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[2003:cf:5702:2000:11ee:99ed:f37:8a51]) by smtp.gmail.com with ESMTPSA id 4fb4d7f45d1cf-647510519efsm15206765a12.29.2025.12.02.03.52.12 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 02 Dec 2025 03:52:12 -0800 (PST) From: Alexander Mikhalitsyn To: kees@kernel.org Cc: linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, Andy Lutomirski , Will Drewry , Jonathan Corbet , Shuah Khan , Aleksa Sarai , Tycho Andersen , Andrei Vagin , Christian Brauner , =?UTF-8?q?St=C3=A9phane=20Graber?= , Tycho Andersen , Alexander Mikhalitsyn Subject: [PATCH v2 1/6] seccomp: remove unused argument from seccomp_do_user_notification Date: Tue, 2 Dec 2025 12:51:53 +0100 Message-ID: <20251202115200.110646-2-aleksandr.mikhalitsyn@canonical.com> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20251202115200.110646-1-aleksandr.mikhalitsyn@canonical.com> References: <20251202115200.110646-1-aleksandr.mikhalitsyn@canonical.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable Remove unused this_syscall argument from seccomp_do_user_notification() and add kdoc for it. Cc: linux-doc@vger.kernel.org Cc: linux-kernel@vger.kernel.org Cc: Kees Cook Cc: Andy Lutomirski Cc: Will Drewry Cc: Jonathan Corbet Cc: Shuah Khan Cc: Aleksa Sarai Cc: Tycho Andersen Cc: Andrei Vagin Cc: Christian Brauner Cc: St=C3=A9phane Graber Reviewed-by: Tycho Andersen (AMD) Signed-off-by: Alexander Mikhalitsyn --- kernel/seccomp.c | 16 +++++++++++++--- 1 file changed, 13 insertions(+), 3 deletions(-) diff --git a/kernel/seccomp.c b/kernel/seccomp.c index 3bbfba30a777..f944ea5a2716 100644 --- a/kernel/seccomp.c +++ b/kernel/seccomp.c @@ -1142,8 +1142,18 @@ static bool should_sleep_killable(struct seccomp_fil= ter *match, return match->wait_killable_recv && n->state >=3D SECCOMP_NOTIFY_SENT; } =20 -static int seccomp_do_user_notification(int this_syscall, - struct seccomp_filter *match, +/** + * seccomp_do_user_notification - sends seccomp notification to the usersp= ace + * listener and waits for a reply. + * @match: seccomp filter we are notifying + * @sd: seccomp data (syscall_nr, args, etc) to be passed to the userspace= listener + * + * Returns + * - -1 on success if userspace provided a reply for the syscall, + * - -1 on interrupted wait, + * - 0 on success if userspace requested to continue the syscall + */ +static int seccomp_do_user_notification(struct seccomp_filter *match, const struct seccomp_data *sd) { int err; 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[2003:cf:5702:2000:11ee:99ed:f37:8a51]) by smtp.gmail.com with ESMTPSA id 4fb4d7f45d1cf-647510519efsm15206765a12.29.2025.12.02.03.52.13 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 02 Dec 2025 03:52:14 -0800 (PST) From: Alexander Mikhalitsyn To: kees@kernel.org Cc: linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, Andy Lutomirski , Will Drewry , Jonathan Corbet , Shuah Khan , Aleksa Sarai , Tycho Andersen , Andrei Vagin , Christian Brauner , =?UTF-8?q?St=C3=A9phane=20Graber?= , Alexander Mikhalitsyn Subject: [PATCH v2 2/6] seccomp: prepare seccomp_run_filters() to support more than one listener Date: Tue, 2 Dec 2025 12:51:54 +0100 Message-ID: <20251202115200.110646-3-aleksandr.mikhalitsyn@canonical.com> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20251202115200.110646-1-aleksandr.mikhalitsyn@canonical.com> References: <20251202115200.110646-1-aleksandr.mikhalitsyn@canonical.