From: Hendrik Borghorst hborghor@amazon.de
commit a44b331614e6f7e63902ed7dff7adc8c85edd8bc upstream.
When serializing and deserializing kvm_sregs, attributes of the segment descriptors are stored by user space. For unusable segments, vmx_segment_access_rights skips all attributes and sets them to 0.
This means we zero out the DPL (Descriptor Privilege Level) for unusable entries.
Unusable segments are - contrary to their name - usable in 64bit mode and are used by guests to for example create a linear map through the NULL selector.
VMENTER checks if SS.DPL is correct depending on the CS segment type. For types 9 (Execute Only) and 11 (Execute Read), CS.DPL must be equal to SS.DPL [1].
We have seen real world guests setting CS to a usable segment with DPL=3 and SS to an unusable segment with DPL=3. Once we go through an sregs get/set cycle, SS.DPL turns to 0. This causes the virtual machine to crash reproducibly.
This commit changes the attribute logic to always preserve attributes for unusable segments. According to [2] SS.DPL is always saved on VM exits, regardless of the unusable bit so user space applications should have saved the information on serialization correctly.
[3] specifies that besides SS.DPL the rest of the attributes of the descriptors are undefined after VM entry if unusable bit is set. So, there should be no harm in setting them all to the previous state.
[1] Intel SDM Vol 3C 26.3.1.2 Checks on Guest Segment Registers [2] Intel SDM Vol 3C 27.3.2 Saving Segment Registers and Descriptor-Table Registers [3] Intel SDM Vol 3C 26.3.2.2 Loading Guest Segment Registers and Descriptor-Table Registers
Cc: Alexander Graf graf@amazon.de Cc: stable@vger.kernel.org Signed-off-by: Hendrik Borghorst hborghor@amazon.de Reviewed-by: Jim Mattson jmattson@google.com Reviewed-by: Alexander Graf graf@amazon.com Message-Id: 20221114164823.69555-1-hborghor@amazon.de Signed-off-by: Paolo Bonzini pbonzini@redhat.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- arch/x86/kvm/vmx/vmx.c | 21 +++++++++------------ 1 file changed, 9 insertions(+), 12 deletions(-)
--- a/arch/x86/kvm/vmx/vmx.c +++ b/arch/x86/kvm/vmx/vmx.c @@ -3412,18 +3412,15 @@ static u32 vmx_segment_access_rights(str { u32 ar;
- if (var->unusable || !var->present) - ar = 1 << 16; - else { - ar = var->type & 15; - ar |= (var->s & 1) << 4; - ar |= (var->dpl & 3) << 5; - ar |= (var->present & 1) << 7; - ar |= (var->avl & 1) << 12; - ar |= (var->l & 1) << 13; - ar |= (var->db & 1) << 14; - ar |= (var->g & 1) << 15; - } + ar = var->type & 15; + ar |= (var->s & 1) << 4; + ar |= (var->dpl & 3) << 5; + ar |= (var->present & 1) << 7; + ar |= (var->avl & 1) << 12; + ar |= (var->l & 1) << 13; + ar |= (var->db & 1) << 14; + ar |= (var->g & 1) << 15; + ar |= (var->unusable || !var->present) << 16;
return ar; }