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Subject[PATCH] signal: don't silently convert SI_USER signals to non-current pidfd
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The current sys_pidfd_send_signal() silently turns signals with explicit
SI_USER context that are sent to non-current tasks into signals with
kernel-generated siginfo.
This is unlike do_rt_sigqueueinfo(), which returns -EPERM in this case.
If a user actually wants to send a signal with kernel-provided siginfo,
they can do that with pidfd_send_signal(pidfd, sig, NULL, 0); so allowing
this case is unnecessary.

Instead of silently replacing the siginfo, just bail out with an error;
this is consistent with other interfaces and avoids special-casing behavior
based on security checks.

Fixes: 3eb39f47934f ("signal: add pidfd_send_signal() syscall")
Signed-off-by: Jann Horn <jannh@google.com>
---
kernel/signal.c | 13 ++++---------
1 file changed, 4 insertions(+), 9 deletions(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index b7953934aa99..f98448cf2def 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -3605,16 +3605,11 @@ 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)) {
- /* Only allow sending arbitrary signals to yourself. */
- ret = -EPERM;
- if (kinfo.si_code != SI_USER)
- goto err;
-
- /* Turn this into a regular kill signal. */
- prepare_kill_siginfo(sig, &kinfo);
- }
+ (kinfo.si_code >= 0 || kinfo.si_code == SI_TKILL))
+ goto err;
} else {
prepare_kill_siginfo(sig, &kinfo);
}
--
2.21.0.392.gf8f6787159e-goog
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