fs: Pass AT_GETATTR_NOSEC flag to getattr interface function

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: <[email protected]>
Fixes: db1d1e8b9867 ("IMA: use vfs_getattr_nosec to get the i_version")
Cc: Alexander Viro <[email protected]>
Cc: <[email protected]>
Cc: Miklos Szeredi <[email protected]>
Cc: Amir Goldstein <[email protected]>
Cc: Tyler Hicks <[email protected]>
Cc: Mimi Zohar <[email protected]>
Suggested-by: Christian Brauner <[email protected]>
Co-developed-by: Amir Goldstein <[email protected]>
Signed-off-by: Stefan Berger <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Reviewed-by: Amir Goldstein <[email protected]>
Signed-off-by: Christian Brauner <[email protected]>

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