04d101270..0e47846ff8ff 100644
--- a/arch/x86/coco/tdx/tdx.c
+++ b/arch/x86/coco/tdx/tdx.c
@@ -38,6 +38,7 @@
#define VE_GET_PORT_NUM(e) ((e) >> 16)
#define VE_IS_IO_STRING(e) ((e) & BIT(4))
+#define ATTR_DEBUG BIT(0)
#define ATTR_SEPT_VE_DISABLE BIT(28)
/* TDX Module call error codes */
@@ -207,8 +208,15 @@ static void tdx_parse_tdinfo(u64 *cc_mask)
* TD-private memory. Only VMM-shared memory (MMIO) will #VE.
*/
td_attr = out.rdx;
- if (!(td_attr & ATTR_SEPT_VE_DISABLE))
- tdx_panic("TD misconfiguration: SEPT_VE_DISABLE attribute must be set.");
+ if (!(td_attr & ATTR_SEPT_VE_DISABLE)) {
+ const char *msg = "TD misconfiguration: SEPT_VE_DISABLE attribute must be set.";
+
+ /* Relax SEPT_VE_DISABLE check for debug TD. */
+ if (td_attr & ATTR_DEBUG)
+ pr_warn("%s\n", msg);
+ else
+ tdx_panic(msg);
+ }
}
/*
@@ -682,6 +690,8 @@ static int virt_exception_kernel(struct pt_regs *regs, struct ve_info *ve)
case EXIT_REASON_CPUID:
return handle_cpuid(regs, ve);
case EXIT_REASON_EPT_VIOLATION:
+ if (ve->gpa != cc_mkdec(ve->gpa))
+ panic("Unexpected EPT-violation on private memory.");
return handle_mmio(regs, ve);
case EXIT_REASON_IO_INSTRUCTION:
return handle_io(regs, ve);
--
2.38.0
On 12/9/22 5:25 AM, Kirill A. Shutemov wrote:
> SEPT_VE_DISABLE check is required to keep the TD protected from VMM
> attacks, but it makes harder to debug guest kernel bugs. If guest
> touches unaccepted memory the TD will get terminated without any
> traces on what has happened.
>
> Relax the SEPT_VE_DISABLE check to warning on debug TD and panic() in
> the #VE handler on EPT-violation on private memory. It will produce
> useful backtrace.
>
> Signed-off-by: Kirill A. Shutemov <[email protected]>
> ---
> arch/x86/coco/tdx/tdx.c | 14 ++++++++++++--
> 1 file changed, 12 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
> index 8ad04d101270..0e47846ff8ff 100644
> --- a/arch/x86/coco/tdx/tdx.c
> +++ b/arch/x86/coco/tdx/tdx.c
> @@ -38,6 +38,7 @@
> #define VE_GET_PORT_NUM(e) ((e) >> 16)
> #define VE_IS_IO_STRING(e) ((e) & BIT(4))
>
> +#define ATTR_DEBUG BIT(0)
> #define ATTR_SEPT_VE_DISABLE BIT(28)
>
> /* TDX Module call error codes */
> @@ -207,8 +208,15 @@ static void tdx_parse_tdinfo(u64 *cc_mask)
> * TD-private memory. Only VMM-shared memory (MMIO) will #VE.
> */
> td_attr = out.rdx;
> - if (!(td_attr & ATTR_SEPT_VE_DISABLE))
> - tdx_panic("TD misconfiguration: SEPT_VE_DISABLE attribute must be set.");
> + if (!(td_attr & ATTR_SEPT_VE_DISABLE)) {
> + const char *msg = "TD misconfiguration: SEPT_VE_DISABLE attribute must be set.";
> +
> + /* Relax SEPT_VE_DISABLE check for debug TD. */
> + if (td_attr & ATTR_DEBUG)
> + pr_warn("%s\n", msg);
> + else
> + tdx_panic(msg);
> + }
> }
>
> /*
> @@ -682,6 +690,8 @@ static int virt_exception_kernel(struct pt_regs *regs, struct ve_info *ve)
> case EXIT_REASON_CPUID:
> return handle_cpuid(regs, ve);
> case EXIT_REASON_EPT_VIOLATION:
> + if (ve->gpa != cc_mkdec(ve->gpa))
> + panic("Unexpected EPT-violation on private memory.");
Why add this change part of TD debug check? Should this be a separate patch?
> return handle_mmio(regs, ve);
> case EXIT_REASON_IO_INSTRUCTION:
> return handle_io(regs, ve);
--
Sathyanarayanan Kuppuswamy
Linux Kernel Developer
On Fri, Dec 09, 2022 at 07:45:34AM -0800, Sathyanarayanan Kuppuswamy wrote:
>
>
> On 12/9/22 5:25 AM, Kirill A. Shutemov wrote:
> > SEPT_VE_DISABLE check is required to keep the TD protected from VMM
> > attacks, but it makes harder to debug guest kernel bugs. If guest
> > touches unaccepted memory the TD will get terminated without any
> > traces on what has happened.
