From: Eric Biggers ebiggers@google.com

Commit 56124d6c87fd ("fsverity: support enabling with tree block size < PAGE_SIZE") changed FS_IOC_ENABLE_VERITY to use __kernel_read() to read the file's data, instead of direct pagecache accesses.

An unintended consequence of this is that the 'WARN_ON_ONCE(!(file->f_mode & FMODE_READ))' in __kernel_read() became reachable by fuzz tests. This happens if FS_IOC_ENABLE_VERITY is called on a fd opened with access mode 3, which means "ioctl access only".

Arguably, FS_IOC_ENABLE_VERITY should work on ioctl-only fds. But ioctl-only fds are a weird Linux extension that is rarely used and that few people even know about. (The documentation for FS_IOC_ENABLE_VERITY even specifically says it requires O_RDONLY.) It's probably not worthwhile to make the ioctl internally open a new fd just to handle this case. Thus, just reject the ioctl on such fds for now.

Fixes: 56124d6c87fd ("fsverity: support enabling with tree block size < PAGE_SIZE") Reported-by: syzbot+51177e4144d764827c45@syzkaller.appspotmail.com Link: https://syzkaller.appspot.com/bug?id=2281afcbbfa8fdb92f9887479cc0e4180f1c6b2... Cc: stable@vger.kernel.org Signed-off-by: Eric Biggers ebiggers@google.com --- fs/verity/enable.c | 7 +++++++ 1 file changed, 7 insertions(+)

diff --git a/fs/verity/enable.c b/fs/verity/enable.c index bbec6f93172cf..fc4c50e5219dc 100644 --- a/fs/verity/enable.c +++ b/fs/verity/enable.c @@ -357,6 +357,13 @@ int fsverity_ioctl_enable(struct file *filp, const void __user *uarg) err = file_permission(filp, MAY_WRITE); if (err) return err; + /* + * __kernel_read() is used while building the Merkle tree. So, we can't + * allow file descriptors that were opened for ioctl access only, using + * the special nonstandard access mode 3. O_RDONLY only, please! + */ + if (!(filp->f_mode & FMODE_READ)) + return -EBADF;

if (IS_APPEND(inode)) return -EPERM;

base-commit: dbd91ed3b5acb7acba2cac2d38e7aec57a5f1e96