x86/mm: Add vsyscall address helper

jira LE-1907
cve CVE-2024-26906
Rebuild_History Non-Buildable kernel-4.18.0-553.8.1.el8_10
commit-author Dave Hansen <[email protected]>
commit 02e983b

We will shortly be using this check in two locations.  Put it in
a helper before we do so.

Let's also insert PAGE_MASK instead of the open-coded ~0xfff.
It is easier to read and also more obviously correct considering
the implicit type conversion that has to happen when it is not
an implicit 'unsigned long'.

	Cc: [email protected]
	Cc: Jann Horn <[email protected]>
	Cc: Sean Christopherson <[email protected]>
	Cc: Thomas Gleixner <[email protected]>
	Cc: Andy Lutomirski <[email protected]>
	Signed-off-by: Dave Hansen <[email protected]>
	Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
(cherry picked from commit 02e983b)
	Signed-off-by: Jonathan Maple <[email protected]>

Author: PlaidCat

arch/x86/mm/fault.c

@@ -823,6 +823,15 @@ show_signal_msg(struct pt_regs *regs, unsigned long error_code,
 	show_opcodes(regs, loglvl);
 }
 
+/*
+ * The (legacy) vsyscall page is the long page in the kernel portion
+ * of the address space that has user-accessible permissions.
+ */
+static bool is_vsyscall_vaddr(unsigned long vaddr)
+{
+	return (vaddr & PAGE_MASK) == VSYSCALL_ADDR;
+}
+
 static void
 __bad_area_nosemaphore(struct pt_regs *regs, unsigned long error_code,
 		       unsigned long address, u32 pkey, int si_code)
@@ -852,7 +861,7 @@ __bad_area_nosemaphore(struct pt_regs *regs, unsigned long error_code,
 		 * emulation.
 		 */
 		if (unlikely((error_code & X86_PF_INSTR) &&
-			     ((address & ~0xfff) == VSYSCALL_ADDR))) {
+			     is_vsyscall_vaddr(address))) {
 			if (emulate_vsyscall(regs, address))
 				return;
 		}