locks: fix TOCTOU race when granting write lease

[ 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>

diff --git a/fs/file_table.c b/fs/file_table.c
index e8c9016..6f297f9 100644 (file)
--- a/fs/file_table.c
+++ b/fs/file_table.c
@@ -284,12 +284,7 @@ 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 4f1fe6d..9075490 100644 (file)
--- a/fs/internal.h
+++ b/fs/internal.h
@@ -100,6 +100,16 @@ 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 1ba1d2a..5e322f1 100644 (file)
--- a/fs/open.c
+++ b/fs/open.c
@@ -786,7 +786,9 @@ 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;
@@ -826,8 +828,6 @@ 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;
@@ -880,10 +880,7 @@ cleanup_all:
        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;