linux-kernel - [PATCH 08/11] x86/fault: Bypass no_context() for implicit kernel faults from usermode

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Date:   Sun, 31 Jan 2021 09:24:39 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     x86@...nel.org
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Yonghong Song <yhs@...com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: [PATCH 08/11] x86/fault: Bypass no_context() for implicit kernel faults from usermode

We can drop an indentation level and remove the last user_mode(regs) == true
caller of no_context() by directly OOPSing for implicit kernel faults from
usermode.

Cc: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Signed-off-by: Andy Lutomirski <luto@...nel.org>
---
 arch/x86/mm/fault.c | 59 ++++++++++++++++++++++++---------------------
 1 file changed, 32 insertions(+), 27 deletions(-)

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 6f43d080e1e8..177b612c7f33 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -789,44 +789,49 @@ __bad_area_nosemaphore(struct pt_regs *regs, unsigned long error_code,
 {
 	struct task_struct *tsk = current;
 
-	/* User mode accesses just cause a SIGSEGV */
-	if (user_mode(regs) && (error_code & X86_PF_USER)) {
-		/*
-		 * It's possible to have interrupts off here:
-		 */
-		local_irq_enable();
+	if (!user_mode(regs)) {
+		no_context(regs, error_code, address, pkey, si_code);
+		return;
+	}
 
-		/*
-		 * Valid to do another page fault here because this one came
-		 * from user space:
-		 */
-		if (is_prefetch(regs, error_code, address))
-			return;
+	if (!(error_code & X86_PF_USER)) {
+		/* Implicit user access to kernel memory -- just oops */
+		page_fault_oops(regs, error_code, address);
+		return;
+	}
 
-		if (is_errata100(regs, address))
-			return;
+	/*
+	 * User mode accesses just cause a SIGSEGV.
+	 * It's possible to have interrupts off here:
+	 */
+	local_irq_enable();
 
-		sanitize_error_code(address, &error_code);
+	/*
+	 * Valid to do another page fault here because this one came
+	 * from user space:
+	 */
+	if (is_prefetch(regs, error_code, address))
+		return;
 
-		if (fixup_vdso_exception(regs, X86_TRAP_PF, error_code, address))
-			return;
+	if (is_errata100(regs, address))
+		return;
 
-		if (likely(show_unhandled_signals))
-			show_signal_msg(regs, error_code, address, tsk);
+	sanitize_error_code(address, &error_code);
 
-		set_signal_archinfo(address, error_code);
+	if (fixup_vdso_exception(regs, X86_TRAP_PF, error_code, address))
+		return;
 
-		if (si_code == SEGV_PKUERR)
-			force_sig_pkuerr((void __user *)address, pkey);
+	if (likely(show_unhandled_signals))
+		show_signal_msg(regs, error_code, address, tsk);
 
-		force_sig_fault(SIGSEGV, si_code, (void __user *)address);
+	set_signal_archinfo(address, error_code);
 
-		local_irq_disable();
+	if (si_code == SEGV_PKUERR)
+		force_sig_pkuerr((void __user *)address, pkey);
 
-		return;
-	}
+	force_sig_fault(SIGSEGV, si_code, (void __user *)address);
 
-	no_context(regs, error_code, address, SIGSEGV, si_code);
+	local_irq_disable();
 }
 
 static noinline void
-- 
2.29.2