[RFC PATCH v4 2/5] x86/fault: Add helper function to sanitize error code

[Sean Christopherson <sean.j.christopherson@intel.com>]

...to prepare for vDSO exception fixup, which will expose the error
code to userspace and runs before set_signal_archinfo(), i.e. squashes
the signal when fixup is successful.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
---
 arch/x86/mm/fault.c | 26 ++++++++++++++------------
 1 file changed, 14 insertions(+), 12 deletions(-)

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 7e8a7558ca07..fefeb745d21d 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -719,18 +719,22 @@ pgtable_bad(struct pt_regs *regs, unsigned long error_code,
 	oops_end(flags, regs, sig);
 }
 
-static void set_signal_archinfo(unsigned long address,
-				unsigned long error_code)
+static void sanitize_error_code(unsigned long address,
+				unsigned long *error_code)
 {
-	struct task_struct *tsk = current;
-
 	/*
 	 * To avoid leaking information about the kernel page
 	 * table layout, pretend that user-mode accesses to
 	 * kernel addresses are always protection faults.
 	 */
 	if (address >= TASK_SIZE_MAX)
-		error_code |= X86_PF_PROT;
+		*error_code |= X86_PF_PROT;
+}
+
+static void set_signal_archinfo(unsigned long address,
+				unsigned long error_code)
+{
+	struct task_struct *tsk = current;
 
 	tsk->thread.trap_nr = X86_TRAP_PF;
 	tsk->thread.error_code = error_code | X86_PF_USER;
@@ -771,6 +775,8 @@ no_context(struct pt_regs *regs, unsigned long error_code,
 		 * faulting through the emulate_vsyscall() logic.
 		 */
 		if (current->thread.sig_on_uaccess_err && signal) {
+			sanitize_error_code(address, &error_code);
+
 			set_signal_archinfo(address, error_code);
 
 			/* XXX: hwpoison faults will set the wrong code. */
@@ -920,13 +926,7 @@ __bad_area_nosemaphore(struct pt_regs *regs, unsigned long error_code,
 		if (is_errata100(regs, address))
 			return;
 
-		/*
-		 * To avoid leaking information about the kernel page table
-		 * layout, pretend that user-mode accesses to kernel addresses
-		 * are always protection faults.
-		 */
-		if (address >= TASK_SIZE_MAX)
-			error_code |= X86_PF_PROT;
+		sanitize_error_code(address, &error_code);
 
 		if (likely(show_unhandled_signals))
 			show_signal_msg(regs, error_code, address, tsk);
@@ -1045,6 +1045,8 @@ do_sigbus(struct pt_regs *regs, unsigned long error_code, unsigned long address,
 	if (is_prefetch(regs, error_code, address))
 		return;
 
+	sanitize_error_code(address, &error_code);
+
 	set_signal_archinfo(address, error_code);
 
 #ifdef CONFIG_MEMORY_FAILURE
-- 
2.19.2