From: "Dr. David Alan Gilbert" <[email protected]>
A live migration under qemu is currently failing when the source
host is ~Nehalem era (pre-xsave) and the destination is much newer,
(configured with a guest CPU type of Nehalem).
QEMU always calls kvm_put_xsave, even on this combination because
KVM_CAP_CHECK_EXTENSION_VM always returns true for KVM_CAP_XSAVE.
When QEMU calls kvm_put_xsave it's rejected by
fpu_copy_uabi_to_guest_fpstate->
copy_uabi_to_xstate->
validate_user_xstate_header
when the validate checks the loaded xfeatures against
user_xfeatures, which it finds to be 0.
I think our initialisation of user_xfeatures is being
too strict here, and we should always allow the base FP/SSE.
Fixes: ad856280ddea ("x86/kvm/fpu: Limit guest user_xfeatures to supported bits of XCR0")
bz: https://bugzilla.redhat.com/show_bug.cgi?id=2079311
Signed-off-by: Dr. David Alan Gilbert <[email protected]>
---
arch/x86/kvm/cpuid.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index de6d44e07e34..3b2319cecfd1 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -298,7 +298,8 @@ static void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
guest_supported_xcr0 =
cpuid_get_supported_xcr0(vcpu->arch.cpuid_entries, vcpu->arch.cpuid_nent);
- vcpu->arch.guest_fpu.fpstate->user_xfeatures = guest_supported_xcr0;
+ vcpu->arch.guest_fpu.fpstate->user_xfeatures = guest_supported_xcr0 |
+ XFEATURE_MASK_FPSSE;
kvm_update_pv_runtime(vcpu);
--
2.37.2
On Tue, Aug 16, 2022, Dr. David Alan Gilbert (git) wrote:
> From: "Dr. David Alan Gilbert" <[email protected]>
>
> A live migration under qemu is currently failing when the source
> host is ~Nehalem era (pre-xsave) and the destination is much newer,
> (configured with a guest CPU type of Nehalem).
> QEMU always calls kvm_put_xsave, even on this combination because
> KVM_CAP_CHECK_EXTENSION_VM always returns true for KVM_CAP_XSAVE.
>
> When QEMU calls kvm_put_xsave it's rejected by
> fpu_copy_uabi_to_guest_fpstate->
> copy_uabi_to_xstate->
> validate_user_xstate_header
>
> when the validate checks the loaded xfeatures against
> user_xfeatures, which it finds to be 0.
>
> I think our initialisation of user_xfeatures is being
> too strict here, and we should always allow the base FP/SSE.
>
> Fixes: ad856280ddea ("x86/kvm/fpu: Limit guest user_xfeatures to supported bits of XCR0")
> bz: https://bugzilla.redhat.com/show_bug.cgi?id=2079311
>
> Signed-off-by: Dr. David Alan Gilbert <[email protected]>
> ---
> arch/x86/kvm/cpuid.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index de6d44e07e34..3b2319cecfd1 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -298,7 +298,8 @@ static void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
> guest_supported_xcr0 =
> cpuid_get_supported_xcr0(vcpu->arch.cpuid_entries, vcpu->arch.cpuid_nent);
>
> - vcpu->arch.guest_fpu.fpstate->user_xfeatures = guest_supported_xcr0;
> + vcpu->arch.guest_fpu.fpstate->user_xfeatures = guest_supported_xcr0 |
> + XFEATURE_MASK_FPSSE;
I don't think this is correct. This will allow the guest to set the SSE bit
even when XSAVE isn't supported due to kvm_guest_supported_xcr0() returning
user_xfeatures.
static inline u64 kvm_guest_supported_xcr0(struct kvm_vcpu *vcpu)
{
return vcpu->arch.guest_fpu.fpstate->user_xfeatures;
}
I believe the right place to fix this is in validate_user_xstate_header(). It's
reachable if and only if XSAVE is supported in the host, and when XSAVE is _not_
supported, the kernel unconditionally allows FP+SSE. So it follows that the kernel
should also allow FP+SSE when using XSAVE too. That would also align the logic
with fpu_copy_guest_fpstate_to_uabi(), which fordces the FPSSE flags. Ditto for
the non-KVM save_xstate_epilog().