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable Prepare seccomp_run_filters() function to support more than one listener in the seccomp tree. In this patch, we only introduce a new struct seccomp_filter_matches with kdoc and modify seccomp_run_filters() signature correspondingly. No functional change intended. Cc: linux-doc@vger.kernel.org Cc: linux-kernel@vger.kernel.org Cc: Kees Cook Cc: Andy Lutomirski Cc: Will Drewry Cc: Jonathan Corbet Cc: Shuah Khan Cc: Aleksa Sarai Cc: Tycho Andersen Cc: Andrei Vagin Cc: Christian Brauner Cc: St=C3=A9phane Graber Signed-off-by: Alexander Mikhalitsyn --- kernel/seccomp.c | 35 +++++++++++++++++++++++++++++++---- 1 file changed, 31 insertions(+), 4 deletions(-) diff --git a/kernel/seccomp.c b/kernel/seccomp.c index f944ea5a2716..c9a1062a53bd 100644 --- a/kernel/seccomp.c +++ b/kernel/seccomp.c @@ -237,6 +237,9 @@ struct seccomp_filter { /* Limit any path through the tree to 256KB worth of instructions. */ #define MAX_INSNS_PER_PATH ((1 << 18) / sizeof(struct sock_filter)) =20 +/* Limit number of listeners through the tree. */ +#define MAX_LISTENERS_PER_PATH 8 + /* * Endianness is explicitly ignored and left for BPF program authors to ma= nage * as per the specific architecture. @@ -391,18 +394,38 @@ static inline bool seccomp_cache_check_allow(const st= ruct seccomp_filter *sfilte } #endif /* SECCOMP_ARCH_NATIVE */ =20 +/** + * struct seccomp_filter_matches - container for seccomp filter match resu= lts + * + * @n: A number of filters matched. + * @filters: An array of (struct seccomp_filter) pointers. + * Holds pointers to filters that matched during evaluation. + * A first one in the array is the one with the least permissive + * action result. + * + * If final action result is less (or more) permissive than SECCOMP_RET_US= ER_NOTIF, + * only the most restrictive filter is stored in the array's first element. + * If final action result is SECCOMP_RET_USER_NOTIF, we need to track + * all filters that resulted in the same action to support multiple listen= ers + * in seccomp tree. + */ +struct seccomp_filter_matches { + unsigned char n; + struct seccomp_filter *filters[MAX_LISTENERS_PER_PATH]; +}; + #define ACTION_ONLY(ret) ((s32)((ret) & (SECCOMP_RET_ACTION_FULL))) /** * seccomp_run_filters - evaluates all seccomp filters against @sd * @sd: optional seccomp data to be passed to filters - * @match: stores struct seccomp_filter that resulted in the return value, + * @matches: array of struct seccomp_filter pointers that resulted in the = return value, * unless filter returned SECCOMP_RET_ALLOW, in which case it will * be unchanged. * * Returns valid seccomp BPF response codes. */ static u32 seccomp_run_filters(const struct seccomp_data *sd, - struct seccomp_filter **match) + struct seccomp_filter_matches *matches) { u32 ret =3D SECCOMP_RET_ALLOW; /* Make sure cross-thread synced filter points somewhere sane. */ @@ -425,7 +448,8 @@ static u32 seccomp_run_filters(const struct seccomp_dat= a *sd, =20 if (ACTION_ONLY(cur_ret) < ACTION_ONLY(ret)) { ret =3D cur_ret; - *match =3D f; + matches->n =3D 1; + matches->filters[0] =3D f; 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[2003:cf:5702:2000:11ee:99ed:f37:8a51]) by smtp.gmail.com with ESMTPSA id 4fb4d7f45d1cf-647510519efsm15206765a12.29.2025.12.02.03.52.15 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 02 Dec 2025 03:52:15 -0800 (PST) From: Alexander Mikhalitsyn To: kees@kernel.org Cc: linux-kernel@vger.kernel.org, bpf@vger.kernel.org, Andy Lutomirski , Will Drewry , Jonathan Corbet , Shuah Khan , Aleksa Sarai , Tycho Andersen , Andrei Vagin , Christian Brauner , =?UTF-8?q?St=C3=A9phane=20Graber?