From: Mateusz Guzik mjguzik@gmail.com

[ Upstream commit f381640e1bd4f2de7ccafbfe8703d33c3718aad9 ]

... 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 Signed-off-by: Sasha Levin sashal@kernel.org --- fs/file.c | 26 +++++++++++++++++--------- 1 file changed, 17 insertions(+), 9 deletions(-)

diff --git a/fs/file.c b/fs/file.c index d868cdb95d1e7..1ba03662ae66f 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); @@ -679,7 +687,7 @@ 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); @@ -1237,7 +1245,7 @@ __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);