Aha! And fpu__init_system_xstate() ensure the host supports FP+SSE when XSAVE
is enabled (knew their had to be a sanity check somewhere).
---
arch/x86/kernel/fpu/xstate.c | 9 +++++++--
1 file changed, 7 insertions(+), 2 deletions(-)
diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index c8340156bfd2..83b9a9653d47 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -399,8 +399,13 @@ int xfeature_size(int xfeature_nr)
static int validate_user_xstate_header(const struct xstate_header *hdr,
struct fpstate *fpstate)
{
- /* No unknown or supervisor features may be set */
- if (hdr->xfeatures & ~fpstate->user_xfeatures)
+ /*
+ * No unknown or supervisor features may be set. Userspace is always
+ * allowed to restore FP+SSE state (XSAVE/XRSTOR are used by the kernel
+ * if and only if FP+SSE are supported in xstate).
+ */
+ if (hdr->xfeatures & ~fpstate->user_xfeatures &
+ ~(XFEATURE_MASK_FP | XFEATURE_MASK_SSE))
return -EINVAL;
/* Userspace must use the uncompacted format */
base-commit: de3d415edca23831c5d1f24f10c74a715af7efdb
--
On Tue, 2022-08-16 at 21:37 +0000, Sean Christopherson wrote:
> On Tue, Aug 16, 2022, Dr. David Alan Gilbert (git) wrote:
> > From: "Dr. David Alan Gilbert" <[email protected]>
> >
> > A live migration under qemu is currently failing when the source
> > host is ~Nehalem era (pre-xsave) and the destination is much newer,
> > (configured with a guest CPU type of Nehalem).
> > QEMU always calls kvm_put_xsave, even on this combination because
> > KVM_CAP_CHECK_EXTENSION_VM always returns true for KVM_CAP_XSAVE.
> >
> > When QEMU calls kvm_put_xsave it's rejected by
> > fpu_copy_uabi_to_guest_fpstate->
> > copy_uabi_to_xstate->
> > validate_user_xstate_header
> >
> > when the validate checks the loaded xfeatures against
> > user_xfeatures, which it finds to be 0.
> >
> > I think our initialisation of user_xfeatures is being
> > too strict here, and we should always allow the base FP/SSE.
> >
> > Fixes: ad856280ddea ("x86/kvm/fpu: Limit guest user_xfeatures to supported bits of XCR0")
Thanks for fixing this, Dave!
> > bz: https://bugzilla.redhat.com/show_bug.cgi?id=2079311
> >
> > Signed-off-by: Dr. David Alan Gilbert <[email protected]>
> > ---
> > arch/x86/kvm/cpuid.c | 3 ++-
> > 1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> > index de6d44e07e34..3b2319cecfd1 100644
> > --- a/arch/x86/kvm/cpuid.c
> > +++ b/arch/x86/kvm/cpuid.c
> > @@ -298,7 +298,8 @@ static void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
> > guest_supported_xcr0 =
> > cpuid_get_supported_xcr0(vcpu->arch.cpuid_entries, vcpu->arch.cpuid_nent);
> >
> > - vcpu->arch.guest_fpu.fpstate->user_xfeatures = guest_supported_xcr0;
> > + vcpu->arch.guest_fpu.fpstate->user_xfeatures = guest_supported_xcr0 |
> > + XFEATURE_MASK_FPSSE;
>
> I don't think this is correct. This will allow the guest to set the SSE bit
> even when XSAVE isn't supported due to kvm_guest_supported_xcr0() returning
> user_xfeatures.
>
> static inline u64 kvm_guest_supported_xcr0(struct kvm_vcpu *vcpu)
> {
> return vcpu->arch.guest_fpu.fpstate->user_xfeatures;
> }
>
> I believe the right place to fix this is in validate_user_xstate_header(). It's
> reachable if and only if XSAVE is supported in the host, and when XSAVE is _not_
> supported, the kernel unconditionally allows FP+SSE. So it follows that the kernel
> should also allow FP+SSE when using XSAVE too. That would also align the logic
> with fpu_copy_guest_fpstate_to_uabi(), which fordces the FPSSE flags. Ditto for
> the non-KVM save_xstate_epilog().
