[ This should work for v5.4 ] From: "Steven Rostedt (Google)" <rostedt@xxxxxxxxxxx> Subject: [PATCH] tracing: Have trace_event_file have ref counters commit bb32500fb9b78215e4ef6ee8b4345c5f5d7eafb4 upstream The following can crash the kernel: # cd /sys/kernel/tracing # echo 'p:sched schedule' > kprobe_events # exec 5>>events/kprobes/sched/enable # > kprobe_events # exec 5>&- The above commands: 1. Change directory to the tracefs directory 2. Create a kprobe event (doesn't matter what one) 3. Open bash file descriptor 5 on the enable file of the kprobe event 4. Delete the kprobe event (removes the files too) 5. Close the bash file descriptor 5 The above causes a crash! BUG: kernel NULL pointer dereference, address: 0000000000000028 #PF: supervisor read access in kernel mode #PF: error_code(0x0000) - not-present page PGD 0 P4D 0 Oops: 0000 [#1] PREEMPT SMP PTI CPU: 6 PID: 877 Comm: bash Not tainted 6.5.0-rc4-test-00008-g2c6b6b1029d4-dirty #186 Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.2-debian-1.16.2-1 04/01/2014 RIP: 0010:tracing_release_file_tr+0xc/0x50 What happens here is that the kprobe event creates a trace_event_file "file" descriptor that represents the file in tracefs to the event. It maintains state of the event (is it enabled for the given instance?). Opening the "enable" file gets a reference to the event "file" descriptor via the open file descriptor. When the kprobe event is deleted, the file is also deleted from the tracefs system which also frees the event "file" descriptor. But as the tracefs file is still opened by user space, it will not be totally removed until the final dput() is called on it. But this is not true with the event "file" descriptor that is already freed. If the user does a write to or simply closes the file descriptor it will reference the event "file" descriptor that was just freed, causing a use-after-free bug. To solve this, add a ref count to the event "file" descriptor as well as a new flag called "FREED". The "file" will not be freed until the last reference is released. But the FREE flag will be set when the event is removed to prevent any more modifications to that event from happening, even if there's still a reference to the event "file" descriptor. Link: https://lore.kernel.org/linux-trace-kernel/20231031000031.1e705592@xxxxxxxxxxxxxxxxxx/ Link: https://lore.kernel.org/linux-trace-kernel/20231031122453.7a48b923@xxxxxxxxxxxxxxxxxx Cc: stable@xxxxxxxxxxxxxxx Cc: Mark Rutland <mark.rutland@xxxxxxx> Fixes: f5ca233e2e66d ("tracing: Increase trace array ref count on enable and filter files") Reported-by: Beau Belgrave <beaub@xxxxxxxxxxxxxxxxxxx> Tested-by: Beau Belgrave <beaub@xxxxxxxxxxxxxxxxxxx> Reviewed-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx> Signed-off-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx> --- include/linux/trace_events.h | 4 +++ kernel/trace/trace.c | 15 ++++++++++++++ kernel/trace/trace.h | 3 ++ kernel/trace/trace_events.c | 38 +++++++++++++++++++++++++------------ kernel/trace/trace_events_filter.c | 3 ++ 5 files changed, 51 insertions(+), 12 deletions(-) Index: linux-trace.git/include/linux/trace_events.h =================================================================== --- linux-trace.git.orig/include/linux/trace_events.h 2023-11-06 14:32:00.422220804 -0500 +++ linux-trace.git/include/linux/trace_events.h 2023-11-06 14:32:23.130563435 -0500 @@ -341,6 +341,7 @@ enum { EVENT_FILE_FL_TRIGGER_COND_BIT, EVENT_FILE_FL_PID_FILTER_BIT, EVENT_FILE_FL_WAS_ENABLED_BIT, + EVENT_FILE_FL_FREED_BIT, }; /* @@ -357,6 +358,7 @@ enum { * TRIGGER_COND - When set, one or more triggers has an associated filter * PID_FILTER - When set, the event is filtered based on pid * WAS_ENABLED - Set when enabled to know to clear trace on module removal + * FREED - File descriptor is freed, all fields should be considered invalid */ enum { EVENT_FILE_FL_ENABLED = (1 << EVENT_FILE_FL_ENABLED_BIT), @@ -370,6 +372,7 @@ enum { EVENT_FILE_FL_TRIGGER_COND = (1 << EVENT_FILE_FL_TRIGGER_COND_BIT), EVENT_FILE_FL_PID_FILTER = (1 << EVENT_FILE_FL_PID_FILTER_BIT), EVENT_FILE_FL_WAS_ENABLED = (1 << EVENT_FILE_FL_WAS_ENABLED_BIT), + EVENT_FILE_FL_FREED = (1 << EVENT_FILE_FL_FREED_BIT), }; struct trace_event_file { @@ -398,6 +401,7 @@ struct trace_event_file { * caching and such. Which is mostly OK ;-) */ unsigned long flags; + atomic_t ref; /* ref count for opened files */ atomic_t sm_ref; /* soft-mode