The F2FS ioctls for starting and committing atomic writes check for inode_owner_or_capable(), but this does not give LSMs like SELinux or Landlock an opportunity to deny the write access - if the caller's FSUID matches the inode's UID, inode_owner_or_capable() immediately returns true.

There are scenarios where LSMs want to deny a process the ability to write particular files, even files that the FSUID of the process owns; but this can currently partially be bypassed using atomic write ioctls in two ways:

- F2FS_IOC_START_ATOMIC_REPLACE + F2FS_IOC_COMMIT_ATOMIC_WRITE can truncate an inode to size 0 - F2FS_IOC_START_ATOMIC_WRITE + F2FS_IOC_ABORT_ATOMIC_WRITE can revert changes another process concurrently made to a file

Fix it by requiring FMODE_WRITE for these operations, just like for F2FS_IOC_MOVE_RANGE. Since any legitimate caller should only be using these ioctls when intending to write into the file, that seems unlikely to break anything.

Fixes: 88b88a667971 ("f2fs: support atomic writes") Cc: stable@vger.kernel.org Signed-off-by: Jann Horn jannh@google.com --- fs/f2fs/file.c | 9 +++++++++ 1 file changed, 9 insertions(+)

diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c index 168f08507004..a662392c5d8b 100644 --- a/fs/f2fs/file.c +++ b/fs/f2fs/file.c @@ -2117,12 +2117,15 @@ static int f2fs_ioc_start_atomic_write(struct file *filp, bool truncate) struct f2fs_inode_info *fi = F2FS_I(inode); struct f2fs_sb_info *sbi = F2FS_I_SB(inode); struct inode *pinode; loff_t isize; int ret;

+ if (!(filp->f_mode & FMODE_WRITE)) + return -EBADF; + if (!inode_owner_or_capable(idmap, inode)) return -EACCES;

if (!S_ISREG(inode->i_mode)) return -EINVAL;

@@ -2225,12 +2228,15 @@ static int f2fs_ioc_start_atomic_write(struct file *filp, bool truncate) static int f2fs_ioc_commit_atomic_write(struct file *filp) { struct inode *inode = file_inode(filp); struct mnt_idmap *idmap = file_mnt_idmap(filp); int ret;

+ if (!(filp->f_mode & FMODE_WRITE)) + return -EBADF; + if (!inode_owner_or_capable(idmap, inode)) return -EACCES;

ret = mnt_want_write_file(filp); if (ret) return ret; @@ -2257,12 +2263,15 @@ static int f2fs_ioc_commit_atomic_write(struct file *filp) static int f2fs_ioc_abort_atomic_write(struct file *filp) { struct inode *inode = file_inode(filp); struct mnt_idmap *idmap = file_mnt_idmap(filp); int ret;

+ if (!(filp->f_mode & FMODE_WRITE)) + return -EBADF; + if (!inode_owner_or_capable(idmap, inode)) return -EACCES;

ret = mnt_want_write_file(filp); if (ret) return ret;

--- base-commit: b446a2dae984fa5bd56dd7c3a02a426f87e05813 change-id: 20240806-f2fs-atomic-write-e019a47823de