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

Stefan Berger <stefanb@xxxxxxxxxxxxx> · Mon, 2 Oct 2023 09:52:57 -0400

On 10/2/23 09:22, Amir Goldstein wrote:
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.

For the other reviewers, the original syzbot message with the 'final 
oops' is here:

https://lore.kernel.org/linux-integrity/bed99e92-cb7c-868d-94f3-ddf53e2b262a@xxxxxxxxxxxxx/T/#m616371f8ac1a316be13865e1699ac59ccf8ce6ef

Regards,

   Stefan



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