= , Alexander Mikhalitsyn Subject: [PATCH v2 3/6] seccomp: limit number of listeners in seccomp tree Date: Tue, 2 Dec 2025 12:51:55 +0100 Message-ID: <20251202115200.110646-4-aleksandr.mikhalitsyn@canonical.com> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20251202115200.110646-1-aleksandr.mikhalitsyn@canonical.com> References: <20251202115200.110646-1-aleksandr.mikhalitsyn@canonical.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable We need to limit number of listeners in seccomp tree to MAX_LISTENERS_PER_PATH, because we don't want to use dynamic memory allocations in a very hot __seccomp_filter() function and we use preallocated static array on the stack. Also, let's return ELOOP to userspace if it attempts to install more than MAX_LISTENERS_PER_PATH listeners, instead of ENOMEM as we do when userspace hits the limit of cBPF instructions. This will make uAPI a bit more convenient. Notice, that has_duplicate_listener() check is still in place, so this change is a preparational. Cc: linux-kernel@vger.kernel.org Cc: bpf@vger.kernel.org Cc: Kees Cook Cc: Andy Lutomirski Cc: Will Drewry Cc: Jonathan Corbet Cc: Shuah Khan Cc: Aleksa Sarai Cc: Tycho Andersen Cc: Andrei Vagin Cc: Christian Brauner Cc: St=C3=A9phane Graber Signed-off-by: Alexander Mikhalitsyn --- kernel/seccomp.c | 12 ++++++++++-- 1 file changed, 10 insertions(+), 2 deletions(-) diff --git a/kernel/seccomp.c b/kernel/seccomp.c index c9a1062a53bd..ded3f6a6430b 100644 --- a/kernel/seccomp.c +++ b/kernel/seccomp.c @@ -931,17 +931,25 @@ static long seccomp_attach_filter(unsigned int flags, struct seccomp_filter *filter) { unsigned long total_insns; + unsigned char total_listeners; struct seccomp_filter *walker; =20 assert_spin_locked(¤t->sighand->siglock); =20 - /* Validate resulting filter length. */ + /* Validate resulting filter length and number of nested listeners. */ total_insns =3D filter->prog->len; - for (walker =3D current->seccomp.filter; walker; walker =3D walker->prev) + total_listeners =3D filter->notif ? 1 : 0; 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[2003:cf:5702:2000:11ee:99ed:f37:8a51]) by smtp.gmail.com with ESMTPSA id 4fb4d7f45d1cf-647510519efsm15206765a12.29.2025.12.02.03.52.16 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 02 Dec 2025 03:52:17 -0800 (PST) From: Alexander Mikhalitsyn To: kees@kernel.org Cc: linux-kernel@vger.kernel.org, bpf@vger.kernel.org, Andy Lutomirski , Will Drewry , Jonathan Corbet , Shuah Khan , Aleksa Sarai , Tycho Andersen , Andrei Vagin , Christian Brauner , =?UTF-8?q?St=C3=A9phane=20Graber?= , Tycho Andersen , Alexander Mikhalitsyn Subject: [PATCH v2 4/6] seccomp: handle multiple listeners case Date: Tue, 2 Dec 2025 12:51:56 +0100 Message-ID: <20251202115200.110646-5-aleksandr.mikhalitsyn@canonical.com> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20251202115200.110646-1-aleksandr.mikhalitsyn@canonical.com> References: <20251202115200.110646-1-aleksandr.mikhalitsyn@canonical.