[PATCH 5.19 351/717] locks: fix TOCTOU race when granting write lease

Greg Kroah-Hartman
2022-10-22T08:06:05Z

From: Amir Goldstein <amir73il@gmail.com>

[ Upstream commit d6da19c9cace63290ccfccb1fc35151ffefc0bec ]

Thread A trying to acquire a write lease checks the value of i_readcount
and i_writecount in check_conflicting_open() to verify that its own fd
is the only fd referencing the file.

Thread B trying to open the file for read will call break_lease() in
do_dentry_open() before incrementing i_readcount, which leaves a small
window where thread A can acquire the write lease and then thread B
completes the open of the file for read without breaking the write lease
that was acquired by thread A.

Fix this race by incrementing i_readcount before checking for existing
leases, same as the case with i_writecount.

Use a helper put_file_access() to decrement i_readcount or i_writecount
in do_dentry_open() and __fput().

Fixes: 387e3746d01c ("locks: eliminate false positive conflicts for write lease")
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
fs/file_table.c | 7 +------
fs/internal.h | 10 ++++++++++
fs/open.c | 11 ++++-------
3 files changed, 15 insertions(+), 13 deletions(-)

diff --git a/fs/file_table.c b/fs/file_table.c
index 5424e3a8df5f..543a501b0247 100644
--- a/fs/file_table.c
+++ b/fs/file_table.c
@@ -321,12 +321,7 @@ static void __fput(struct file *file)
}
fops_put(file->f_op);
put_pid(file->f_owner.pid);
- if ((mode & (FMODE_READ | FMODE_WRITE)) == FMODE_READ)
- i_readcount_dec(inode);
- if (mode & FMODE_WRITER) {
- put_write_access(inode);
- __mnt_drop_write(mnt);
- }
+ put_file_access(file);
dput(dentry);
if (unlikely(mode & FMODE_NEED_UNMOUNT))
dissolve_on_fput(mnt);
diff --git a/fs/internal.h b/fs/internal.h
index 3e206d3e317c..4372d67a3753 100644
--- a/fs/internal.h
+++ b/fs/internal.h
@@ -102,6 +102,16 @@ extern void chroot_fs_refs(const struct path *, const struct path *);
extern struct file *alloc_empty_file(int, const struct cred *);
extern struct file *alloc_empty_file_noaccount(int, const struct cred *);

+static inline void put_file_access(struct file *file)
+{
+ if ((file->f_mode & (FMODE_READ | FMODE_WRITE)) == FMODE_READ) {
+ i_readcount_dec(file->f_inode);
+ } else if (file->f_mode & FMODE_WRITER) {
+ put_write_access(file->f_inode);
+ __mnt_drop_write(file->f_path.mnt);
+ }
+}
+
/*
* super.c
*/
diff --git a/fs/open.c b/fs/open.c
index 1d57fbde2feb..5874258b54bd 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -810,7 +810,9 @@ static int do_dentry_open(struct file *f,
return 0;
}

- if (f->f_mode & FMODE_WRITE && !special_file(inode->i_mode)) {
+ if ((f->f_mode & (FMODE_READ | FMODE_WRITE)) == FMODE_READ) {
+ i_readcount_inc(inode);
+ } else if (f->f_mode & FMODE_WRITE && !special_file(inode->i_mode)) {
error = get_write_access(inode);
if (unlikely(error))
goto cleanup_file;
@@ -850,8 +852,6 @@ static int do_dentry_open(struct file *f,
goto cleanup_all;
}
f->f_mode |= FMODE_OPENED;
- if ((f->f_mode & (FMODE_READ | FMODE_WRITE)) == FMODE_READ)
- i_readcount_inc(inode);
if ((f->f_mode & FMODE_READ) &&
likely(f->f_op->read || f->f_op->read_iter))
f->f_mode |= FMODE_CAN_READ;
@@ -902,10 +902,7 @@ static int do_dentry_open(struct file *f,
if (WARN_ON_ONCE(error > 0))
error = -EINVAL;
fops_put(f->f_op);
- if (f->f_mode & FMODE_WRITER) {
- put_write_access(inode);
- __mnt_drop_write(f->f_path.mnt);
- }
+ put_file_access(f);
cleanup_file:
path_put(&f->f_path);
f->f_path.mnt = NULL;
--
2.35.1