From 88e4607034ee49e09e32d91d083dced5c2f4f127 Mon Sep 17 00:00:00 2001 From: Vladimir Sementsov-Ogievskiy Date: Thu, 20 Apr 2023 15:04:09 +0300 Subject: coredump: require O_WRONLY instead of O_RDWR The motivation for this patch has been to enable using a stricter apparmor profile to prevent programs from reading any coredump in the system. However, this became something else. The following details are based on Christian's and Linus' archeology into the history of the number "2" in the coredump handling code. To make sure we're not accidently introducing some subtle behavioral change into the coredump code we set out on a voyage into the depths of history.git to figure out why this was O_RDWR in the first place. Coredump handling was introduced over 30 years ago in commit ddc733f452e0 ("[PATCH] Linux-0.97 (August 1, 1992)"). The original code used O_WRONLY: open_namei("core",O_CREAT | O_WRONLY | O_TRUNC,0600,&inode,NULL) However, this changed in 1993 and starting with commit 9cb9f18b5d26 ("[PATCH] Linux-0.99.10 (June 7, 1993)") the coredump code suddenly used the constant "2": open_namei("core",O_CREAT | 2 | O_TRUNC,0600,&inode,NULL) This was curious as in the same commit the kernel switched from constants to proper defines in other places such as KERNEL_DS and USER_DS and O_RDWR did already exist. So why was "2" used? It turns out that open_namei() - an early version of what later turned into filp_open() - didn't accept O_RDWR. A semantic quirk of the open() uapi is the definition of the O_RDONLY flag. It would seem natural to define: #define O_RDWR (O_RDONLY | O_WRONLY) but that isn't possible because: #define O_RDONLY 0 This makes O_RDONLY effectively meaningless when passed to the kernel. In other words, there has never been a way - until O_PATH at least - to open a file without any permission; O_RDONLY was always implied on the uapi side while the kernel does in fact allow opening files without permissions. The trouble comes when trying to map the uapi flags onto the corresponding file mode flags FMODE_{READ,WRITE}. This mapping still happens today and is causing issues to this day (We ran into this during additions for openat2() for example.). So the special value "3" was used to indicate that the file was opened for special access: f->f_flags = flag = flags; f->f_mode = (flag+1) & O_ACCMODE; if (f->f_mode) flag++; This allowed the file mode to be set to FMODE_READ | FMODE_WRITE mapping the O_{RDONLY,WRONLY,RDWR} flags into the FMODE_{READ,WRITE} flags. The special access then required read-write permissions and 0 was used to access symlinks. But back when ddc733f452e0 ("[PATCH] Linux-0.97 (August 1, 1992)") added coredump handling open_namei() took the FMODE_{READ,WRITE} flags as an argument. So the coredump handling introduced in ddc733f452e0 ("[PATCH] Linux-0.97 (August 1, 1992)") was buggy because O_WRONLY shouldn't have been passed. Since O_WRONLY is 1 but open_namei() took FMODE_{READ,WRITE} it was passed FMODE_READ on accident. So 9cb9f18b5d26 ("[PATCH] Linux-0.99.10 (June 7, 1993)") was a bugfix for this and the 2 didn't really mean O_RDWR, it meant FMODE_WRITE which was correct. The clue is that FMODE_{READ,WRITE} didn't exist yet and thus a raw "2" value was passed. Fast forward 5 years when around 2.2.4pre4 (February 16, 1999) this code was changed to: - dentry = open_namei(corefile,O_CREAT | 2 | O_TRUNC | O_NOFOLLOW, 0600); ... + file = filp_open(corefile,O_CREAT | 2 | O_TRUNC | O_NOFOLLOW, 0600); At this point the raw "2" should have become O_WRONLY again as filp_open() didn't take FMODE_{READ,WRITE} but O_{RDONLY,WRONLY,RDWR}. Another 17 years later, the code was changed again cementing the mistake and making it almost impossible to detect when commit 378c6520e7d2 ("fs/coredump: prevent fsuid=0 