[RFC PATCH v2 3/4] x86/traps: Attempt to fixup exceptions in vDSO before signaling

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

Call fixup_vdso_exception() in all trap flows that generate signals to
userspace immediately prior to generating any such signal.  If the
exception is fixed, return cleanly and do not generate a signal.

The goal of vDSO fixup is not to fixup all faults, nor is it to avoid
all signals, but rather to report faults directly to userspace when the
fault would otherwise directly result in a signal being sent to the
process.

Suggested-by: Andy Lutomirski <luto@amacapital.net>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
---
 arch/x86/kernel/traps.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 9b7c4ca8f0a7..f813481a85ff 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -61,6 +61,7 @@
 #include <asm/mpx.h>
 #include <asm/vm86.h>
 #include <asm/umip.h>
+#include <asm/vdso.h>
 
 #ifdef CONFIG_X86_64
 #include <asm/x86_init.h>
@@ -223,6 +224,10 @@ do_trap_no_signal(struct task_struct *tsk, int trapnr, const char *str,
 	tsk->thread.error_code = error_code;
 	tsk->thread.trap_nr = trapnr;
 
+	if (user_mode(regs) &&
+	    fixup_vdso_exception(regs, trapnr, error_code, 0))
+		return 0;
+
 	return -1;
 }
 
@@ -563,6 +568,9 @@ do_general_protection(struct pt_regs *regs, long error_code)
 	tsk->thread.error_code = error_code;
 	tsk->thread.trap_nr = X86_TRAP_GP;
 
+	if (fixup_vdso_exception(regs, X86_TRAP_GP, error_code, 0))
+		return;
+
 	show_signal(tsk, SIGSEGV, "", desc, regs, error_code);
 
 	force_sig(SIGSEGV, tsk);
@@ -791,6 +799,10 @@ dotraplinkage void do_debug(struct pt_regs *regs, long error_code)
 		goto exit;
 	}
 
+	if (user_mode(regs) &&
+	    fixup_vdso_exception(regs, X86_TRAP_DB, error_code, 0))
+		goto exit;
+
 	if (WARN_ON_ONCE((dr6 & DR_STEP) && !user_mode(regs))) {
 		/*
 		 * Historical junk that used to handle SYSENTER single-stepping.
@@ -854,6 +866,9 @@ static void math_error(struct pt_regs *regs, int error_code, int trapnr)
 	if (!si_code)
 		return;
 
+	if (fixup_vdso_exception(regs, trapnr, error_code, 0))
+		return;
+
 	force_sig_fault(SIGFPE, si_code,
 			(void __user *)uprobe_get_trap_addr(regs), task);
 }
-- 
2.19.2