> >
> > Relax the SEPT_VE_DISABLE check to warning on debug TD and panic() in
> > the #VE handler on EPT-violation on private memory. It will produce
> > useful backtrace.
> >
> > Signed-off-by: Kirill A. Shutemov <[email protected]>
> > ---
> > arch/x86/coco/tdx/tdx.c | 14 ++++++++++++--
> > 1 file changed, 12 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
> > index 8ad04d101270..0e47846ff8ff 100644
> > --- a/arch/x86/coco/tdx/tdx.c
> > +++ b/arch/x86/coco/tdx/tdx.c
> > @@ -38,6 +38,7 @@
> > #define VE_GET_PORT_NUM(e) ((e) >> 16)
> > #define VE_IS_IO_STRING(e) ((e) & BIT(4))
> >
> > +#define ATTR_DEBUG BIT(0)
> > #define ATTR_SEPT_VE_DISABLE BIT(28)
> >
> > /* TDX Module call error codes */
> > @@ -207,8 +208,15 @@ static void tdx_parse_tdinfo(u64 *cc_mask)
> > * TD-private memory. Only VMM-shared memory (MMIO) will #VE.
> > */
> > td_attr = out.rdx;
> > - if (!(td_attr & ATTR_SEPT_VE_DISABLE))
> > - tdx_panic("TD misconfiguration: SEPT_VE_DISABLE attribute must be set.");
> > + if (!(td_attr & ATTR_SEPT_VE_DISABLE)) {
> > + const char *msg = "TD misconfiguration: SEPT_VE_DISABLE attribute must be set.";
> > +
> > + /* Relax SEPT_VE_DISABLE check for debug TD. */
> > + if (td_attr & ATTR_DEBUG)
> > + pr_warn("%s\n", msg);
> > + else
> > + tdx_panic(msg);
> > + }
> > }
> >
> > /*
> > @@ -682,6 +690,8 @@ static int virt_exception_kernel(struct pt_regs *regs, struct ve_info *ve)
> > case EXIT_REASON_CPUID:
> > return handle_cpuid(regs, ve);
> > case EXIT_REASON_EPT_VIOLATION:
> > + if (ve->gpa != cc_mkdec(ve->gpa))
> > + panic("Unexpected EPT-violation on private memory.");
>
> Why add this change part of TD debug check? Should this be a separate patch?
This code is never reachable if ATTR_SEPT_VE_DISABLE is set. And the panic
provides backtrace useful for debug.
>
> > return handle_mmio(regs, ve);
> > case EXIT_REASON_IO_INSTRUCTION:
> > return handle_io(regs, ve);
>
> --
> Sathyanarayanan Kuppuswamy
> Linux Kernel Developer
--
Kiryl Shutsemau / Kirill A. Shutemov
On 12/9/22 05:25, Kirill A. Shutemov wrote:
> SEPT_VE_DISABLE check is required to keep the TD protected from VMM
> attacks, but it makes harder to debug guest kernel bugs. If guest
> touches unaccepted memory the TD will get terminated without any
> traces on what has happened.
This is a bit sparse.
--
A "SEPT #VE" occurs when a TDX guest touches memory that is not properly
mapped into the "secure EPT". This can be the result of hypervisor
attacks or bugs, *OR* guest bugs. Most notably, buggy guests might
touch unaccepted memory for lots of different memory safety bugs like
buffer overflows.
TDX guests do not want to continue in the face of hypervisor attacks or
hypervisor bugs. They want to terminate as fast and safely as possible.
SEPT_VE_DISABLE ensures that TDX guests *can't* continue in the face of
these kinds of issues.
But, that causes a problem. TDX guests that can't continue can't spit
out oopses or other debugging info. In essence SEPT_VE_DISABLE=1 guests
are not debuggable. That's a problem.
--
Eh?
> Relax the SEPT_VE_DISABLE check to warning on debug TD and panic() in
> the #VE handler on EPT-violation on private memory. It will produce
> useful backtrace.
>
> Signed-off-by: Kirill A. Shutemov <[email protected]>
> ---
> arch/x86/coco/tdx/tdx.c | 14 ++++++++++++--
> 1 file changed, 12 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
> index 8ad04d101270..0e47846ff8ff 100644
> --- a/arch/x86/coco/tdx/tdx.c
> +++ b/arch/x86/coco/tdx/tdx.c
> @@ -38,6 +38,7 @@
> #define VE_GET_PORT_NUM(e) ((e) >> 16)
> #define VE_IS_IO_STRING(e) ((e) & BIT(4))
>
> +#define ATTR_DEBUG BIT(0)
> #define ATTR_SEPT_VE_DISABLE BIT(28)
>
> /* TDX Module call error codes */
> @@ -207,8 +208,15 @@ static void tdx_parse_tdinfo(u64 *cc_mask)
> * TD-private memory. Only VMM-shared memory (MMIO) will #VE.