dumps into user-controlled directories") replaced the raw "2" with O_RDWR. And now, here we are with this patch that sent us on a quest to answer the big questions in life such as "Why are coredump files opened with O_RDWR?" and "Is it safe to just use O_WRONLY?". So with this commit we're reintroducing O_WRONLY again and bringing this code back to its original state when it was first introduced in commit ddc733f452e0 ("[PATCH] Linux-0.97 (August 1, 1992)") over 30 years ago. Signed-off-by: Vladimir Sementsov-Ogievskiy Message-Id: <20230420120409.602576-1-vsementsov@yandex-team.ru> [brauner@kernel.org: completely rewritten commit message] Signed-off-by: Linus Torvalds Signed-off-by: Christian Brauner --- fs/coredump.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) (limited to 'fs/coredump.c') diff --git a/fs/coredump.c b/fs/coredump.c index ece7badf701b..ead3b05fb8f4 100644 --- a/fs/coredump.c +++ b/fs/coredump.c @@ -646,7 +646,7 @@ void do_coredump(const kernel_siginfo_t *siginfo) } else { struct mnt_idmap *idmap; struct inode *inode; - int open_flags = O_CREAT | O_RDWR | O_NOFOLLOW | + int open_flags = O_CREAT | O_WRONLY | O_NOFOLLOW | O_LARGEFILE | O_EXCL; if (cprm.limit < binfmt->min_coredump) -- cgit v1.2.3 From f9010dbdce911ee1f1af1398a24b1f9f992e0080 Mon Sep 17 00:00:00 2001 From: Mike Christie Date: Thu, 1 Jun 2023 13:32:32 -0500 Subject: fork, vhost: Use CLONE_THREAD to fix freezer/ps regression When switching from kthreads to vhost_tasks two bugs were added: 1. The vhost worker tasks's now show up as processes so scripts doing ps or ps a would not incorrectly detect the vhost task as another process. 2. kthreads disabled freeze by setting PF_NOFREEZE, but vhost tasks's didn't disable or add support for them. To fix both bugs, this switches the vhost task to be thread in the process that does the VHOST_SET_OWNER ioctl, and has vhost_worker call get_signal to support SIGKILL/SIGSTOP and freeze signals. Note that SIGKILL/STOP support is required because CLONE_THREAD requires CLONE_SIGHAND which requires those 2 signals to be supported. This is a modified version of the patch written by Mike Christie which was a modified version of patch originally written by Linus. Much of what depended upon PF_IO_WORKER now depends on PF_USER_WORKER. Including ignoring signals, setting up the register state, and having get_signal return instead of calling do_group_exit. Tidied up the vhost_task abstraction so that the definition of vhost_task only needs to be visible inside of vhost_task.c. Making it easier to review the code and tell what needs to be done where. As part of this the main loop has been moved from vhost_worker into vhost_task_fn. vhost_worker now returns true if work was done. The main loop has been updated to call get_signal which handles SIGSTOP, freezing, and collects the message that tells the thread to exit as part of process exit. This collection clears __fatal_signal_pending. This collection is not guaranteed to clear signal_pending() so clear that explicitly so the schedule() sleeps. For now the vhost thread continues to exist and run work until the last file descriptor is closed and the release function is called as part of freeing struct file. To avoid hangs in the coredump rendezvous and when killing threads in a multi-threaded exec. The coredump code and de_thread have been modified to ignore vhost threads. Remvoing the special case for exec appears to require teaching vhost_dev_flush how to directly complete transactions in case the vhost thread is no longer running. Removing the special case for coredump rendezvous requires either the above fix needed for exec or moving the coredump rendezvous into get_signal. Fixes: 6e890c5d5021 ("vhost: use vhost_tasks for worker threads") Signed-off-by: Eric W. Biederman Co-developed-by: Mike Christie Signed-off-by: Mike Christie Acked-by: Michael S. Tsirkin Signed-off-by: Linus Torvalds --- arch/x86/include/asm/fpu/sched.h | 2 +- arch/x86/kernel/fpu/context.h | 2 +- arch/x86/kernel/fpu/core.c | 2 +- drivers/vhost/vhost.c | 22 +++------- fs/coredump.c | 4 +- include/linux/sched/task.h | 1 - include/linux/sched/vhost_task.h | 15 ++----- kernel/exit.c | 5 ++- kernel/fork.c | 13 +++--- kernel/signal.c | 8 ++-- kernel/vhost_task.c | 92 ++++++++++++++++++++++++++-------------- 11 files changed, 89 insertions(+), 77 deletions(-) (limited to 'fs/coredump.c') diff --git a/arch/x86/include/asm/fpu/sched.h b/arch/x86/include/asm/fpu/sched.h index c2d6cd78ed0c..78fcde7b1f07 100644 --- a/arch/x86/include/asm/fpu/sched.h +++ b/arch/x86/include/asm/fpu/sched.h @@ -39,7 +39,7 @@ extern void fpu_flush_thread(void); static inline void switch_fpu_prepare(struct fpu *old_fpu, int cpu) { if (cpu_feature_enabled(X86_FEATURE_FPU) && - !(current->flags & (PF_KTHREAD | PF_IO_WORKER))) { + !(current->flags & (PF_KTHREAD | PF_USER_WORKER))) { save_fpregs_to_fpstate(old_fpu); /* * The save operation preserved register state, so the diff --git a/arch/x86/kernel/fpu/context.h b/arch/x86/kernel/fpu/context.h index 9fcfa5c4dad7..af5cbdd9bd29 100644 --- a/arch/x86/kernel/fpu/context.h +++ b/arch/x86/kernel/fpu/context.h @@ -57,7 +57,7 @@ static inline void fpregs_restore_userregs(void) struct fpu *fpu = ¤t->thread.fpu; int cpu = smp_processor_id(); - if (WARN_ON_ONCE(current->flags & (PF_KTHREAD | PF_IO_WORKER))) + if (WARN_ON_ONCE(current->flags & (PF_KTHREAD | PF_USER_WORKER))) return; if (!fpregs_state_valid(fpu, cpu)) { diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c index caf33486dc5e..1015af1ae562 100644 --- a/arch/x86/kernel/fpu/core.c +++ b/arch/x86/kernel/fpu/core.c @@ -426,7 +426,7 @@ void kernel_fpu_begin_mask(unsigned int kfpu_mask) this_cpu_write(in_kernel_fpu, true); - if (!(current->flags & (PF_KTHREAD | PF_IO_WORKER)) && + if (!(current->flags & (PF_KTHREAD | PF_USER_WORKER)) && !test_thread_flag(TIF_NEED_FPU_LOAD)) { set_thread_flag(TIF_NEED_FPU_LOAD); save_fpregs_to_fpstate(¤t->thread.fpu); diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c index a92af08e7864..074273020849 100644 --- a/drivers/vhost/vhost.c +++ b/drivers/vhost/vhost.c @@ -256,7 +256,7 @@ void vhost_work_queue(struct vhost_dev *dev, struct vhost_work *work) * test_and_set_bit() implies a memory barrier. */ llist_add(&work->node, &dev->worker->work_list); - wake_up_process(dev->worker->vtsk->task); + vhost_task_wake(dev->worker->vtsk); } } EXPORT_SYMBOL_GPL(vhost_work_queue); @@ -333,31 +333,19 @@ static void vhost_vq_reset(struct vhost_dev *dev, __vhost_vq_meta_reset(vq); } -static int vhost_worker(void *data) +static bool vhost_worker(void *data) { struct vhost_worker *worker = data; struct vhost_work *work, *work_next; struct llist_node *node; - for (;;) { - /* mb paired w/ kthread_stop */ - set_current_state(TASK_INTERRUPTIBLE); - - if (vhost_task_should_stop(worker->vtsk)) { - __set_current_state(TASK_RUNNING); - break; - } - - node = llist_del_all(&worker->work_list); - if (!node) - schedule(); - + node = llist_del_all(&worker->work_list); + if (node) { node = llist_reverse_order(node); /* make sure flag is seen after deletion */ smp_wmb(); llist_for_each_entry_safe(work, work_next, node, node) { clear_bit(VHOST_WORK_QUEUED, &work->flags); - __set_current_state(TASK_RUNNING); kcov_remote_start_common(worker->kcov_handle); work->fn(work); kcov_remote_stop(); @@ -365,7 +353,7 @@ static int vhost_worker(void *data) } } - return 0; + return !!node; } static void vhost_vq_free_iovecs(struct vhost_virtqueue *vq) diff --git a/fs/coredump.c b/fs/coredump.c index ece7badf701b..88740c51b942 100644 --- a/fs/coredump.c +++ b/fs/coredump.c @@ -371,7 +371,9 @@ static int zap_process(struct task_struct *start, int exit_code) if (t != current && !