reference counter */ atomic_t tm_ref; /* trigger-mode reference counter */ }; Index: linux-trace.git/kernel/trace/trace.c =================================================================== --- linux-trace.git.orig/kernel/trace/trace.c 2023-11-06 14:32:00.422220804 -0500 +++ linux-trace.git/kernel/trace/trace.c 2023-11-06 14:32:00.410220623 -0500 @@ -4257,6 +4257,20 @@ int tracing_open_file_tr(struct inode *i if (ret) return ret; + mutex_lock(&event_mutex); + + /* Fail if the file is marked for removal */ + if (file->flags & EVENT_FILE_FL_FREED) { + trace_array_put(file->tr); + ret = -ENODEV; + } else { + event_file_get(file); + } + + mutex_unlock(&event_mutex); + if (ret) + return ret; + filp->private_data = inode->i_private; return 0; @@ -4267,6 +4281,7 @@ int tracing_release_file_tr(struct inode struct trace_event_file *file = inode->i_private; trace_array_put(file->tr); + event_file_put(file); return 0; } Index: linux-trace.git/kernel/trace/trace.h =================================================================== --- linux-trace.git.orig/kernel/trace/trace.h 2023-11-06 14:32:00.422220804 -0500 +++ linux-trace.git/kernel/trace/trace.h 2023-11-06 14:32:00.414220683 -0500 @@ -1696,6 +1696,9 @@ extern int register_event_command(struct extern int unregister_event_command(struct event_command *cmd); extern int register_trigger_hist_enable_disable_cmds(void); +extern void event_file_get(struct trace_event_file *file); +extern void event_file_put(struct trace_event_file *file); + /** * struct event_trigger_ops - callbacks for trace event triggers * Index: linux-trace.git/kernel/trace/trace_events.c =================================================================== --- linux-trace.git.orig/kernel/trace/trace_events.c 2023-11-06 14:32:00.422220804 -0500 +++ linux-trace.git/kernel/trace/trace_events.c 2023-11-06 14:39:32.657041546 -0500 @@ -698,21 +698,34 @@ static void remove_subsystem(struct trac } } +void event_file_get(struct trace_event_file *file) +{ + atomic_inc(&file->ref); +} + +void event_file_put(struct trace_event_file *file) +{ + if (WARN_ON_ONCE(!atomic_read(&file->ref))) { + if (file->flags & EVENT_FILE_FL_FREED) + kmem_cache_free(file_cachep, file); + return; + } + + if (atomic_dec_and_test(&file->ref)) { + /* Count should only go to zero when it is freed */ + if (WARN_ON_ONCE(!(file->flags & EVENT_FILE_FL_FREED))) + return; + kmem_cache_free(file_cachep, file); + } +} + static void remove_event_file_dir(struct trace_event_file *file) { struct dentry *dir = file->dir; struct dentry *child; - if (dir) { - spin_lock(&dir->d_lock); /* probably unneeded */ - list_for_each_entry(child, &dir->d_subdirs, d_child) { - if (d_really_is_positive(child)) /* probably unneeded */ - d_inode(child)->i_private = NULL; - } - spin_unlock(&dir->d_lock); - + if (dir) tracefs_remove_recursive(dir); - } list_del(&file->list); remove_subsystem(file->system); @@ -1033,7 +1046,7 @@ event_enable_read(struct file *filp, cha flags = file->flags; mutex_unlock(&event_mutex); - if (!file) + if (!file || flags & EVENT_FILE_FL_FREED) return -ENODEV; if (flags & EVENT_FILE_FL_ENABLED && @@ -1071,7 +1084,7 @@ event_enable_write(struct file *filp, co ret = -ENODEV; mutex_lock(&event_mutex); file = event_file_data(filp); - if (likely(file)) + if (likely(file && !(file->flags & EVENT_FILE_FL_FREED))) ret = ftrace_event_enable_disable(file, val); mutex_unlock(&event_mutex); break; @@ -1340,7 +1353,7 @@ event_filter_read(struct file *filp, cha mutex_lock(&event_mutex); file = event_file_data(filp); - if (file) + if (file && !(file->flags & EVENT_FILE_FL_FREED)) print_event_filter(file, s); mutex_unlock(&event_mutex); @@ -2264,6 +2277,7 @@ trace_create_new_event(struct trace_even atomic_set(&file->tm_ref, 0); INIT_LIST_HEAD(&file->triggers); list_add(&file->list, &tr->events); + event_file_get(file); return file; } Index: linux-trace.git/kernel/trace/trace_events_filter.c =================================================================== --- linux-trace.git.orig/kernel/trace/trace_events_filter.c 2023-11-06 14:32:00.422220804 -0500 +++ linux-trace.git/kernel/trace/trace_events_filter.c 2023-11-06 14:32:00.414220683 -0500 @@ -1800,6 +1800,9 @@ int apply_event_filter(struct trace_even struct event_filter *filter = NULL; int err; + if (file->flags & EVENT_FILE_FL_FREED) + return -ENODEV; + if (!strcmp(strstrip(filter_string), "0")) { filter_disable(file); filter = event_filter(file);