>
> Aha! And fpu__init_system_xstate() ensure the host supports FP+SSE when XSAVE
> is enabled (knew their had to be a sanity check somewhere).
Thanks for the feedback Sean!
I have near to no experience in this code, and I hope you can help me with a
question I have, based in Dave's commit message:
> > QEMU always calls kvm_put_xsave, even on this combination because
> > KVM_CAP_CHECK_EXTENSION_VM always returns true for KVM_CAP_XSAVE.
Any particular reason why it always returns true for KVM_CAP_XSAVE, even when
the CPU does not support it?
IIUC, if it returns false to this capability, kvm_put_xsave() should never be
called, and thus it can avoid bug reproduction.
Thanks in advance,
Leo
>
> ---
> arch/x86/kernel/fpu/xstate.c | 9 +++++++--
> 1 file changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
> index c8340156bfd2..83b9a9653d47 100644
> --- a/arch/x86/kernel/fpu/xstate.c
> +++ b/arch/x86/kernel/fpu/xstate.c
> @@ -399,8 +399,13 @@ int xfeature_size(int xfeature_nr)
> static int validate_user_xstate_header(const struct xstate_header *hdr,
> struct fpstate *fpstate)
> {
> - /* No unknown or supervisor features may be set */
> - if (hdr->xfeatures & ~fpstate->user_xfeatures)
> + /*
> + * No unknown or supervisor features may be set. Userspace is always
> + * allowed to restore FP+SSE state (XSAVE/XRSTOR are used by the kernel
> + * if and only if FP+SSE are supported in xstate).
> + */
> + if (hdr->xfeatures & ~fpstate->user_xfeatures &
> + ~(XFEATURE_MASK_FP | XFEATURE_MASK_SSE))
> return -EINVAL;
>
> /* Userspace must use the uncompacted format */
>
> base-commit: de3d415edca23831c5d1f24f10c74a715af7efdb
> --
>
On 8/17/22 05:29, Leonardo Brás wrote:
>>> QEMU always calls kvm_put_xsave, even on this combination because
>>> KVM_CAP_CHECK_EXTENSION_VM always returns true for KVM_CAP_XSAVE.
> Any particular reason why it always returns true for KVM_CAP_XSAVE, even when
> the CPU does not support it?
>
> IIUC, if it returns false to this capability, kvm_put_xsave() should never be
> called, and thus it can avoid bug reproduction.
Because it allows userspace to have a single path for saving/restoring
FPU state. See for example the "migration" code in
tools/testing/selftests/kvm/lib/x86_64/processor.c (the vcpu_save_state
and vcpu_load_state functions).
In fact, the QEMU code that uses KVM_GET_FPU/KVM_SET_FPU in x86 is
obsolete, because it's not been used since Linux 2.6.36.
Paolo
* Sean Christopherson ([email protected]) wrote:
> On Tue, Aug 16, 2022, Dr. David Alan Gilbert (git) wrote:
> > From: "Dr. David Alan Gilbert" <[email protected]>
> >
> > A live migration under qemu is currently failing when the source
> > host is ~Nehalem era (pre-xsave) and the destination is much newer,
> > (configured with a guest CPU type of Nehalem).
> > QEMU always calls kvm_put_xsave, even on this combination because
> > KVM_CAP_CHECK_EXTENSION_VM always returns true for KVM_CAP_XSAVE.
> >
> > When QEMU calls kvm_put_xsave it's rejected by
> > fpu_copy_uabi_to_guest_fpstate->
> > copy_uabi_to_xstate->
> > validate_user_xstate_header
> >
> > when the validate checks the loaded xfeatures against
> > user_xfeatures, which it finds to be 0.
> >
> > I think our initialisation of user_xfeatures is being
> > too strict here, and we should always allow the base FP/SSE.
