[PATCH 5.15 010/123] fs: use acquire ordering in __fget_light()

Greg Kroah-Hartman
2022-12-12T13:25:24Z

From: Jann Horn <jannh@google.com>

[ Upstream commit 7ee47dcfff1835ff75a794d1075b6b5f5462cfed ]

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 <jannh@google.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
fs/file.c | 11 ++++++++++-
1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/fs/file.c b/fs/file.c
index ee9317346702..214364e19d76 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -1029,7 +1029,16 @@ 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;
--
2.35.1