[patch v4 02/27] signal: Prevent user space from setting si_sys_private

[Thomas Gleixner]

From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

The si_sys_private member of siginfo is used to handle posix-timer rearming
from the signal delivery path. Prevent user space from setting it as that
creates inconsistent state.

Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>

---
 kernel/signal.c | 8 ++++++++
 1 file changed, 8 insertions(+)
---
diff --git a/kernel/signal.c b/kernel/signal.c
index a83ea99f9389..7706cd304785 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -3354,6 +3354,14 @@ int copy_siginfo_to_user(siginfo_t __user *to, const kernel_siginfo_t *from)
 static int post_copy_siginfo_from_user(kernel_siginfo_t *info,
 				       const siginfo_t __user *from)
 {
+	/*
+	 * Clear the si_sys_private field for timer signals as that's the
+	 * indicator for rearming a posix timer. User space submitted
+	 * signals are not allowed to inject that.
+	 */
+	if (info->si_code == SI_TIMER)
+		info->si_sys_private = 0;
+
 	if (unlikely(!known_siginfo_layout(info->si_signo, info->si_code))) {
 		char __user *expansion = si_expansion(from);
 		char buf[SI_EXPANSION_SIZE];