> >
> > Fixes: ad856280ddea ("x86/kvm/fpu: Limit guest user_xfeatures to supported bits of XCR0")
> > bz: https://bugzilla.redhat.com/show_bug.cgi?id=2079311
> >
> > Signed-off-by: Dr. David Alan Gilbert <[email protected]>
> > ---
> > arch/x86/kvm/cpuid.c | 3 ++-
> > 1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> > index de6d44e07e34..3b2319cecfd1 100644
> > --- a/arch/x86/kvm/cpuid.c
> > +++ b/arch/x86/kvm/cpuid.c
> > @@ -298,7 +298,8 @@ static void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
> > guest_supported_xcr0 =
> > cpuid_get_supported_xcr0(vcpu->arch.cpuid_entries, vcpu->arch.cpuid_nent);
> >
> > - vcpu->arch.guest_fpu.fpstate->user_xfeatures = guest_supported_xcr0;
> > + vcpu->arch.guest_fpu.fpstate->user_xfeatures = guest_supported_xcr0 |
> > + XFEATURE_MASK_FPSSE;
Hi Sean,
Thanks for the reply,
> I don't think this is correct. This will allow the guest to set the SSE bit
> even when XSAVE isn't supported due to kvm_guest_supported_xcr0() returning
> user_xfeatures.
>
> static inline u64 kvm_guest_supported_xcr0(struct kvm_vcpu *vcpu)
> {
> return vcpu->arch.guest_fpu.fpstate->user_xfeatures;
> }
>
> I believe the right place to fix this is in validate_user_xstate_header(). It's
> reachable if and only if XSAVE is supported in the host, and when XSAVE is _not_
> supported, the kernel unconditionally allows FP+SSE. So it follows that the kernel
> should also allow FP+SSE when using XSAVE too. That would also align the logic
> with fpu_copy_guest_fpstate_to_uabi(), which fordces the FPSSE flags. Ditto for
> the non-KVM save_xstate_epilog().
OK, yes, I'd followed the check that failed down to this test; although
by itself this test works until Leo's patch came along later; so I
wasn't sure where to fix it.
> Aha! And fpu__init_system_xstate() ensure the host supports FP+SSE when XSAVE
> is enabled (knew their had to be a sanity check somewhere).
>
> ---
> arch/x86/kernel/fpu/xstate.c | 9 +++++++--
> 1 file changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
> index c8340156bfd2..83b9a9653d47 100644
> --- a/arch/x86/kernel/fpu/xstate.c
> +++ b/arch/x86/kernel/fpu/xstate.c
> @@ -399,8 +399,13 @@ int xfeature_size(int xfeature_nr)
> static int validate_user_xstate_header(const struct xstate_header *hdr,
> struct fpstate *fpstate)
> {
> - /* No unknown or supervisor features may be set */
> - if (hdr->xfeatures & ~fpstate->user_xfeatures)
> + /*
> + * No unknown or supervisor features may be set. Userspace is always
> + * allowed to restore FP+SSE state (XSAVE/XRSTOR are used by the kernel
> + * if and only if FP+SSE are supported in xstate).
> + */
> + if (hdr->xfeatures & ~fpstate->user_xfeatures &
> + ~(XFEATURE_MASK_FP | XFEATURE_MASK_SSE))
> return -EINVAL;
>
> /* Userspace must use the uncompacted format */
That passes the small smoke test for me; will you repost that then?
Thanks,
Dave
> base-commit: de3d415edca23831c5d1f24f10c74a715af7efdb
> --
>
--
Dr. David Alan Gilbert / [email protected] / Manchester, UK
On Wed, Aug 17, 2022, Dr. David Alan Gilbert wrote:
> That passes the small smoke test for me; will you repost that then?
Yep, will do.
* Sean Christopherson ([email protected]) wrote:
> On Wed, Aug 17, 2022, Dr. David Alan Gilbert wrote:
> > That passes the small smoke test for me; will you repost that then?
>
> Yep, will do.
Thanks.
