[PATCH 15/17] ptrace: don't let PTRACE_SETSIGINFO override __SI_TRAP siginfo

[Tejun Heo <tj@kernel.org>]

__SI_TRAP siginfo is special in the operation of ptrace.  It reports
group stop related information and will also interact with
notification retraps.  Don't let userland mess with it.

Signed-off-by: Tejun Heo <tj@kernel.org>
---
 kernel/ptrace.c |   31 ++++++++++++++++++++++---------
 1 files changed, 22 insertions(+), 9 deletions(-)

diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index d5e38ac..f1efe07 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -545,16 +545,29 @@ static int ptrace_getsiginfo(struct task_struct *child, siginfo_t *info)
 static int ptrace_setsiginfo(struct task_struct *child, const siginfo_t *info)
 {
 	unsigned long flags;
-	int error = -ESRCH;
+	int error;
 
-	if (lock_task_sighand(child, &flags)) {
-		error = -EINVAL;
-		if (likely(child->last_siginfo != NULL)) {
-			*child->last_siginfo = *info;
-			error = 0;
-		}
-		unlock_task_sighand(child, &flags);
-	}
+	if (!lock_task_sighand(child, &flags))
+		return -ESRCH;
+
+	error = -EINVAL;
+	if (unlikely(!child->last_siginfo))
+		goto out_unlock;
+
+	/*
+	 * If seized, __SI_TRAP siginfo is used to communicate information
+	 * regarding traps and contains dynamic information generated on
+	 * GETSIGINFO.  Don't let userland override or fake it.
+	 */
+	if ((child->ptrace & PT_SEIZED) &&
+	    unlikely((child->last_siginfo->si_code & __SI_MASK) == __SI_TRAP ||
+		     (info->si_code & __SI_MASK) == __SI_TRAP))
+		goto out_unlock;
+
+	*child->last_siginfo = *info;
+	error = 0;
+out_unlock:
+	unlock_task_sighand(child, &flags);
 	return error;
 }
 
-- 
1.7.5.2