diff mbox series

[kvm-unit-tests,v2,2/4] x86: Align L2's stacks

Message ID 20211214011823.3277011-3-aaronlewis@google.com (mailing list archive)
State New, archived
Headers show
Series Add additional testing for routing L2 exceptions | expand

Commit Message

Aaron Lewis Dec. 14, 2021, 1:18 a.m. UTC 
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 | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

Comments

Sean Christopherson Jan. 12, 2022, 7:38 p.m. UTC | #1 
On Tue, Dec 14, 2021, Aaron Lewis wrote:
> 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>
> ---

Reviewed-by: Sean Christopherson <seanjc@google.com>
diff mbox series

Patch

diff --git a/x86/vmx.c b/x86/vmx.c
index 6dc9a55..f4fbb94 100644
--- a/x86/vmx.c
+++ b/x86/vmx.c
@@ -42,7 +42,7 @@ 
 u64 *bsp_vmxon_region;
 struct vmcs *vmcs_root;
 u32 vpid_cnt;
-void *guest_stack, *guest_syscall_stack;
+u64 guest_stack_top, guest_syscall_stack_top;
 u32 ctrl_pin, ctrl_enter, ctrl_exit, ctrl_cpu[2];
 struct regs regs;
 
@@ -1241,8 +1241,7 @@  static void init_vmcs_guest(void)
 	vmcs_write(GUEST_CR3, guest_cr3);
 	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));
+	vmcs_write(GUEST_SYSENTER_ESP, guest_syscall_stack_top);
 	vmcs_write(GUEST_SYSENTER_EIP, (u64)(&entry_sysenter));
 	vmcs_write(GUEST_DR7, 0);
 	vmcs_write(GUEST_EFER, rdmsr(MSR_EFER));
@@ -1292,7 +1291,7 @@  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, guest_stack_top);
 	vmcs_write(GUEST_RFLAGS, X86_EFLAGS_FIXED);
 
 	/* 26.3.1.5 */
@@ -1388,8 +1387,8 @@  void init_vmx(u64 *vmxon_region)
 static void alloc_bsp_vmx_pages(void)
 {
 	bsp_vmxon_region = alloc_page();
-	guest_stack = alloc_page();
-	guest_syscall_stack = alloc_page();
+	guest_stack_top = (uintptr_t)alloc_page() + PAGE_SIZE;
+	guest_syscall_stack_top = (uintptr_t)alloc_page() + PAGE_SIZE;
 	vmcs_root = alloc_page();
 }