(t->flags & PF_POSTCOREDUMP)) { sigaddset(&t->pending.signal, SIGKILL); signal_wake_up(t, 1); - nr++; + /* The vhost_worker does not particpate in coredumps */ + if ((t->flags & (PF_USER_WORKER | PF_IO_WORKER)) != PF_USER_WORKER) + nr++; } } diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h index 537cbf9a2ade..e0f5ac90a228 100644 --- a/include/linux/sched/task.h +++ b/include/linux/sched/task.h @@ -29,7 +29,6 @@ struct kernel_clone_args { u32 io_thread:1; u32 user_worker:1; u32 no_files:1; - u32 ignore_signals:1; unsigned long stack; unsigned long stack_size; unsigned long tls; diff --git a/include/linux/sched/vhost_task.h b/include/linux/sched/vhost_task.h index 6123c10b99cf..837a23624a66 100644 --- a/include/linux/sched/vhost_task.h +++ b/include/linux/sched/vhost_task.h @@ -2,22 +2,13 @@ #ifndef _LINUX_VHOST_TASK_H #define _LINUX_VHOST_TASK_H -#include -struct task_struct; +struct vhost_task; -struct vhost_task { - int (*fn)(void *data); - void *data; - struct completion exited; - unsigned long flags; - struct task_struct *task; -}; - -struct vhost_task *vhost_task_create(int (*fn)(void *), void *arg, +struct vhost_task *vhost_task_create(bool (*fn)(void *), void *arg, const char *name); void vhost_task_start(struct vhost_task *vtsk); void vhost_task_stop(struct vhost_task *vtsk); -bool vhost_task_should_stop(struct vhost_task *vtsk); +void vhost_task_wake(struct vhost_task *vtsk); #endif diff --git a/kernel/exit.c b/kernel/exit.c index 34b90e2e7cf7..edb50b4c9972 100644 --- a/kernel/exit.c +++ b/kernel/exit.c @@ -411,7 +411,10 @@ static void coredump_task_exit(struct task_struct *tsk) tsk->flags |= PF_POSTCOREDUMP; core_state = tsk->signal->core_state; spin_unlock_irq(&tsk->sighand->siglock); - if (core_state) { + + /* The vhost_worker does not particpate in coredumps */ + if (core_state && + ((tsk->flags & (PF_IO_WORKER | PF_USER_WORKER)) != PF_USER_WORKER)) { struct core_thread self; self.task = current; diff --git a/kernel/fork.c b/kernel/fork.c index ed4e01daccaa..81cba91f30bb 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -2336,16 +2336,16 @@ __latent_entropy struct task_struct *copy_process( p->flags &= ~PF_KTHREAD; if (args->kthread) p->flags |= PF_KTHREAD; - if (args->user_worker) - p->flags |= PF_USER_WORKER; - if (args->io_thread) { + if (args->user_worker) { /* - * Mark us an IO worker, and block any signal that isn't + * Mark us a user worker, and block any signal that isn't * fatal or STOP */ - p->flags |= PF_IO_WORKER; + p->flags |= PF_USER_WORKER; siginitsetinv(&p->blocked, sigmask(SIGKILL)|sigmask(SIGSTOP)); } + if (args->io_thread) + p->flags |= PF_IO_WORKER; if (args->name) strscpy_pad(p->comm, args->name, sizeof(p->comm)); @@ -2517,9 +2517,6 @@ __latent_entropy struct task_struct *copy_process( if (retval) goto bad_fork_cleanup_io; - if (args->ignore_signals) - ignore_signals(p); - stackleak_task_init(p); if (pid != &init_struct_pid) { diff --git a/kernel/signal.c b/kernel/signal.c index 8f6330f0e9ca..2547fa73bde5 100644 --- a/kernel/signal.c +++ b/kernel/signal.c @@ -1368,7 +1368,9 @@ int zap_other_threads(struct task_struct *p) while_each_thread(p, t) { task_clear_jobctl_pending(t, JOBCTL_PENDING_MASK); - count++; + /* Don't require de_thread to wait for the vhost_worker */ + if ((t->flags & (PF_IO_WORKER | PF_USER_WORKER)) != PF_USER_WORKER) + count++; /* Don't bother with already dead threads */ if (t->exit_state) @@ -2861,11 +2863,11 @@ relock: } /* - * PF_IO_WORKER threads will catch and exit on fatal signals + * PF_USER_WORKER threads will catch and exit on fatal signals * themselves. They have cleanup that must be performed, so * we