linux-kernel - [PATCH v2] signal: restore the override

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Message-ID: <20241104195419.3962584-1-roman.gushchin@linux.dev>
Date: Mon,  4 Nov 2024 19:54:19 +0000
From: Roman Gushchin <roman.gushchin@...ux.dev>
To: linux-kernel@...r.kernel.org
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Roman Gushchin <roman.gushchin@...ux.dev>,
	Andrei Vagin <avagin@...gle.com>,
	Kees Cook <kees@...nel.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Alexey Gladkov <legion@...nel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	stable@...r.kernel.org
Subject: [PATCH v2] signal: restore the override_rlimit logic

Prior to commit d64696905554 ("Reimplement RLIMIT_SIGPENDING on top of
ucounts") UCOUNT_RLIMIT_SIGPENDING rlimit was not enforced for a class
of signals. However now it's enforced unconditionally, even if
override_rlimit is set. This behavior change caused production issues.

For example, if the limit is reached and a process receives a SIGSEGV
signal, sigqueue_alloc fails to allocate the necessary resources for the
signal delivery, preventing the signal from being delivered with
siginfo. This prevents the process from correctly identifying the fault
address and handling the error. From the user-space perspective,
applications are unaware that the limit has been reached and that the
siginfo is effectively 'corrupted'. This can lead to unpredictable
behavior and crashes, as we observed with java applications.

Fix this by passing override_rlimit into inc_rlimit_get_ucounts() and
skip the comparison to max there if override_rlimit is set. This
effectively restores the old behavior.

v2: refactor to make the logic simpler (Eric, Oleg, Alexey)

Fixes: d64696905554 ("Reimplement RLIMIT_SIGPENDING on top of ucounts")
Signed-off-by: Roman Gushchin <roman.gushchin@...ux.dev>
Co-developed-by: Andrei Vagin <avagin@...gle.com>
Signed-off-by: Andrei Vagin <avagin@...gle.com>
Cc: Kees Cook <kees@...nel.org>
Cc: "Eric W. Biederman" <ebiederm@...ssion.com>
Cc: Alexey Gladkov <legion@...nel.org>
Cc: Oleg Nesterov <oleg@...hat.com>
Cc: <stable@...r.kernel.org>
---
 include/linux/user_namespace.h | 3 ++-
 kernel/signal.c                | 3 ++-
 kernel/ucount.c                | 6 ++++--
 3 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
index 3625096d5f85..7183e5aca282 100644
--- a/include/linux/user_namespace.h
+++ b/include/linux/user_namespace.h
@@ -141,7 +141,8 @@ static inline long get_rlimit_value(struct ucounts *ucounts, enum rlimit_type ty
 
 long inc_rlimit_ucounts(struct ucounts *ucounts, enum rlimit_type type, long v);
 bool dec_rlimit_ucounts(struct ucounts *ucounts, enum rlimit_type type, long v);
-long inc_rlimit_get_ucounts(struct ucounts *ucounts, enum rlimit_type type);
+long inc_rlimit_get_ucounts(struct ucounts *ucounts, enum rlimit_type type,
+			    bool override_rlimit);
 void dec_rlimit_put_ucounts(struct ucounts *ucounts, enum rlimit_type type);
 bool is_rlimit_overlimit(struct ucounts *ucounts, enum rlimit_type type, unsigned long max);
 
diff --git a/kernel/signal.c b/kernel/signal.c
index 4344860ffcac..cbabb2d05e0a 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -419,7 +419,8 @@ __sigqueue_alloc(int sig, struct task_struct *t, gfp_t gfp_flags,
 	 */
 	rcu_read_lock();
 	ucounts = task_ucounts(t);
-	sigpending = inc_rlimit_get_ucounts(ucounts, UCOUNT_RLIMIT_SIGPENDING);
+	sigpending = inc_rlimit_get_ucounts(ucounts, UCOUNT_RLIMIT_SIGPENDING,
+					    override_rlimit);
 	rcu_read_unlock();
 	if (!sigpending)
 		return NULL;
diff --git a/kernel/ucount.c b/kernel/ucount.c
index 16c0ea1cb432..49fcec41e5b4 100644
--- a/kernel/ucount.c
+++ b/kernel/ucount.c
@@ -307,7 +307,8 @@ void dec_rlimit_put_ucounts(struct ucounts *ucounts, enum rlimit_type type)
 	do_dec_rlimit_put_ucounts(ucounts, NULL, type);
 }
 
-long inc_rlimit_get_ucounts(struct ucounts *ucounts, enum rlimit_type type)
+long inc_rlimit_get_ucounts(struct ucounts *ucounts, enum rlimit_type type,
+			    bool override_rlimit)
 {
 	/* Caller must hold a reference to ucounts */
 	struct ucounts *iter;
@@ -320,7 +321,8 @@ long inc_rlimit_get_ucounts(struct ucounts *ucounts, enum rlimit_type type)
 			goto unwind;
 		if (iter == ucounts)
 			ret = new;
-		max = get_userns_rlimit_max(iter->ns, type);
+		if (!override_rlimit)
+			max = get_userns_rlimit_max(iter->ns, type);
 		/*
 		 * Grab an extra ucount reference for the caller when
 		 * the rlimit count was previously 0.
-- 
2.47.0.199.ga7371fff76-goog