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Wed, 15 Jul 2026 14:20:38 -0700 (PDT) From: Cong Wang To: Andy Lutomirski Cc: Kees Cook , linux-kernel@vger.kernel.org, Will Drewry , Christian Brauner , Andrew Morton , linux-mm@kvack.org, Cong Wang Subject: [PATCH v6 3/8] seccomp: introduce SECCOMP_IOCTL_NOTIF_PIN_INSTALL Date: Wed, 15 Jul 2026 14:20:02 -0700 Message-ID: <20260715212007.2382846-4-xiyou.wangcong@gmail.com> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20260715212007.2382846-1-xiyou.wangcong@gmail.com> References: <20260715212007.2382846-1-xiyou.wangcong@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: Cong Wang SECCOMP_IOCTL_NOTIF_PIN_INSTALL maps a supervisor-owned @memfd at @target_addr in the trapped task's mm via vm_mmap_remote(), PROT_READ, MAP_SHARED, MAP_FIXED_NOREPLACE and VM_SEALED. Because the mapping is sealed, neither the target nor a CLONE_VM peer can munmap, mremap, mprotect or MAP_FIXED-stomp it; its contents are immutable from the target's side while the supervisor retains write access through its own mapping of the same memfd. The install needs no target-side cooperation, which is what makes the feature usable for fork+execve sandbox wrappers (Sandlock, Firejail, Bubblewrap-style) that have no trusted post-exec window to install their own mappings. The pin is just a sealed VMA owned by the target's mm: it persists until the task execve()s or exits (a sealed VMA cannot be unmapped piecemeal), and the kernel keeps no per-pin bookkeeping. A supervisor reuses one region across many redirects. Assisted-by: Claude:claude-opus-4.8 Signed-off-by: Cong Wang --- include/linux/seccomp.h | 5 ++ include/uapi/linux/seccomp.h | 34 ++++++++++ kernel/seccomp.c | 117 +++++++++++++++++++++++++++++++++++ 3 files changed, 156 insertions(+) diff --git a/include/linux/seccomp.h b/include/linux/seccomp.h index 9b959972bf4a..a91d1fc8a2b8 100644 --- a/include/linux/seccomp.h +++ b/include/linux/seccomp.h @@ -16,6 +16,11 @@ #define SECCOMP_NOTIFY_ADDFD_SIZE_VER0 24 #define SECCOMP_NOTIFY_ADDFD_SIZE_LATEST SECCOMP_NOTIFY_ADDFD_SIZE_VER0 +/* sizeof() the first published struct seccomp_notif_pin_install */ +#define SECCOMP_NOTIFY_PIN_INSTALL_SIZE_VER0 32 /* up to @size */ +#define SECCOMP_NOTIFY_PIN_INSTALL_SIZE_VER1 40 /* adds @offset */ +#define SECCOMP_NOTIFY_PIN_INSTALL_SIZE_LATEST SECCOMP_NOTIFY_PIN_INSTALL_SIZE_VER1 + #ifdef CONFIG_SECCOMP #include diff --git a/include/uapi/linux/seccomp.h b/include/uapi/linux/seccomp.h index dbfc9b37fcae..d3249294788b 100644 --- a/include/uapi/linux/seccomp.h +++ b/include/uapi/linux/seccomp.h @@ -137,6 +137,37 @@ struct seccomp_notif_addfd { __u32 newfd_flags; }; +/** + * struct seccomp_notif_pin_install - have the kernel install a sealed + * MAP_SHARED mapping of @memfd into the trapped task's mm at @target_addr. + * + * The supervisor owns @memfd and the kernel installs the mapping without + * target-side cooperation. It is read-only and VM_SEALED, so the target and + * any CLONE_VM peer cannot munmap, mremap, mprotect or MAP_FIXED-stomp it. + * @memfd must be write-sealed (F_SEAL_WRITE or F_SEAL_FUTURE_WRITE, -EINVAL + * otherwise) so its bytes cannot be rewritten through any other reference to + * the same memfd. + * + * @id: The ID of an active seccomp notification on this listener, + * identifying the trapped task whose mm receives the pin. + * @flags: Reserved, must be 0. + * @memfd: Supervisor-side fd for the backing memfd. Must be write-sealed. + * @target_addr: Page-aligned address in the trapped task's mm to install at. + * If non-zero it is MAP_FIXED (no existing mapping may overlap + * [@target_addr, @target_addr + @size)); if zero the kernel + * picks a free area. The actual address is written back here. + * @size: Size of the pin in bytes. Must be page-aligned. + * @offset: Page-aligned byte offset into @memfd to map from. + */ +struct seccomp_notif_pin_install { + __u64 id; + __u32 flags; + __u32 memfd; + __u64 target_addr; + __u64 size; + __u64 offset; +}; + #define SECCOMP_IOC_MAGIC '!' #define SECCOMP_IO(nr) _IO(SECCOMP_IOC_MAGIC, nr) #define SECCOMP_IOR(nr, type) _IOR(SECCOMP_IOC_MAGIC, nr, type) @@ -154,4 +185,7 @@ struct seccomp_notif_addfd { #define SECCOMP_IOCTL_NOTIF_SET_FLAGS SECCOMP_IOW(4, __u64) +#define SECCOMP_IOCTL_NOTIF_PIN_INSTALL SECCOMP_IOWR(5, \ + struct seccomp_notif_pin_install) + #endif /* _UAPI_LINUX_SECCOMP_H */ diff --git a/kernel/seccomp.c b/kernel/seccomp.c index 066909393c38..12135c5df9b1 100644 --- a/kernel/seccomp.c +++ b/kernel/seccomp.c @@ -37,12 +37,18 @@ #ifdef CONFIG_SECCOMP_FILTER #include #include +#include #include #include #include #include #include #include +#include +#include +#include +#include +#include /* * When SECCOMP_IOCTL_NOTIF_ID_VALID was first introduced, it had the @@ -1823,6 +1829,114 @@ static long seccomp_notify_addfd(struct seccomp_filter *filter, return ret; } +static unsigned long seccomp_install_pin(struct mm_struct *mm, + struct file *memfd_file, + unsigned long target_addr, size_t size, + unsigned long offset) +{ + unsigned long ret; + + if (!VM_SEALED) + return -EOPNOTSUPP; + + /* + * Install a sealed, read-only mapping. A fixed request (@target_addr + * != 0) is MAP_FIXED_NOREPLACE: an existing mapping yields -EEXIST + * rather than being silently clobbered. A request of 0 lets the kernel + * pick a free area in the target mm. + */ + ret = vm_mmap_remote(mm, memfd_file, target_addr, size, PROT_READ, + MAP_SHARED | MAP_FIXED_NOREPLACE, + offset >> PAGE_SHIFT, VM_SEALED); + if (IS_ERR_VALUE(ret)) + return ret; + if (target_addr && ret != target_addr) + return -ENOMEM; + return ret; +} + +static long seccomp_notify_pin_install(struct seccomp_filter *filter, + struct seccomp_notif_pin_install __user *upin, + unsigned int size) +{ + struct seccomp_notif_pin_install pin; + struct seccomp_knotif *knotif; + struct task_struct *target; + struct file *memfd_file; + struct mm_struct *mm; + unsigned long addr; + int seals; + long ret; + + BUILD_BUG_ON(sizeof(pin) < SECCOMP_NOTIFY_PIN_INSTALL_SIZE_VER0); + BUILD_BUG_ON(sizeof(pin) != SECCOMP_NOTIFY_PIN_INSTALL_SIZE_LATEST); + + if (size < SECCOMP_NOTIFY_PIN_INSTALL_SIZE_VER0 || size >= PAGE_SIZE) + return -EINVAL; + + ret = copy_struct_from_user(&pin, sizeof(pin), upin, size); + if (ret) + return ret; + + if (pin.flags) + return -EINVAL; + if (!pin.size || !IS_ALIGNED(pin.target_addr, PAGE_SIZE) || + !IS_ALIGNED(pin.size, PAGE_SIZE) || !IS_ALIGNED(pin.offset, PAGE_SIZE)) + return -EINVAL; + if (pin.target_addr + pin.size < pin.target_addr) + return -EINVAL; + if (pin.offset + pin.size < pin.offset) + return -EINVAL; + + memfd_file = fget(pin.memfd); + if (!memfd_file) + return -EBADF; + + seals = memfd_get_seals(memfd_file); + if (seals < 0 || !(seals & (F_SEAL_WRITE | F_SEAL_FUTURE_WRITE))) { + ret = -EINVAL; + goto out_fput; + } + + ret = mutex_lock_interruptible(&filter->notify_lock); + if (ret < 0) + goto out_fput; + + knotif = find_notification(filter, pin.id); + if (!knotif) { + ret = -ENOENT; + goto out_unlock; + } + if (knotif->state != SECCOMP_NOTIFY_SENT) { + ret = -EINPROGRESS; + goto out_unlock; + } + + target = knotif->task; + mm = get_task_mm(target); + mutex_unlock(&filter->notify_lock); + if (!mm) { + ret = -ESRCH; + goto out_fput; + } + addr = seccomp_install_pin(mm, memfd_file, pin.target_addr, + pin.size, pin.offset); + mmput(mm); + if (IS_ERR_VALUE(addr)) + ret = addr; + else if (put_user(addr, &upin->target_addr)) + ret = -EFAULT; + else + ret = 0; + goto out_fput; + +out_unlock: + mutex_unlock(&filter->notify_lock); +out_fput: + fput(memfd_file); + return ret; +} + static long seccomp_notify_ioctl(struct file *file, unsigned int cmd, unsigned long arg) { @@ -1847,6 +1961,9 @@ static long seccomp_notify_ioctl(struct file *file, unsigned int cmd, switch (EA_IOCTL(cmd)) { case EA_IOCTL(SECCOMP_IOCTL_NOTIF_ADDFD): return seccomp_notify_addfd(filter, buf, _IOC_SIZE(cmd)); + case EA_IOCTL(SECCOMP_IOCTL_NOTIF_PIN_INSTALL): + return seccomp_notify_pin_install(filter, buf, + _IOC_SIZE(cmd)); default: return -EINVAL; } -- 2.43.0