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable If we have more than one listener in the tree and lower listener wants us to continue syscall (SECCOMP_USER_NOTIF_FLAG_CONTINUE) we must consult with upper listeners first, otherwise it is a clear seccomp restrictions bypass scenario. Cc: linux-kernel@vger.kernel.org Cc: bpf@vger.kernel.org Cc: Kees Cook Cc: Andy Lutomirski Cc: Will Drewry Cc: Jonathan Corbet Cc: Shuah Khan Cc: Aleksa Sarai Cc: Tycho Andersen Cc: Andrei Vagin Cc: Christian Brauner Cc: St=C3=A9phane Graber Reviewed-by: Tycho Andersen (AMD) Signed-off-by: Alexander Mikhalitsyn --- kernel/seccomp.c | 33 +++++++++++++++++++++++++++++++-- 1 file changed, 31 insertions(+), 2 deletions(-) diff --git a/kernel/seccomp.c b/kernel/seccomp.c index ded3f6a6430b..262390451ff1 100644 --- a/kernel/seccomp.c +++ b/kernel/seccomp.c @@ -448,8 +448,21 @@ static u32 seccomp_run_filters(const struct seccomp_da= ta *sd, =20 if (ACTION_ONLY(cur_ret) < ACTION_ONLY(ret)) { ret =3D cur_ret; + /* + * No matter what we had before in matches->filters[], + * we need to overwrite it, because current action is more + * restrictive than any previous one. + */ matches->n =3D 1; matches->filters[0] =3D f; + } else if ((ACTION_ONLY(cur_ret) =3D=3D ACTION_ONLY(ret)) && + ACTION_ONLY(cur_ret) =3D=3D SECCOMP_RET_USER_NOTIF) { + /* + * For multiple SECCOMP_RET_USER_NOTIF results, we need to + * track all filters that resulted in the same action, because + * we might need to notify a few of them to get a final decision. + */ + matches->filters[matches->n++] =3D f; 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[2003:cf:5702:2000:11ee:99ed:f37:8a51]) by smtp.gmail.com with ESMTPSA id 4fb4d7f45d1cf-647510519efsm15206765a12.29.2025.12.02.03.52.18 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 02 Dec 2025 03:52:19 -0800 (PST) From: Alexander Mikhalitsyn To: kees@kernel.org Cc: linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, bpf@vger.kernel.org, Andy Lutomirski , Will Drewry , Jonathan Corbet , Shuah Khan , Aleksa Sarai , Tycho Andersen , Andrei Vagin , Christian Brauner , =?UTF-8?q?St=C3=A9phane=20Graber?= , Alexander Mikhalitsyn , Alexander Mikhalitsyn Subject: [PATCH v2 5/6] seccomp: relax has_duplicate_listeners check Date: Tue, 2 Dec 2025 12:51:57 +0100 Message-ID: <20251202115200.110646-6-aleksandr.mikhalitsyn@canonical.com> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20251202115200.110646-1-aleksandr.mikhalitsyn@canonical.com> References: <20251202115200.110646-1-aleksandr.mikhalitsyn@canonical.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable Now everything is ready to get rid of "only one listener per tree" limitation. Let's introduce a new uAPI flag SECCOMP_FILTER_FLAG_ALLOW_NESTED_LISTENERS, so userspace may explicitly allow nested listeners when installing a listener. Note, that to install n-th listener, this flag must be set on all the listeners up the tree. Cc: linux-doc@vger.kernel.org Cc: linux-kernel@vger.kernel.org Cc: bpf@vger.kernel.org Cc: Kees Cook Cc: Andy Lutomirski Cc: Will Drewry Cc: Jonathan Corbet Cc: Shuah Khan Cc: Aleksa Sarai Cc: Tycho Andersen Cc: Andrei Vagin Cc: Christian Brauner Cc: St=C3=A9phane Graber Signed-off-by: Alexander Mikhalitsyn --- .../userspace-api/seccomp_filter.rst | 6 +++++ include/linux/seccomp.h | 3 ++- include/uapi/linux/seccomp.h | 13 ++++++----- kernel/seccomp.c | 22 +++++++++++++++---- tools/include/uapi/linux/seccomp.h | 13 ++++++----- 5 files changed, 40 insertions(+), 17 deletions(-) diff --git a/Documentation/userspace-api/seccomp_filter.rst b/Documentation= /userspace-api/seccomp_filter.rst index cff0fa7f3175..b9633ab1ed47 100644 --- a/Documentation/userspace-api/seccomp_filter.rst +++ b/Documentation/userspace-api/seccomp_filter.rst @@ -210,6 +210,12 @@ notifications from both tasks will appear on the same = filter fd. Reads and writes to/from a filter fd are also synchronized, so a filter fd can safely have many readers. =20 +By default, only one listener within seccomp filters tree is allowed. On a= ttempt +to add a new listener when one already exists in the filter tree, the +``seccomp()`` call will fail with ``-EBUSY``. To allow multiple listeners,= the +``SECCOMP_FILTER_FLAG_ALLOW_NESTED_LISTENERS`` flag can be passed in addit= ion to +the ``SECCOMP_FILTER_FLAG_NEW_LISTENER`` flag. + The interface for a seccomp notification fd consists of two structures: =20 .. code-block:: c diff --git a/include/linux/seccomp.h b/include/linux/seccomp.h index 9b959972bf4a..9b060946019d 100644 --- a/include/linux/seccomp.h +++ b/include/linux/seccomp.h @@ -10,7 +10,8 @@ SECCOMP_FILTER_FLAG_SPEC_ALLOW | \ SECCOMP_FILTER_FLAG_NEW_LISTENER | \ SECCOMP_FILTER_FLAG_TSYNC_ESRCH | \ - SECCOMP_FILTER_FLAG_WAIT_KILLABLE_RECV) + SECCOMP_FILTER_FLAG_WAIT_KILLABLE_RECV | \ + SECCOMP_FILTER_FLAG_ALLOW_NESTED_LISTENERS) =20 /* sizeof() the first published struct seccomp_notif_addfd */ #define SECCOMP_NOTIFY_ADDFD_SIZE_VER0 24 diff --git a/include/uapi/linux/seccomp.h b/include/uapi/linux/seccomp.h index dbfc9b37fcae..de78d8e7a70b 100644 --- a/include/uapi/linux/seccomp.h +++ b/include/uapi/linux/seccomp.h @@ -18,13 +18,14 @@ #define SECCOMP_GET_NOTIF_SIZES 3 =20 /* Valid flags for SECCOMP_SET_MODE_FILTER */ -#define SECCOMP_FILTER_FLAG_TSYNC (1UL << 0) -#define SECCOMP_FILTER_FLAG_LOG (1UL << 1) -#define SECCOMP_FILTER_FLAG_SPEC_ALLOW (1UL << 2) -#define SECCOMP_FILTER_FLAG_NEW_LISTENER (1UL << 3) -#define SECCOMP_FILTER_FLAG_TSYNC_ESRCH (1UL << 4) +#define SECCOMP_FILTER_FLAG_TSYNC (1UL << 0) +#define SECCOMP_FILTER_FLAG_LOG (1UL << 1) +#define SECCOMP_FILTER_FLAG_SPEC_ALLOW (1UL << 2) +#define SECCOMP_FILTER_FLAG_NEW_LISTENER (1UL << 3) +#define SECCOMP_FILTER_FLAG_TSYNC_ESRCH (1UL << 4) /* Received notifications wait in killable state (only respond to fatal si= gnals) */ -#define SECCOMP_FILTER_FLAG_WAIT_KILLABLE_RECV (1UL << 5) +#define SECCOMP_FILTER_FLAG_WAIT_KILLABLE_RECV (1UL << 5) +#define SECCOMP_FILTER_FLAG_ALLOW_NESTED_LISTENERS (1UL << 6) =20 /* * All BPF programs must return a 32-bit value. diff --git a/kernel/seccomp.c b/kernel/seccomp.c index 262390451ff1..a59276afc5b4 100644 --- a/kernel/seccomp.c +++ b/kernel/seccomp.c @@ -205,6 +205,7 @@ static inline void seccomp_cache_prepare(struct seccomp= _filter *sfilter) * @log: true if all actions except for SECCOMP_RET_ALLOW should be logged * @wait_killable_recv: Put notifying process in killable state once the * notification is received by the userspace listener. + * @allow_nested_listeners: Allow nested seccomp listeners. * @prev: points to a previously installed, or inherited, filter * @prog: the BPF program to evaluate * @notif: the struct that holds all notification related information @@ -226,6 +227,7 @@ struct seccomp_filter { refcount_t users; bool log; bool