> */
> td_attr = out.rdx;
> - if (!(td_attr & ATTR_SEPT_VE_DISABLE))
> - tdx_panic("TD misconfiguration: SEPT_VE_DISABLE attribute must be set.");
> + if (!(td_attr & ATTR_SEPT_VE_DISABLE)) {
> + const char *msg = "TD misconfiguration: SEPT_VE_DISABLE attribute must be set.";
> +
> + /* Relax SEPT_VE_DISABLE check for debug TD. */
> + if (td_attr & ATTR_DEBUG)
> + pr_warn("%s\n", msg);
> + else
> + tdx_panic(msg);
> + }
> }
>
> /*
> @@ -682,6 +690,8 @@ static int virt_exception_kernel(struct pt_regs *regs, struct ve_info *ve)
> case EXIT_REASON_CPUID:
> return handle_cpuid(regs, ve);
> case EXIT_REASON_EPT_VIOLATION:
> + if (ve->gpa != cc_mkdec(ve->gpa))
> + panic("Unexpected EPT-violation on private memory.");
What's the cc_mkdec() doing?
On Tue, Dec 13, 2022 at 03:13:43PM -0800, Dave Hansen wrote:
> On 12/9/22 05:25, Kirill A. Shutemov wrote:
> > SEPT_VE_DISABLE check is required to keep the TD protected from VMM
> > attacks, but it makes harder to debug guest kernel bugs. If guest
> > touches unaccepted memory the TD will get terminated without any
> > traces on what has happened.
>
> This is a bit sparse.
>
> --
>
> A "SEPT #VE" occurs when a TDX guest touches memory that is not properly
> mapped into the "secure EPT". This can be the result of hypervisor
> attacks or bugs, *OR* guest bugs. Most notably, buggy guests might
> touch unaccepted memory for lots of different memory safety bugs like
> buffer overflows.
>
> TDX guests do not want to continue in the face of hypervisor attacks or
> hypervisor bugs. They want to terminate as fast and safely as possible.
> SEPT_VE_DISABLE ensures that TDX guests *can't* continue in the face of
> these kinds of issues.
>
> But, that causes a problem. TDX guests that can't continue can't spit
> out oopses or other debugging info. In essence SEPT_VE_DISABLE=1 guests
> are not debuggable. That's a problem.
>
> --
>
> Eh?
Thanks!
> > Relax the SEPT_VE_DISABLE check to warning on debug TD and panic() in
> > the #VE handler on EPT-violation on private memory. It will produce
> > useful backtrace.
> >
> > Signed-off-by: Kirill A. Shutemov <[email protected]>
> > ---
> > arch/x86/coco/tdx/tdx.c | 14 ++++++++++++--
> > 1 file changed, 12 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
> > index 8ad04d101270..0e47846ff8ff 100644
> > --- a/arch/x86/coco/tdx/tdx.c
> > +++ b/arch/x86/coco/tdx/tdx.c
> > @@ -38,6 +38,7 @@
> > #define VE_GET_PORT_NUM(e) ((e) >> 16)
> > #define VE_IS_IO_STRING(e) ((e) & BIT(4))
> >
> > +#define ATTR_DEBUG BIT(0)
> > #define ATTR_SEPT_VE_DISABLE BIT(28)
> >
> > /* TDX Module call error codes */
> > @@ -207,8 +208,15 @@ static void tdx_parse_tdinfo(u64 *cc_mask)
> > * TD-private memory. Only VMM-shared memory (MMIO) will #VE.
> > */
> > td_attr = out.rdx;
> > - if (!(td_attr & ATTR_SEPT_VE_DISABLE))
> > - tdx_panic("TD misconfiguration: SEPT_VE_DISABLE attribute must be set.");
> > + if (!(td_attr & ATTR_SEPT_VE_DISABLE)) {
> > + const char *msg = "TD misconfiguration: SEPT_VE_DISABLE attribute must be set.";
> > +
> > + /* Relax SEPT_VE_DISABLE check for debug TD. */
> > + if (td_attr & ATTR_DEBUG)
> > + pr_warn("%s\n", msg);
> > + else
> > + tdx_panic(msg);
> > + }
> > }
> >
> > /*
> > @@ -682,6 +690,8 @@ static int virt_exception_kernel(struct pt_regs *regs, struct ve_info *ve)
> > case EXIT_REASON_CPUID:
> > return handle_cpuid(regs, ve);
> > case EXIT_REASON_EPT_VIOLATION:
> > + if (ve->gpa != cc_mkdec(ve->gpa))
> > + panic("Unexpected EPT-violation on private memory.");
>
> What's the cc_mkdec() doing?
Checks if the GPA is private. I will move it to helper. Like this:
static inline bool is_private_gpa(u64 gpa)
{
return gpa == cc_mkenc(gpa);
}
--
Kiryl Shutsemau / Kirill A. Shutemov