linux-kernel - [PATCH 1/2] x86/context-tracking: Remove exception_enter/exit() from do_page

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Message-Id: <20191227163612.10039-2-frederic@kernel.org>
Date:   Fri, 27 Dec 2019 17:36:11 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     linux-kernel@...r.kernel.org
Cc:     Frederic Weisbecker <frederic@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Ingo Molnar <mingo@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Andy Lutomirski <luto@...nel.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Subject: [PATCH 1/2] x86/context-tracking: Remove exception_enter/exit() from do_page_fault()

do_page_fault(), like other exceptions, is already covered by
user_enter() and user_exit() when the exception triggers in userspace.

As explained in 8c84014f3bbb112d07e73f30a10ac8a3a72f8649
("x86/entry: Remove exception_enter() from most trap handlers"),
exception_enter/exit() only remained to handle possible page fault from
kernel mode while context tracking is in CONTEXT_USER mode, ie: on
kernel entry before we manage to call user_exit(). And the only known
offender was do_fast_syscall_32() fetching EBP register from where
vDSO stashed it.

Meanwhile this got fixed with 9999c8c01f34c918a57d6e5ba2f5d8b79aa04801
("x86/entry: Call enter_from_user_mode() with IRQs off") that moved
enter_from_user_mode() before the call to get_user().

So we can safely remove it now.

Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
Cc: Andy Lutomirski <luto@...nel.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Peter Zijlstra <peterz@...radead.org>
---
 arch/x86/mm/fault.c | 39 ++++++++++++---------------------------
 1 file changed, 12 insertions(+), 27 deletions(-)

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 304d31d8cbbc..2b4ab2862eda 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -1486,27 +1486,6 @@ void do_user_addr_fault(struct pt_regs *regs,
 }
 NOKPROBE_SYMBOL(do_user_addr_fault);
 
-/*
- * Explicitly marked noinline such that the function tracer sees this as the
- * page_fault entry point.
- */
-static noinline void
-__do_page_fault(struct pt_regs *regs, unsigned long hw_error_code,
-		unsigned long address)
-{
-	prefetchw(&current->mm->mmap_sem);
-
-	if (unlikely(kmmio_fault(regs, address)))
-		return;
-
-	/* Was the fault on kernel-controlled part of the address space? */
-	if (unlikely(fault_in_kernel_space(address)))
-		do_kern_addr_fault(regs, hw_error_code, address);
-	else
-		do_user_addr_fault(regs, hw_error_code, address);
-}
-NOKPROBE_SYMBOL(__do_page_fault);
-
 static __always_inline void
 trace_page_fault_entries(struct pt_regs *regs, unsigned long error_code,
 			 unsigned long address)
@@ -1521,13 +1500,19 @@ trace_page_fault_entries(struct pt_regs *regs, unsigned long error_code,
 }
 
 dotraplinkage void
-do_page_fault(struct pt_regs *regs, unsigned long error_code, unsigned long address)
+do_page_fault(struct pt_regs *regs, unsigned long hw_error_code,
+		unsigned long address)
 {
-	enum ctx_state prev_state;
+	prefetchw(&current->mm->mmap_sem);
+	trace_page_fault_entries(regs, hw_error_code, address);
 
-	prev_state = exception_enter();
-	trace_page_fault_entries(regs, error_code, address);
-	__do_page_fault(regs, error_code, address);
-	exception_exit(prev_state);
+	if (unlikely(kmmio_fault(regs, address)))
+		return;
+
+	/* Was the fault on kernel-controlled part of the address space? */
+	if (unlikely(fault_in_kernel_space(address)))
+		do_kern_addr_fault(regs, hw_error_code, address);
+	else
+		do_user_addr_fault(regs, hw_error_code, address);
 }
 NOKPROBE_SYMBOL(do_page_fault);
-- 
2.23.0