wait_killable_recv; + bool allow_nested_listeners; struct action_cache cache; struct seccomp_filter *prev; struct bpf_prog *prog; @@ -984,6 +986,10 @@ static long seccomp_attach_filter(unsigned int flags, if (flags & SECCOMP_FILTER_FLAG_WAIT_KILLABLE_RECV) filter->wait_killable_recv =3D true; =20 + /* Set nested listeners allow flag, if present. */ + if (flags & SECCOMP_FILTER_FLAG_ALLOW_NESTED_LISTENERS) + filter->allow_nested_listeners =3D true; + /* * If there is an existing filter, make it the prev and don't drop its * task reference. @@ -1972,7 +1978,8 @@ static struct file *init_listener(struct seccomp_filt= er *filter) } =20 /* - * Does @new_child have a listener while an ancestor also has a listener? + * Does @new_child have a listener while an ancestor also has a listener + * and hasn't allowed nesting? * If so, we'll want to reject this filter. * This only has to be tested for the current process, even in the TSYNC c= ase, * because TSYNC installs @child with the same parent on all threads. @@ -1990,7 +1997,12 @@ static bool has_duplicate_listener(struct seccomp_fi= lter *new_child) return false; for (cur =3D current->seccomp.filter; cur; cur =3D cur->prev) { if (cur->notif) - return true; + /* + * We don't need to go up further, because if there is a + * listener with nesting allowed, then all the listeners + * up the tree have allowed nesting as well. + */ + return !cur->allow_nested_listeners; } =20 return false; @@ -2035,10 +2047,12 @@ static long seccomp_set_mode_filter(unsigned int fl= ags, return -EINVAL; =20 /* - * The SECCOMP_FILTER_FLAG_WAIT_KILLABLE_SENT flag doesn't make sense + * The SECCOMP_FILTER_FLAG_WAIT_KILLABLE_SENT and + * SECCOMP_FILTER_FLAG_ALLOW_NESTED_LISTENERS flags don't make sense * without the SECCOMP_FILTER_FLAG_NEW_LISTENER flag. */ - if ((flags & SECCOMP_FILTER_FLAG_WAIT_KILLABLE_RECV) && + if (((flags & SECCOMP_FILTER_FLAG_WAIT_KILLABLE_RECV) || + (flags & SECCOMP_FILTER_FLAG_ALLOW_NESTED_LISTENERS)) && ((flags & SECCOMP_FILTER_FLAG_NEW_LISTENER) =3D=3D 0)) return -EINVAL; =20 diff --git a/tools/include/uapi/linux/seccomp.h b/tools/include/uapi/linux/= seccomp.h index dbfc9b37fcae..de78d8e7a70b 100644 --- a/tools/include/uapi/linux/seccomp.h +++ b/tools/include/uapi/linux/seccomp.h @@ -18,13 +18,14 @@ #define SECCOMP_GET_NOTIF_SIZES 3 =20 /* Valid flags for SECCOMP_SET_MODE_FILTER */ -#define SECCOMP_FILTER_FLAG_TSYNC (1UL << 0) -#define SECCOMP_FILTER_FLAG_LOG (1UL << 1) -#define SECCOMP_FILTER_FLAG_SPEC_ALLOW (1UL << 2) -#define SECCOMP_FILTER_FLAG_NEW_LISTENER (1UL << 3) -#define SECCOMP_FILTER_FLAG_TSYNC_ESRCH (1UL << 4) +#define SECCOMP_FILTER_FLAG_TSYNC (1UL << 0) +#define SECCOMP_FILTER_FLAG_LOG (1UL << 1) +#define SECCOMP_FILTER_FLAG_SPEC_ALLOW (1UL << 2) +#define SECCOMP_FILTER_FLAG_NEW_LISTENER (1UL << 3) +#define SECCOMP_FILTER_FLAG_TSYNC_ESRCH (1UL << 4) /* Received notifications wait in killable state (only respond to fatal si= gnals) */ -#define SECCOMP_FILTER_FLAG_WAIT_KILLABLE_RECV (1UL << 5) +#define SECCOMP_FILTER_FLAG_WAIT_KILLABLE_RECV (1UL << 5) +#define SECCOMP_FILTER_FLAG_ALLOW_NESTED_LISTENERS (1UL << 6) =20 /* * All BPF programs must return a 32-bit value. --=20 2.43.0 From nobody Sun Feb 8 23:26:00 2026 Received: from smtp-relay-internal-1.canonical.com (smtp-relay-internal-1.canonical.com [185.125.188.123]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 