Re: [PATCH] fs: Pass AT_GETATTR_NOSEC flag to getattr interface function

From: Amir Goldstein
Date: Mon Oct 02 2023 - 09:22:43 EST

On Mon, Oct 2, 2023 at 3:57 PM Stefan Berger <stefanb@xxxxxxxxxxxxxxxxxx> wrote:
>
> From: Stefan Berger <stefanb@xxxxxxxxxxxxx>
>
> When vfs_getattr_nosec() calls a filesystem's getattr interface function
> then the 'nosec' should propagate into this function so that
> vfs_getattr_nosec() can again be called from the filesystem's gettattr
> rather than vfs_getattr(). The latter would add unnecessary security
> checks that the initial vfs_getattr_nosec() call wanted to avoid.
> Therefore, introduce the getattr flag GETATTR_NOSEC and allow to pass
> with the new getattr_flags parameter to the getattr interface function.
> In overlayfs and ecryptfs use this flag to determine which one of the
> two functions to call.
>
> In a recent code change introduced to IMA vfs_getattr_nosec() ended up
> calling vfs_getattr() in overlayfs, which in turn called
> security_inode_getattr() on an exiting process that did not have
> current->fs set anymore, which then caused a kernel NULL pointer
> dereference. With this change the call to security_inode_getattr() can
> be avoided, thus avoiding the NULL pointer dereference.
>
> Reported-by: syzbot+a67fc5321ffb4b311c98@xxxxxxxxxxxxxxxxxxxxxxxxx
> Fixes: db1d1e8b9867 ("IMA: use vfs_getattr_nosec to get the i_version")
> Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
> Cc: linux-fsdevel@xxxxxxxxxxxxxxx
> Cc: Miklos Szeredi <miklos@xxxxxxxxxx>
> Cc: Amir Goldstein <amir73il@xxxxxxxxx>
> Cc: Tyler Hicks <code@xxxxxxxxxxx>
> Cc: Mimi Zohar <zohar@xxxxxxxxxxxxx>
> Suggested-by: Christian Brauner <brauner@xxxxxxxxxx>
> Co-developed-by: Amir Goldstein <amir73il@xxxxxxxxx>
> Signed-off-by: Stefan Berger <stefanb@xxxxxxxxxxxxx>
> ---

Reviewed-by: Amir Goldstein <amir73il@xxxxxxxxx>

Now let's see what vfs maintainers think about this...

Thanks,
Amir.

