x86/entry/vdso32: Don't rely on int80_landing_pad for adjusting ip

There is no fundamental reason to use the int80_landing_pad symbol to
adjust ip when moving the vdso. If ip falls within the vdso, and the
vdso is moved, we should change the ip accordingly, regardless of mode
or location within the vdso. This *currently* can only happen on 32
bits, but there isn't any reason not to do so generically.

Note that if this is ever possible from a vdso-internal call, then the
user space stack will also needed to be adjusted (as well as the
shadow stack, if enabled.) Fortunately this is not currently the case.

At the moment, we don't even consider other threads when moving the
vdso. The assumption is that it is only used by process freeze/thaw
for migration, where this is not an issue.

Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://patch.msgid.link/20251216212606.1325678-5-hpa@zytor.com

Author: H. Peter Anvin

arch/x86/entry/vdso/vma.c

@@ -65,16 +65,12 @@ static vm_fault_t vdso_fault(const struct vm_special_mapping *sm,
 static void vdso_fix_landing(const struct vdso_image *image,
 		struct vm_area_struct *new_vma)
 {
-	if (in_ia32_syscall() && image == &vdso32_image) {
-		struct pt_regs *regs = current_pt_regs();
-		unsigned long vdso_land = image->sym_int80_landing_pad;
-		unsigned long old_land_addr = vdso_land +
-			(unsigned long)current->mm->context.vdso;
-
-		/* Fixing userspace landing - look at do_fast_syscall_32 */
-		if (regs->ip == old_land_addr)
-			regs->ip = new_vma->vm_start + vdso_land;
-	}
+	struct pt_regs *regs = current_pt_regs();
+	unsigned long ipoffset = regs->ip -
+		(unsigned long)current->mm->context.vdso;
+
+	if (ipoffset < image->size)
+		regs->ip = new_vma->vm_start + ipoffset;
 }
 
 static int vdso_mremap(const struct vm_special_mapping *sm,