68257313E20 for ; 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[2003:cf:5702:2000:11ee:99ed:f37:8a51]) by smtp.gmail.com with ESMTPSA id 4fb4d7f45d1cf-647510519efsm15206765a12.29.2025.12.02.03.52.20 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 02 Dec 2025 03:52:20 -0800 (PST) From: Alexander Mikhalitsyn To: kees@kernel.org Cc: linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, bpf@vger.kernel.org, Andy Lutomirski , Will Drewry , Jonathan Corbet , Shuah Khan , Aleksa Sarai , Tycho Andersen , Andrei Vagin , Christian Brauner , =?UTF-8?q?St=C3=A9phane=20Graber?= , Alexander Mikhalitsyn Subject: [PATCH v2 6/6] tools/testing/selftests/seccomp: test nested listeners Date: Tue, 2 Dec 2025 12:51:58 +0100 Message-ID: <20251202115200.110646-7-aleksandr.mikhalitsyn@canonical.com> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20251202115200.110646-1-aleksandr.mikhalitsyn@canonical.com> References: <20251202115200.110646-1-aleksandr.mikhalitsyn@canonical.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable Add some basic tests for nested listeners. Cc: linux-kernel@vger.kernel.org Cc: linux-kselftest@vger.kernel.org Cc: bpf@vger.kernel.org Cc: Kees Cook Cc: Andy Lutomirski Cc: Will Drewry Cc: Jonathan Corbet Cc: Shuah Khan Cc: Aleksa Sarai Cc: Tycho Andersen Cc: Andrei Vagin Cc: Christian Brauner Cc: St=C3=A9phane Graber Signed-off-by: Alexander Mikhalitsyn --- tools/testing/selftests/seccomp/seccomp_bpf.c | 162 ++++++++++++++++++ 1 file changed, 162 insertions(+) diff --git a/tools/testing/selftests/seccomp/seccomp_bpf.c b/tools/testing/= selftests/seccomp/seccomp_bpf.c index fc4910d35342..0bf02d04fe15 100644 --- a/tools/testing/selftests/seccomp/seccomp_bpf.c +++ b/tools/testing/selftests/seccomp/seccomp_bpf.c @@ -293,6 +293,10 @@ struct seccomp_notif_addfd_big { #define SECCOMP_FILTER_FLAG_WAIT_KILLABLE_RECV (1UL << 5) #endif =20 +#ifndef SECCOMP_FILTER_FLAG_ALLOW_NESTED_LISTENERS +#define SECCOMP_FILTER_FLAG_ALLOW_NESTED_LISTENERS (1UL << 6) +#endif + #ifndef seccomp int seccomp(unsigned int op, unsigned int flags, void *args) { @@ -4408,6 +4412,164 @@ TEST(user_notification_sync) ASSERT_EQ(status, 0); } =20 +/* from kernel/seccomp.c */ +#define MAX_LISTENERS_PER_PATH 8 + +TEST(user_notification_nested_limits) +{ + pid_t pid; + long ret; + int i, status, listeners[MAX_LISTENERS_PER_PATH]; + + struct sock_filter filter[] =3D { + BPF_STMT(BPF_RET|BPF_K, SECCOMP_RET_ALLOW), + }; + struct sock_fprog prog =3D { + .len =3D (unsigned short)ARRAY_SIZE(filter), + .filter =3D filter, + }; + + ret =3D prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0); + ASSERT_EQ(0, ret) { + TH_LOG("Kernel does not support PR_SET_NO_NEW_PRIVS!"); + } + + /* Install 6 levels of listeners and allow nesting. */ + for (i =3D 0; i < 6; i++) { + listeners[i] =3D user_notif_syscall(__NR_getppid, + SECCOMP_FILTER_FLAG_NEW_LISTENER | + SECCOMP_FILTER_FLAG_ALLOW_NESTED_LISTENERS); + ASSERT_GE(listeners[i], 0); + + /* Add some no-op filters for grins. */ + EXPECT_EQ(seccomp(SECCOMP_SET_MODE_FILTER, 0, &prog), 0); + } + + /* Check behavior when nesting is not allowed. */ + pid =3D fork(); + ASSERT_GE(pid, 0); + if (pid =3D=3D 0) { + /* Install a next listener in the chain without nesting allowed. */ + listeners[6] =3D user_notif_syscall(__NR_getppid, + SECCOMP_FILTER_FLAG_NEW_LISTENER); + if (listeners[6] < 0) + exit(1); + + /* Add some no-op filters for grins. */ + ret =3D seccomp(SECCOMP_SET_MODE_FILTER, 0, &prog); + if (ret !