[PATCH RFC v9 4/7] x86/entry: Erase kernel stack in syscall_trace_enter()

[Alexander Popov <alex.popov@linux.com>]

Make STACKLEAK erase kernel stack after ptrace/seccomp/auditing
not to leave any sensitive information on the stack for the syscall code.

This code is modified from Brad Spengler/PaX Team's code in the last
public patch of grsecurity/PaX based on our understanding of the code.
Changes or omissions from the original code are ours and don't reflect
the original grsecurity/PaX code.

Signed-off-by: Alexander Popov <alex.popov@linux.com>
---
 arch/x86/entry/common.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
index 74f6eee..b4be776 100644
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -46,6 +46,12 @@ __visible inline void enter_from_user_mode(void)
 static inline void enter_from_user_mode(void) {}
 #endif
 
+#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+asmlinkage void erase_kstack(void);
+#else
+static void erase_kstack(void) {}
+#endif
+
 static void do_audit_syscall_entry(struct pt_regs *regs, u32 arch)
 {
 #ifdef CONFIG_X86_64
@@ -128,6 +134,7 @@ static long syscall_trace_enter(struct pt_regs *regs)
 
 	do_audit_syscall_entry(regs, arch);
 
+	erase_kstack();
 	return ret ?: regs->orig_ax;
 }
 
-- 
2.7.4