From: Alexey Dobriyan Current /proc creation interfaces suffer from at least two types of races: -------------------------------------------------------- 1. Write via ->write_proc sleeps in copy_from_user(). Module disappears meanwhile. pde = create_proc_entry() if (!pde) return -ENOMEM; pde->write_proc = ... open write copy_from_user pde = create_proc_entry(); if (!pde) { remove_proc_entry(); return -ENOMEM; /* module unloaded */ } *boom* -------------------------------------------------------- 2. Read/write happens when PDE only partially initialized. ->data is NULL when create_proc_entry() returns. Almost all ->read_proc and ->write_proc handlers assume that ->data is valid. pde = create_proc_entry(); if (pde) { /* which dereferences ->data */ pde->write_proc = ... open write pde->data = ... } -------------------------------------------------------- The following plan is going to be executed (as per Al Viro's explanations): PDE gets counter counting reads and writes in progress done via ->read_proc, ->write_proc, ->get_info . Generic proc code will bump PDE's counter before calling into module-specific method and decrement it after it returns. remove_proc_entry() will wait until all readers and writers are done. To do this reliably it will set ->proc_fops to NULL and generic proc code won't call into module it it sees NULL ->proc_fops. This patch implements part above. So far, no changes in proc users required except that users dynamically creating ->proc_fops need to be careful to not get leak. Patch fixes races of type 1. Patch survives infinite modprobe/rmmod loop in parallel with infinite read loops with many debugging options turned on including lockdep (albeit on UP-PREEMPT box). Signed-off-by: Alexey Dobriyan Cc: Al Viro Cc: "Eric W. Biederman" Cc: Oleg Nesterov Signed-off-by: Andrew Morton --- fs/proc/generic.c | 83 +++++++++++++++++++++++++++++++++++++- include/linux/proc_fs.h | 19 ++++++++ 2 files changed, 100 insertions(+), 2 deletions(-) diff -puN fs/proc/generic.c~fix-rmmod-read-write-races-in-proc-entries fs/proc/generic.c --- a/fs/proc/generic.c~fix-rmmod-read-write-races-in-proc-entries +++ a/fs/proc/generic.c @@ -20,6 +20,7 @@ #include #include #include +#include #include #include "internal.h" @@ -76,6 +77,21 @@ proc_file_read(struct file *file, char _ if (!(page = (char*) __get_free_page(GFP_KERNEL))) return -ENOMEM; + spin_lock(&dp->pde_unload_lock); + if (!dp->proc_fops) + /* + * remove_proc_entry() marked PDE as "going away". + * No new readers allowed. + */ + goto out_unlock; + /* + * We are going to call into module's code via ->get_info or + * ->read_proc. Bump refcount so that remove_proc_entry() will + * wait for read to complete. + */ + dp->pde_users++; + spin_unlock(&dp->pde_unload_lock); + while ((nbytes > 0) && !eof) { count = min_t(size_t, PROC_BLOCK_SIZE, nbytes); @@ -195,6 +211,13 @@ proc_file_read(struct file *file, char _ buf += n; retval += n; } + + spin_lock(&dp->pde_unload_lock); + dp->pde_users--; + if (dp->pde_unload_completion && dp->pde_users == 0) + complete(dp->pde_unload_completion); +out_unlock: + spin_unlock(&dp->pde_unload_lock); free_page((unsigned long) page); return retval; } @@ -205,14 +228,41 @@ proc_file_write(struct file *file, const { struct inode *inode = file->f_path.dentry->d_inode; struct proc_dir_entry * dp; + ssize_t rv = -EIO; dp = PDE(inode); if (!dp->write_proc) - return -EIO; + goto out; + spin_lock(&dp->pde_unload_lock); + if (!dp->proc_fops) + /* + * remove_proc_entry() marked PDE as "going away". + * No new writers allowed. + */ + goto out_unlock; + /* + * We are going to call into module's code via ->write_proc . + * Bump refcount so that module won't dissapear while ->write_proc + * sleeps in copy_from_user(). remove_proc_entry() will wait for + * write to complete. + */ + dp->pde_users++; + spin_unlock(&dp->pde_unload_lock); + + /* PDE is ready, refcount bumped, call into module. */ /* FIXME: does this routine need ppos? probably... */ - return dp->write_proc(file, buffer, count, dp->data); + rv = dp->write_proc(file, buffer, count, dp->data); + + spin_lock(&dp->pde_unload_lock); + dp->pde_users--; + if (dp->pde_unload_completion && dp->pde_users == 0) + complete(dp->pde_unload_completion); +out_unlock: + spin_unlock(&dp->pde_unload_lock); +out: + return rv; } @@ -607,6 +657,9 @@ static struct proc_dir_entry *proc_creat ent->namelen = len; ent->mode = mode; ent->nlink = nlink; + ent->pde_users = 0; + spin_lock_init(&ent->pde_unload_lock); + ent->pde_unload_completion = NULL; out: return ent; } @@ -728,6 +781,32 @@ void remove_proc_entry(const char *name, de = *p; *p = de->next; de->next = NULL; + + spin_lock(&de->pde_unload_lock); + /* + * Stop accepting new readers/writers. If you're dynamically + * allocating ->proc_fops, save a pointer somewhere. + */ + de->proc_fops = NULL; + /* Wait until all existing readers/writers are done. */ + if (de->pde_users > 0) { + struct completion c; + + init_completion(&c); + if (!de->pde_unload_completion) + de->pde_unload_completion = &c; + + spin_unlock(&de->pde_unload_lock); + spin_unlock(&proc_subdir_lock); + + wait_for_completion(de->pde_unload_completion); + + spin_lock(&proc_subdir_lock); + goto continue_removing; + } + spin_unlock(&de->pde_unload_lock); + +continue_removing: if (S_ISDIR(de->mode)) parent->nlink--; proc_kill_inodes(de); diff -puN include/linux/proc_fs.h~fix-rmmod-read-write-races-in-proc-entries include/linux/proc_fs.h --- a/include/linux/proc_fs.h~fix-rmmod-read-write-races-in-proc-entries +++ a/include/linux/proc_fs.h @@ -7,6 +7,8 @@ #include #include +struct completion; + /* * The proc filesystem constants/structures */ @@ -56,6 +58,19 @@ struct proc_dir_entry { gid_t gid; loff_t size; const struct inode_operations *proc_iops; + /* + * NULL ->proc_fops means "PDE is going away RSN" or + * "PDE is just created". In either case ->get_info, ->read_proc, + * ->write_proc won't be called because it's too late or too early, + * respectively. + * + * Valid ->proc_fops means "use this file_operations" or + * "->data is setup, it's safe to call ->read_proc, ->write_proc which + * can dereference it". + * + * If you're allocating ->proc_fops dynamically, save a pointer + * somewhere. + */ const struct file_operations *proc_fops; get_info_t *get_info; struct module *owner; @@ -66,6 +81,10 @@ struct proc_dir_entry { atomic_t count; /* use count */ int deleted; /* delete flag */ void *set; + int pde_users; /* number of readers + number of writers via + * ->read_proc, ->write_proc, ->get_info */ + spinlock_t pde_unload_lock; + struct completion *pde_unload_completion; }; struct kcore_list { _