security: pass down mount idmapping to setattr hook

Before this change we used to take a shortcut and place the actual
values that would be written to inode->i_{g,u}id into struct iattr. This
had the advantage that we moved idmappings mostly out of the picture
early on but it made reasoning about changes more difficult than it
should be.

The filesystem was never explicitly told that it dealt with an idmapped
mount. The transition to the value that needed to be stored in
inode->i_{g,u}id appeared way too early and increased the probability of
bugs in various codepaths.

We know place the same value in struct iattr no matter if this is an
idmapped mount or not. The vfs will only deal with type safe
vfs{g,u}id_t. This makes it massively safer to perform permission checks
as the type will tell us what checks we need to perform and what helpers
we need to use.

Adapt the security_inode_setattr() helper to pass down the mount's
idmapping to account for that change.

Link: https://lore.kernel.org/r/20220621141454.2914719-8-brauner@kernel.org
Cc: Seth Forshee <sforshee@digitalocean.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Aleksa Sarai <cyphar@cyphar.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
CC: linux-fsdevel@vger.kernel.org
Reviewed-by: Seth Forshee <sforshee@digitalocean.com>
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>

1 files changed, 5 insertions, 3 deletions

diff --git a/include/linux/security.h b/include/linux/security.h
index 7fc4e9f49f54..4d0baf30266e 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -353,7 +353,8 @@ int security_inode_readlink(struct dentry *dentry);
 int security_inode_follow_link(struct dentry *dentry, struct inode *inode,
 			       bool rcu);
 int security_inode_permission(struct inode *inode, int mask);
-int security_inode_setattr(struct dentry *dentry, struct iattr *attr);
+int security_inode_setattr(struct user_namespace *mnt_userns,
+			   struct dentry *dentry, struct iattr *attr);
 int security_inode_getattr(const struct path *path);
 int security_inode_setxattr(struct user_namespace *mnt_userns,
 			    struct dentry *dentry, const char *name,
@@ -848,8 +849,9 @@ static inline int security_inode_permission(struct inode *inode, int mask)
 	return 0;
 }
 
-static inline int security_inode_setattr(struct dentry *dentry,
-					  struct iattr *attr)
+static inline int security_inode_setattr(struct user_namespace *mnt_userns,
+					 struct dentry *dentry,
+					 struct iattr *attr)
 {
 	return 0;
 }