=3D 0) + exit(2); + + ret =3D user_notif_syscall(__NR_getppid, + SECCOMP_FILTER_FLAG_NEW_LISTENER); + /* Installing a next listener in the chain should result in EBUSY. */ + exit((ret >=3D 0 || errno !=3D EBUSY) ? 3 : 0); + } + + EXPECT_EQ(waitpid(pid, &status, 0), pid); + EXPECT_EQ(true, WIFEXITED(status)); + EXPECT_EQ(0, WEXITSTATUS(status)); + + /* Install more filters with listeners to reach nesting levels limit. */ + for (; i < MAX_LISTENERS_PER_PATH; i++) { + listeners[i] =3D user_notif_syscall(__NR_getppid, + SECCOMP_FILTER_FLAG_NEW_LISTENER | + SECCOMP_FILTER_FLAG_ALLOW_NESTED_LISTENERS); + ASSERT_GE(listeners[i], 0); + + /* Add some no-op filters for grins. */ + EXPECT_EQ(seccomp(SECCOMP_SET_MODE_FILTER, 0, &prog), 0); + } + + /* Installing a next listener in the chain should result in ELOOP. */ + EXPECT_EQ(user_notif_syscall(__NR_getppid, + SECCOMP_FILTER_FLAG_NEW_LISTENER), + -1); + EXPECT_EQ(errno, ELOOP); +} + +TEST(user_notification_nested) +{ + pid_t pid; + long ret; + int i, status, listeners[6]; + struct seccomp_notif req =3D {}; + struct seccomp_notif_resp resp =3D {}; + + struct sock_filter filter[] =3D { + BPF_STMT(BPF_RET|BPF_K, SECCOMP_RET_ALLOW), + }; + struct sock_fprog prog =3D { + .len =3D (unsigned short)ARRAY_SIZE(filter), + .filter =3D filter, + }; + + ret =3D prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0); + ASSERT_EQ(0, ret) { + TH_LOG("Kernel does not support PR_SET_NO_NEW_PRIVS!"); + } + + /* Install 6 levels of listeners and allow nesting. */ + for (i =3D 0; i < 6; i++) { + listeners[i] =3D user_notif_syscall(__NR_getppid, + SECCOMP_FILTER_FLAG_NEW_LISTENER | + SECCOMP_FILTER_FLAG_ALLOW_NESTED_LISTENERS); + ASSERT_GE(listeners[i], 0); + + /* Add some no-op filters for grins. */ + EXPECT_EQ(seccomp(SECCOMP_SET_MODE_FILTER, 0, &prog), 0); + } + + pid =3D fork(); + ASSERT_GE(pid, 0); + + if (pid =3D=3D 0) { + ret =3D syscall(__NR_getppid); + exit(ret !=3D (USER_NOTIF_MAGIC-3)); + } + + /* + * We want to have the following picture: + * + * | Listener level (i) | Listener decision | + * |--------------------|-------------------| + * | 0 | WHATEVER | + * | 1 | WHATEVER | + * | 2 | WHATEVER | + * | 3 | RETURN | <-- stop here + * | 4 | CONTINUE SYSCALL | + * | 5 | CONTINUE SYSCALL | <- start here (current->seccomp.filter) + * + * First listener who receives a notification is level 5, then 4, + * then we expect to stop on level 3 and return from syscall with + * (USER_NOTIF_MAGIC - 3) return value. + */ + for (i =3D 6 - 1; i >=3D 3; i--) { + memset(&req, 0, sizeof(req)); + EXPECT_EQ(ioctl(listeners[i], SECCOMP_IOCTL_NOTIF_RECV, &req), 0); + EXPECT_EQ(req.pid, pid); + EXPECT_EQ(req.data.nr, __NR_getppid); + + memset(&resp, 0, sizeof(resp)); + resp.id =3D req.id; + + if (i =3D=3D 5 || i =3D=3D 4) { + resp.flags =3D SECCOMP_USER_NOTIF_FLAG_CONTINUE; + } else { + resp.error =3D 0; + resp.val =3D USER_NOTIF_MAGIC - i; + } + + EXPECT_EQ(ioctl(listeners[i], SECCOMP_IOCTL_NOTIF_SEND, &resp), 0); + } + + EXPECT_EQ(waitpid(pid, &status, 0), pid); + EXPECT_EQ(true, WIFEXITED(status)); + EXPECT_EQ(0, WEXITSTATUS(status)); + + for (i =3D 0; i < 6; i++) + close(listeners[i]); +} =20 /* Make sure PTRACE_O_SUSPEND_SECCOMP requires CAP_SYS_ADMIN. */ FIXTURE(O_SUSPEND_SECCOMP) { --=20 2.43.0