[PATCH v4 14/17] x86/umip: Force a page fault when unable to copy emulated result to user

[Ricardo Neri]

fixup_umip_exception will be called from do_general_protection. If the
former returns false, the latter will issue a SIGSEGV with SEND_SIG_PRIV.
However, when emulation is successful but the emulated result cannot be
copied to user space memory, it is more accurate to issue a SIGSEGV with
SEGV_MAPERR with the offending address. A new function is inspired in
force_sig_info_fault is introduced to model the page fault.

Signed-off-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
---
 arch/x86/kernel/umip.c | 45 +++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 43 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/umip.c b/arch/x86/kernel/umip.c
index b16542a..93bc80d 100644
--- a/arch/x86/kernel/umip.c
+++ b/arch/x86/kernel/umip.c
@@ -170,6 +170,41 @@ static int __emulate_umip_insn(struct insn *insn, enum umip_insn umip_inst,
 }
 
 /**
+ * __force_sig_info_umip_fault - Force a SIGSEGV with SEGV_MAPERR
+ * @address:	Address that caused the signal
+ * @regs:	Register set containing the instruction pointer
+ *
+ * Force a SIGSEGV signal with SEGV_MAPERR as the error code. This function is
+ * intended to be used to provide a segmentation fault when the result of the
+ * UMIP emulation could not be copied to the user space memory.
+ *
+ * Return: none
+ */
+static void __force_sig_info_umip_fault(void __user *address,
+					struct pt_regs *regs)
+{
+	siginfo_t info;
+	struct task_struct *tsk = current;
+
+	if (show_unhandled_signals && unhandled_signal(tsk, SIGSEGV)) {
+		printk_ratelimited("%s[%d] umip emulation segfault ip:%lx sp:%lx error:%lx in %lx\n",
+				   tsk->comm, task_pid_nr(tsk), regs->ip,
+				   regs->sp, UMIP_PF_USER | UMIP_PF_WRITE,
+				   regs->ip);
+	}
+
+	tsk->thread.cr2		= (unsigned long)address;
+	tsk->thread.error_code	= UMIP_PF_USER | UMIP_PF_WRITE;
+	tsk->thread.trap_nr	= X86_TRAP_PF;
+
+	info.si_signo	= SIGSEGV;
+	info.si_errno	= 0;
+	info.si_code	= SEGV_MAPERR;
+	info.si_addr	= address;
+	force_sig_info(SIGSEGV, &info, tsk);
+}
+
+/**
  * fixup_umip_exception - Fixup #GP faults caused by UMIP
  * @regs:	Registers as saved when entering the #GP trap
  *
@@ -252,8 +287,14 @@ bool fixup_umip_exception(struct pt_regs *regs)
 	} else {
 		uaddr = insn_get_addr_ref(&insn, regs);
 		nr_copied = copy_to_user(uaddr, dummy_data, dummy_data_size);
-		if (nr_copied  > 0)
-			return false;
+		if (nr_copied  > 0) {
+			/*
+			 * If copy fails, send a signal and tell caller that
+			 * fault was fixed up
+			 */
+			__force_sig_info_umip_fault(uaddr, regs);
+			return true;
+		}
 	}
 
 	/* increase IP to let the program keep going */
-- 
2.9.3