cannot call do_exit() on their behalf. */ - if (current->flags & PF_IO_WORKER) + if (current->flags & PF_USER_WORKER) goto out; /* diff --git a/kernel/vhost_task.c b/kernel/vhost_task.c index b7cbd66f889e..f80d5c51ae67 100644 --- a/kernel/vhost_task.c +++ b/kernel/vhost_task.c @@ -12,58 +12,88 @@ enum vhost_task_flags { VHOST_TASK_FLAGS_STOP, }; +struct vhost_task { + bool (*fn)(void *data); + void *data; + struct completion exited; + unsigned long flags; + struct task_struct *task; +}; + static int vhost_task_fn(void *data) { struct vhost_task *vtsk = data; - int ret; + bool dead = false; + + for (;;) { + bool did_work; + + /* mb paired w/ vhost_task_stop */ + if (test_bit(VHOST_TASK_FLAGS_STOP, &vtsk->flags)) + break; + + if (!dead && signal_pending(current)) { + struct ksignal ksig; + /* + * Calling get_signal will block in SIGSTOP, + * or clear fatal_signal_pending, but remember + * what was set. + * + * This thread won't actually exit until all + * of the file descriptors are closed, and + * the release function is called. + */ + dead = get_signal(&ksig); + if (dead) + clear_thread_flag(TIF_SIGPENDING); + } + + did_work = vtsk->fn(vtsk->data); + if (!did_work) { + set_current_state(TASK_INTERRUPTIBLE); + schedule(); + } + } - ret = vtsk->fn(vtsk->data); complete(&vtsk->exited); - do_exit(ret); + do_exit(0); +} + +/** + * vhost_task_wake - wakeup the vhost_task + * @vtsk: vhost_task to wake + * + * wake up the vhost_task worker thread + */ +void vhost_task_wake(struct vhost_task *vtsk) +{ + wake_up_process(vtsk->task); } +EXPORT_SYMBOL_GPL(vhost_task_wake); /** * vhost_task_stop - stop a vhost_task * @vtsk: vhost_task to stop * - * Callers must call vhost_task_should_stop and return from their worker - * function when it returns true; + * vhost_task_fn ensures the worker thread exits after + * VHOST_TASK_FLAGS_SOP becomes true. */ void vhost_task_stop(struct vhost_task *vtsk) { - pid_t pid = vtsk->task->pid; - set_bit(VHOST_TASK_FLAGS_STOP, &vtsk->flags); - wake_up_process(vtsk->task); + vhost_task_wake(vtsk); /* * Make sure vhost_task_fn is no longer accessing the vhost_task before - * freeing it below. If userspace crashed or exited without closing, - * then the vhost_task->task could already be marked dead so - * kernel_wait will return early. + * freeing it below. */ wait_for_completion(&vtsk->exited); - /* - * If we are just closing/removing a device and the parent process is - * not exiting then reap the task. - */ - kernel_wait4(pid, NULL, __WCLONE, NULL); kfree(vtsk); } EXPORT_SYMBOL_GPL(vhost_task_stop); /** - * vhost_task_should_stop - should the vhost task return from the work function - * @vtsk: vhost_task to stop - */ -bool vhost_task_should_stop(struct vhost_task *vtsk) -{ - return test_bit(VHOST_TASK_FLAGS_STOP, &vtsk->flags); -} -EXPORT_SYMBOL_GPL(vhost_task_should_stop); - -/** - * vhost_task_create - create a copy of a process to be used by the kernel - * @fn: thread stack + * vhost_task_create - create a copy of a task to be used by the kernel + * @fn: vhost worker function * @arg: data to be passed to fn * @name: the thread's name * @@ -71,17 +101,17 @@ EXPORT_SYMBOL_GPL(vhost_task_should_stop); * failure. The returned task is inactive, and the caller must fire it up * through vhost_task_start(). */ -struct vhost_task *vhost_task_create(int (*fn)(void *), void *arg, +struct vhost_task *vhost_task_create(bool (*fn)(void *), void *arg, const char *name) { struct kernel_clone_args args = { - .flags = CLONE_FS | CLONE_UNTRACED | CLONE_VM, + .flags = CLONE_FS | CLONE_UNTRACED | CLONE_VM | + CLONE_THREAD | CLONE_SIGHAND, .exit_signal = 0, .fn = vhost_task_fn, .name = name, .user_worker = 1, .no_files = 1, - .ignore_signals = 1, }; struct vhost_task *vtsk; struct task_struct *tsk; -- cgit v1.2.3