[kvm-unit-tests PATCH 2/3] x86: Align L2's stacks

Aaron Lewis
2021-12-09T18:26:51Z

Setting the stack to PAGE_SIZE - 1 sets the stack to being 1-byte
aligned, which fails in usermode with alignment checks enabled (ie: with
flags cr0.am set and eflags.ac set). This was causing an #AC in
usermode.c when preparing to call the callback in run_in_user().
Aligning the stack fixes the issue.

For the purposes of fixing the #AC in usermode.c the stack has to be
aligned to at least an 8-byte boundary. Setting it to a page aligned
boundary ensures any stack alignment requirements are met as x86_64
stacks generally want to be 16-byte aligned.

Signed-off-by: Aaron Lewis <aaronlewis@google.com>
---
x86/vmx.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/x86/vmx.c b/x86/vmx.c
index 6dc9a55..44f4861 100644
--- a/x86/vmx.c
+++ b/x86/vmx.c
@@ -1242,7 +1242,7 @@ static void init_vmcs_guest(void)
vmcs_write(GUEST_CR4, guest_cr4);
vmcs_write(GUEST_SYSENTER_CS, KERNEL_CS);
vmcs_write(GUEST_SYSENTER_ESP,
- (u64)(guest_syscall_stack + PAGE_SIZE - 1));
+ (u64)(guest_syscall_stack + PAGE_SIZE));
vmcs_write(GUEST_SYSENTER_EIP, (u64)(&entry_sysenter));
vmcs_write(GUEST_DR7, 0);
vmcs_write(GUEST_EFER, rdmsr(MSR_EFER));
@@ -1292,7 +1292,7 @@ static void init_vmcs_guest(void)

/* 26.3.1.4 */
vmcs_write(GUEST_RIP, (u64)(&guest_entry));
- vmcs_write(GUEST_RSP, (u64)(guest_stack + PAGE_SIZE - 1));
+ vmcs_write(GUEST_RSP, (u64)(guest_stack + PAGE_SIZE));
vmcs_write(GUEST_RFLAGS, X86_EFLAGS_FIXED);

/* 26.3.1.5 */
--
2.34.1.173.g76aa8bc2d0-goog