Dave
--
Dr. David Alan Gilbert / [email protected] / Manchester, UK
On Wed, Aug 17, 2022, Dr. David Alan Gilbert wrote:
> * Sean Christopherson ([email protected]) wrote:
> > On Tue, Aug 16, 2022, Dr. David Alan Gilbert (git) wrote:
> > > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> > > index de6d44e07e34..3b2319cecfd1 100644
> > > --- a/arch/x86/kvm/cpuid.c
> > > +++ b/arch/x86/kvm/cpuid.c
> > > @@ -298,7 +298,8 @@ static void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
> > > guest_supported_xcr0 =
> > > cpuid_get_supported_xcr0(vcpu->arch.cpuid_entries, vcpu->arch.cpuid_nent);
> > >
> > > - vcpu->arch.guest_fpu.fpstate->user_xfeatures = guest_supported_xcr0;
> > > + vcpu->arch.guest_fpu.fpstate->user_xfeatures = guest_supported_xcr0 |
> > > + XFEATURE_MASK_FPSSE;
>
> Hi Sean,
> Thanks for the reply,
>
> > I don't think this is correct. This will allow the guest to set the SSE bit
> > even when XSAVE isn't supported due to kvm_guest_supported_xcr0() returning
> > user_xfeatures.
> >
> > static inline u64 kvm_guest_supported_xcr0(struct kvm_vcpu *vcpu)
> > {
> > return vcpu->arch.guest_fpu.fpstate->user_xfeatures;
> > }
> >
> > I believe the right place to fix this is in validate_user_xstate_header(). It's
> > reachable if and only if XSAVE is supported in the host, and when XSAVE is _not_
> > supported, the kernel unconditionally allows FP+SSE. So it follows that the kernel
> > should also allow FP+SSE when using XSAVE too. That would also align the logic
> > with fpu_copy_guest_fpstate_to_uabi(), which fordces the FPSSE flags. Ditto for
> > the non-KVM save_xstate_epilog().
>
> OK, yes, I'd followed the check that failed down to this test; although
> by itself this test works until Leo's patch came along later; so I
> wasn't sure where to fix it.
>
> > Aha! And fpu__init_system_xstate() ensure the host supports FP+SSE when XSAVE
> > is enabled (knew their had to be a sanity check somewhere).
> >
> > ---
> > arch/x86/kernel/fpu/xstate.c | 9 +++++++--
> > 1 file changed, 7 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
> > index c8340156bfd2..83b9a9653d47 100644
> > --- a/arch/x86/kernel/fpu/xstate.c
> > +++ b/arch/x86/kernel/fpu/xstate.c
> > @@ -399,8 +399,13 @@ int xfeature_size(int xfeature_nr)
> > static int validate_user_xstate_header(const struct xstate_header *hdr,
> > struct fpstate *fpstate)
> > {
> > - /* No unknown or supervisor features may be set */
> > - if (hdr->xfeatures & ~fpstate->user_xfeatures)
> > + /*
> > + * No unknown or supervisor features may be set. Userspace is always
> > + * allowed to restore FP+SSE state (XSAVE/XRSTOR are used by the kernel
> > + * if and only if FP+SSE are supported in xstate).
> > + */
> > + if (hdr->xfeatures & ~fpstate->user_xfeatures &
> > + ~(XFEATURE_MASK_FP | XFEATURE_MASK_SSE))
> > return -EINVAL;
> >
> > /* Userspace must use the uncompacted format */
>
> That passes the small smoke test for me; will you repost that then?
*sigh*
The bug is more subtle than just failing to restore. Saving can also "fail". If
XSAVE is hidden from the guest on an XSAVE-capable host, __copy_xstate_to_uabi_buf()
will happily reinitialize FP+SSE state and thus corrupt guest FPU state on migration.
And not that it matters now, but before realizing that KVM_GET_XSAVE is also broken,
I decided I like Dave's patch better because KVM really should separate what userspace
can save/restore from what the guest can access.
Amusingly, there's actually another bug lurking with respect to usurping user_xfeatures
to represent supported_guest_xcr0. The latter is zero-initialized, whereas
user_xfeatures is set to the "default" features on initialization, i.e. migrating a
VM without ever doing KVM_SET_CPUID2 would do odd things.
Sending a v2 shortly to reinstate guest_supported_xcr0 before landing Dave's patch.