> fs/ecryptfs/inode.c | 12 ++++++++++--
> fs/overlayfs/inode.c | 10 +++++-----
> fs/overlayfs/overlayfs.h | 8 ++++++++
> fs/stat.c | 6 +++++-
> include/uapi/linux/fcntl.h | 3 +++
> 5 files changed, 31 insertions(+), 8 deletions(-)
>
> diff --git a/fs/ecryptfs/inode.c b/fs/ecryptfs/inode.c
> index 992d9c7e64ae..5ab4b87888a7 100644
> --- a/fs/ecryptfs/inode.c
> +++ b/fs/ecryptfs/inode.c
> @@ -998,6 +998,14 @@ static int ecryptfs_getattr_link(struct mnt_idmap *idmap,
> return rc;
> }
>
> +static int ecryptfs_do_getattr(const struct path *path, struct kstat *stat,
> + u32 request_mask, unsigned int flags)
> +{
> + if (flags & AT_GETATTR_NOSEC)
> + return vfs_getattr_nosec(path, stat, request_mask, flags);
> + return vfs_getattr(path, stat, request_mask, flags);
> +}
> +
> static int ecryptfs_getattr(struct mnt_idmap *idmap,
> const struct path *path, struct kstat *stat,
> u32 request_mask, unsigned int flags)
> @@ -1006,8 +1014,8 @@ static int ecryptfs_getattr(struct mnt_idmap *idmap,
> struct kstat lower_stat;
> int rc;
>
> - rc = vfs_getattr(ecryptfs_dentry_to_lower_path(dentry), &lower_stat,
> - request_mask, flags);
> + rc = ecryptfs_do_getattr(ecryptfs_dentry_to_lower_path(dentry),
> + &lower_stat, request_mask, flags);
> if (!rc) {
> fsstack_copy_attr_all(d_inode(dentry),
> ecryptfs_inode_to_lower(d_inode(dentry)));
> diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
> index 83ef66644c21..fca29dba7b14 100644
> --- a/fs/overlayfs/inode.c
> +++ b/fs/overlayfs/inode.c
> @@ -171,7 +171,7 @@ int ovl_getattr(struct mnt_idmap *idmap, const struct path *path,
>
> type = ovl_path_real(dentry, &realpath);
> old_cred = ovl_override_creds(dentry->d_sb);
> - err = vfs_getattr(&realpath, stat, request_mask, flags);
> + err = ovl_do_getattr(&realpath, stat, request_mask, flags);
> if (err)
> goto out;
>
> @@ -196,8 +196,8 @@ int ovl_getattr(struct mnt_idmap *idmap, const struct path *path,
> (!is_dir ? STATX_NLINK : 0);
>
> ovl_path_lower(dentry, &realpath);
> - err = vfs_getattr(&realpath, &lowerstat,
> - lowermask, flags);
> + err = ovl_do_getattr(&realpath, &lowerstat, lowermask,
> + flags);
> if (err)
> goto out;
>
> @@ -249,8 +249,8 @@ int ovl_getattr(struct mnt_idmap *idmap, const struct path *path,
>
> ovl_path_lowerdata(dentry, &realpath);
> if (realpath.dentry) {
> - err = vfs_getattr(&realpath, &lowerdatastat,
> - lowermask, flags);
> + err = ovl_do_getattr(&realpath, &lowerdatastat,
> + lowermask, flags);
> if (err)
> goto out;
> } else {
> diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h
> index 9817b2dcb132..09ca82ed0f8c 100644
> --- a/fs/overlayfs/overlayfs.h
> +++ b/fs/overlayfs/overlayfs.h
> @@ -397,6 +397,14 @@ static inline bool ovl_open_flags_need_copy_up(int flags)
> return ((OPEN_FMODE(flags) & FMODE_WRITE) || (flags & O_TRUNC));
> }
>
> +static inline int ovl_do_getattr(const struct path *path, struct kstat *stat,
> + u32 request_mask, unsigned int flags)
> +{
> + if (flags & AT_GETATTR_NOSEC)
> + return vfs_getattr_nosec(path, stat, request_mask, flags);
> + return vfs_getattr(path, stat, request_mask, flags);
> +}
> +
> /* util.c */
> int ovl_want_write(struct dentry *dentry);
> void ovl_drop_write(struct dentry *dentry);
> diff --git a/fs/stat.c b/fs/stat.c
> index d43a5cc1bfa4..5375be5f97cc 100644
> --- a/fs/stat.c
> +++ b/fs/stat.c
> @@ -133,7 +133,8 @@ int vfs_getattr_nosec(const struct path *path, struct kstat *stat,
> idmap = mnt_idmap(path->mnt);
> if (inode->i_op->getattr)
> return inode->i_op->getattr(idmap, path, stat,
> - request_mask, query_flags);
> + request_mask,
> + query_flags | AT_GETATTR_NOSEC);
>
> generic_fillattr(idmap, request_mask, inode, stat);
> return 0;
> @@ -166,6 +167,9 @@ int vfs_getattr(const struct path *path, struct kstat *stat,
> {
> int retval;
>
> + if (WARN_ON_ONCE(query_flags & AT_GETATTR_NOSEC))
> + return -EPERM;
> +
> retval = security_inode_getattr(path);
> if (retval)
> return retval;
> diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h
> index 6c80f96049bd..282e90aeb163 100644
> --- a/include/uapi/linux/fcntl.h
> +++ b/include/uapi/linux/fcntl.h
> @@ -116,5 +116,8 @@
> #define AT_HANDLE_FID AT_REMOVEDIR /* file handle is needed to
> compare object identity and may not
> be usable to open_by_handle_at(2) */
> +#if defined(__KERNEL__)
> +#define AT_GETATTR_NOSEC 0x80000000
> +#endif
>
> #endif /* _UAPI_LINUX_FCNTL_H */
> --
> 2.40.1
>