[PATCH] signal: disallow non-SI_USER signals in pidfd_send_signal()

[Christian Brauner]

Oleg pointed out that the following condition:

/* Only allow sending arbitrary signals to yourself. */
ret = -EPERM;
if ((task_pid(current) != pid) &&
    (kinfo.si_code >= 0 || kinfo.si_code == SI_TKILL))
        goto err;

doesn't account for PIDFD_SIGNAL_PROCESS_GROUP. He suggested:

/* Only allow sending arbitrary signals to yourself. */
ret = -EPERM;
if ((task_pid(current) != pid || type == PIDTYPE_PGID) &&
    (kinfo.si_code >= 0 || kinfo.si_code == SI_TKILL)
        goto err;

but I think we should just go all the way and error out if userspace
specifies anything else than SI_USER as si_code. It's probably an unused
feature right now anyway and if someone needs it than it's easy to add
back.

Reported-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20240214123655.GB16265@xxxxxxxxxx
Signed-off-by: Christian Brauner <brauner@xxxxxxxxxx>
---
 kernel/signal.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index cf6539a6b1cb..92a80e8d6b22 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -3954,10 +3954,7 @@ SYSCALL_DEFINE4(pidfd_send_signal, int, pidfd, int, sig,
 		if (unlikely(sig != kinfo.si_signo))
 			goto err;
 
-		/* Only allow sending arbitrary signals to yourself. */
-		ret = -EPERM;
-		if ((task_pid(current) != pid) &&
-		    (kinfo.si_code >= 0 || kinfo.si_code == SI_TKILL))
+		if (kinfo.si_code != SI_USER)
 			goto err;
 	} else {
 		prepare_kill_siginfo(sig, &kinfo, type);
-- 
2.43.0


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