From: Christian Brauner <brauner@xxxxxxxxxx> NFS server is exporting a sticky directory (mode 01777) with root squashing enabled. Client has protect_regular enabled and then tries to open a file as root in that directory. File is created (with ownership set to nobody:nobody) but the open syscall returns an error. The problem is may_create_in_sticky which rejects the open even though the file has already been created. Add a new condition to may_create_in_sticky. If the file was just created, then allow bypassing the ownership check if the task has CAP_FOWNER. With this change, the initial open of a file by root works, but later opens of the same file will fail. Note that we can contrive a similar situation by exporting with all_squash and opening the file as an unprivileged user. This patch does not fix that case. I suspect that that configuration is likely to be fundamentally incompatible with the protect_* sysctls enabled on the clients. Link: https://bugzilla.redhat.com/show_bug.cgi?id=1976829 Reported-by: Yongchen Yang <yoyang@xxxxxxxxxx> Suggested-by: Christian Brauner <brauner@xxxxxxxxxx> Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx> --- fs/namei.c | 9 ++++++--- 1 file changed, 6 insertions(+), 3 deletions(-) Hi Christian, I left you as author here since this is basically identical to the patch you suggested. Let me know if that's an issue. -- Jeff diff --git a/fs/namei.c b/fs/namei.c index 1f28d3f463c3..26b602d1152b 100644 --- a/fs/namei.c +++ b/fs/namei.c @@ -1221,7 +1221,8 @@ int may_linkat(struct user_namespace *mnt_userns, struct path *link) * Returns 0 if the open is allowed, -ve on error. */ static int may_create_in_sticky(struct user_namespace *mnt_userns, - struct nameidata *nd, struct inode *const inode) + struct nameidata *nd, struct inode *const inode, + bool created) { umode_t dir_mode = nd->dir_mode; kuid_t dir_uid = nd->dir_uid; @@ -1230,7 +1231,8 @@ static int may_create_in_sticky(struct user_namespace *mnt_userns, (!sysctl_protected_regular && S_ISREG(inode->i_mode)) || likely(!(dir_mode & S_ISVTX)) || uid_eq(i_uid_into_mnt(mnt_userns, inode), dir_uid) || - uid_eq(current_fsuid(), i_uid_into_mnt(mnt_userns, inode))) + uid_eq(current_fsuid(), i_uid_into_mnt(mnt_userns, inode)) || + (created && inode_owner_or_capable(mnt_userns, inode))) return 0; if (likely(dir_mode & 0002) || @@ -3496,7 +3498,8 @@ static int do_open(struct nameidata *nd, if (d_is_dir(nd->path.dentry)) return -EISDIR; error = may_create_in_sticky(mnt_userns, nd, - d_backing_inode(nd->path.dentry)); + d_backing_inode(nd->path.dentry), + (file->f_mode & FMODE_CREATED)); if (unlikely(error)) return error; } -- 2.37.1
