| Message ID | 20241004193442.189774-1-ebiggers@kernel.org (mailing list archive) |
|---|---|
| State | Superseded |
| Headers | show |
| Series | [f2fs-dev,6.1] f2fs: Require FMODE_WRITE for atomic write ioctls | expand |
On Fri, Oct 04, 2024 at 07:34:41PM +0000, Eric Biggers wrote: >From: Jann Horn <jannh@google.com> > >commit 4f5a100f87f32cb65d4bb1ad282a08c92f6f591e upstream. > >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> >Reviewed-by: Chao Yu <chao@kernel.org> >Reviewed-by: Eric Biggers <ebiggers@google.com> >Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org> >Signed-off-by: Eric Biggers <ebiggers@google.com> I've queued these backports, thanks!
diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c index 62f2521cd955e..2982cc5b37d78 100644 --- a/fs/f2fs/file.c +++ b/fs/f2fs/file.c @@ -2103,10 +2103,13 @@ static int f2fs_ioc_start_atomic_write(struct file *filp) 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(mnt_userns, inode)) return -EACCES; if (!S_ISREG(inode->i_mode)) return -EINVAL; @@ -2200,10 +2203,13 @@ static int f2fs_ioc_commit_atomic_write(struct file *filp) { struct inode *inode = file_inode(filp); struct user_namespace *mnt_userns = file_mnt_user_ns(filp); int ret; + if (!(filp->f_mode & FMODE_WRITE)) + return -EBADF; + if (!inode_owner_or_capable(mnt_userns, inode)) return -EACCES; ret = mnt_want_write_file(filp); if (ret) @@ -2232,10 +2238,13 @@ static int f2fs_ioc_abort_atomic_write(struct file *filp) { struct inode *inode = file_inode(filp); struct user_namespace *mnt_userns = file_mnt_user_ns(filp); int ret; + if (!(filp->f_mode & FMODE_WRITE)) + return -EBADF; + if (!inode_owner_or_capable(mnt_userns, inode)) return -EACCES; ret = mnt_want_write_file(filp); if (ret)