From 7ee47dcfff1835ff75a794d1075b6b5f5462cfed Mon Sep 17 00:00:00 2001 From: Jann Horn Date: Mon, 31 Oct 2022 18:52:56 +0100 Subject: [PATCH] fs: use acquire ordering in __fget_light() We must prevent the CPU from reordering the files->count read with the FD table access like this, on architectures where read-read reordering is possible: files_lookup_fd_raw() close_fd() put_files_struct() atomic_read(&files->count) I would like to mark this for stable, but the stable rules explicitly say "no theoretical races", and given that the FD table pointer and files->count are explicitly stored in the same cacheline, this sort of reordering seems quite unlikely in practice... Signed-off-by: Jann Horn Signed-off-by: Al Viro --- fs/file.c | 11 ++++++++++- 1 file changed, 10 insertions(+), 1 deletion(-) diff --git a/fs/file.c b/fs/file.c index 5f9c802..c942c89c 100644 --- a/fs/file.c +++ b/fs/file.c @@ -1003,7 +1003,16 @@ static unsigned long __fget_light(unsigned int fd, fmode_t mask) struct files_struct *files = current->files; struct file *file; - if (atomic_read(&files->count) == 1) { + /* + * If another thread is concurrently calling close_fd() followed + * by put_files_struct(), we must not observe the old table + * entry combined with the new refcount - otherwise we could + * return a file that is concurrently being freed. + * + * atomic_read_acquire() pairs with atomic_dec_and_test() in + * put_files_struct(). + */ + if (atomic_read_acquire(&files->count) == 1) { file = files_lookup_fd_raw(files, fd); if (!file || unlikely(file->f_mode & mask)) return 0; -- 2.7.4