diff options
author | Mateusz Guzik <mjguzik@gmail.com> | 2025-03-13 14:57:25 +0100 |
---|---|---|
committer | Christian Brauner <brauner@kernel.org> | 2025-03-18 15:34:27 +0100 |
commit | f381640e1bd4f2de7ccafbfe8703d33c3718aad9 (patch) | |
tree | 090384ca8c7dbb6609364afebeb7a354a9942533 /fs/file.c | |
parent | 64a56f635aae25381ea35dce418ce8dd5ceb2f3d (diff) |
fs: consistently deref the files table with rcu_dereference_raw()
... except when the table is known to be only used by one thread.
A file pointer can get installed at any moment despite the ->file_lock
being held since the following:
8a81252b774b53e6 ("fs/file.c: don't acquire files->file_lock in fd_install()")
Accesses subject to such a race can in principle suffer load tearing.
While here redo the comment in dup_fd -- it only covered a race against
files showing up, still assuming fd_install() takes the lock.
Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Link: https://lore.kernel.org/r/20250313135725.1320914-1-mjguzik@gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>
Diffstat (limited to 'fs/file.c')
-rw-r--r-- | fs/file.c | 26 |
1 files changed, 17 insertions, 9 deletions
diff --git a/fs/file.c b/fs/file.c index c8214975e248..2d66816af105 100644 --- a/fs/file.c +++ b/fs/file.c @@ -418,17 +418,25 @@ struct files_struct *dup_fd(struct files_struct *oldf, struct fd_range *punch_ho old_fds = old_fdt->fd; new_fds = new_fdt->fd; + /* + * We may be racing against fd allocation from other threads using this + * files_struct, despite holding ->file_lock. + * + * alloc_fd() might have already claimed a slot, while fd_install() + * did not populate it yet. Note the latter operates locklessly, so + * the file can show up as we are walking the array below. + * + * At the same time we know no files will disappear as all other + * operations take the lock. + * + * Instead of trying to placate userspace racing with itself, we + * ref the file if we see it and mark the fd slot as unused otherwise. + */ for (i = open_files; i != 0; i--) { - struct file *f = *old_fds++; + struct file *f = rcu_dereference_raw(*old_fds++); if (f) { get_file(f); } else { - /* - * The fd may be claimed in the fd bitmap but not yet - * instantiated in the files array if a sibling thread - * is partway through open(). So make sure that this - * fd is available to the new process. - */ __clear_open_fd(open_files - i, new_fdt); } rcu_assign_pointer(*new_fds++, f); @@ -680,7 +688,7 @@ struct file *file_close_fd_locked(struct files_struct *files, unsigned fd) return NULL; fd = array_index_nospec(fd, fdt->max_fds); - file = fdt->fd[fd]; + file = rcu_dereference_raw(fdt->fd[fd]); if (file) { rcu_assign_pointer(fdt->fd[fd], NULL); __put_unused_fd(files, fd); @@ -1248,7 +1256,7 @@ __releases(&files->file_lock) */ fdt = files_fdtable(files); fd = array_index_nospec(fd, fdt->max_fds); - tofree = fdt->fd[fd]; + tofree = rcu_dereference_raw(fdt->fd[fd]); if (!tofree && fd_is_open(fd, fdt)) goto Ebusy; get_file(file); |