[PATCH v3 19/38] open: handle idmapped mounts in do_truncate()

[Christian Brauner <christian.brauner@ubuntu.com>]

When truncating files the vfs will verify that the caller is privileged over the
inode. Extend it to handle idmapped mounts. If the inode is accessed through an
idmapped mount it is mapped according to the mount's user namespace. Afterwards
the permissions checks are identical to non-idmapped mounts. If the initial user
namespace is passed nothing changes so non-idmapped mounts will see identical
behavior as before.

Cc: Christoph Hellwig <hch@lst.de>
Cc: David Howells <dhowells@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
---
/* v2 */
unchanged

/* v3 */
unchanged
---
 fs/coredump.c      | 12 +++++++++---
 fs/inode.c         | 13 +++++++++----
 fs/namei.c         |  6 +++---
 fs/open.c          | 21 +++++++++++++--------
 include/linux/fs.h |  4 ++--
 5 files changed, 36 insertions(+), 20 deletions(-)

diff --git a/fs/coredump.c b/fs/coredump.c
index 0cd9056d79cc..25beac7230ff 100644
--- a/fs/coredump.c
+++ b/fs/coredump.c
@@ -703,6 +703,7 @@ void do_coredump(const kernel_siginfo_t *siginfo)
 			goto close_fail;
 		}
 	} else {
+		struct user_namespace *user_ns;
 		struct inode *inode;
 		int open_flags = O_CREAT | O_RDWR | O_NOFOLLOW |
 				 O_LARGEFILE | O_EXCL;
@@ -786,7 +787,8 @@ void do_coredump(const kernel_siginfo_t *siginfo)
 			goto close_fail;
 		if (!(cprm.file->f_mode & FMODE_CAN_WRITE))
 			goto close_fail;
-		if (do_truncate(cprm.file->f_path.dentry, 0, 0, cprm.file))
+		user_ns = mnt_user_ns(cprm.file->f_path.mnt);
+		if (do_truncate(user_ns, cprm.file->f_path.dentry, 0, 0, cprm.file))
 			goto close_fail;
 	}
 
@@ -931,8 +933,12 @@ void dump_truncate(struct coredump_params *cprm)
 
 	if (file->f_op->llseek && file->f_op->llseek != no_llseek) {
 		offset = file->f_op->llseek(file, 0, SEEK_CUR);
-		if (i_size_read(file->f_mapping->host) < offset)
-			do_truncate(file->f_path.dentry, offset, 0, file);
+		if (i_size_read(file->f_mapping->host) < offset) {
+			struct user_namespace *user_ns;
+
+			user_ns = mnt_user_ns(file->f_path.mnt);
+			do_truncate(user_ns, file->f_path.dentry, offset, 0, file);
+		}
 	}
 }
 EXPORT_SYMBOL(dump_truncate);
diff --git a/fs/inode.c b/fs/inode.c
index 75c64f003c45..0ccdd673636d 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -1904,7 +1904,8 @@ int dentry_needs_remove_privs(struct dentry *dentry)
 	return mask;
 }
 
-static int __remove_privs(struct dentry *dentry, int kill)
+static int __remove_privs(struct user_namespace *user_ns, struct dentry *dentry,
+			  int kill)
 {
 	struct iattr newattrs;
 
@@ -1913,7 +1914,7 @@ static int __remove_privs(struct dentry *dentry, int kill)
 	 * Note we call this on write, so notify_change will not
 	 * encounter any conflicting delegations:
 	 */
-	return notify_change(&init_user_ns, dentry, &newattrs, NULL);
+	return notify_change(user_ns, dentry, &newattrs, NULL);
 }
 
 /*
@@ -1939,8 +1940,12 @@ int file_remove_privs(struct file *file)
 	kill = dentry_needs_remove_privs(dentry);
 	if (kill < 0)
 		return kill;
-	if (kill)
-		error = __remove_privs(dentry, kill);
+	if (kill) {
+		struct user_namespace *user_ns;
+
+		user_ns = mnt_user_ns(file->f_path.mnt);
+		error = __remove_privs(user_ns, dentry, kill);
+	}
 	if (!error)
 		inode_has_no_xattr(inode);
 
diff --git a/fs/namei.c b/fs/namei.c
index b91bf923d22c..5601b6680d4c 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2940,9 +2940,9 @@ static int handle_truncate(struct file *filp)
 	if (!error)
 		error = security_path_truncate(path);
 	if (!error) {
-		error = do_truncate(path->dentry, 0,
-				    ATTR_MTIME|ATTR_CTIME|ATTR_OPEN,
-				    filp);
+		error = do_truncate(mnt_user_ns(filp->f_path.mnt),
+				    path->dentry, 0,
+				    ATTR_MTIME | ATTR_CTIME | ATTR_OPEN, filp);
 	}
 	put_write_access(inode);
 	return error;
diff --git a/fs/open.c b/fs/open.c
index 2dc94689a7dc..137dcc52d2f8 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -35,8 +35,8 @@
 
 #include "internal.h"
 
-int do_truncate(struct dentry *dentry, loff_t length, unsigned int time_attrs,
-	struct file *filp)
+int do_truncate(struct user_namespace *user_ns, struct dentry *dentry,
+		loff_t length, unsigned int time_attrs, struct file *filp)
 {
 	int ret;
 	struct iattr newattrs;
@@ -61,13 +61,14 @@ int do_truncate(struct dentry *dentry, loff_t length, unsigned int time_attrs,
 
 	inode_lock(dentry->d_inode);
 	/* Note any delegations or leases have already been broken: */
-	ret = notify_change(&init_user_ns, dentry, &newattrs, NULL);
+	ret = notify_change(user_ns, dentry, &newattrs, NULL);
 	inode_unlock(dentry->d_inode);
 	return ret;
 }
 
 long vfs_truncate(const struct path *path, loff_t length)
 {
+	struct user_namespace *user_ns;
 	struct inode *inode;
 	long error;
 
@@ -83,7 +84,8 @@ long vfs_truncate(const struct path *path, loff_t length)
 	if (error)
 		goto out;
 
-	error = inode_permission(&init_user_ns, inode, MAY_WRITE);
+	user_ns = mnt_user_ns(path->mnt);
+	error = inode_permission(user_ns, inode, MAY_WRITE);
 	if (error)
 		goto mnt_drop_write_and_out;
 
@@ -107,7 +109,7 @@ long vfs_truncate(const struct path *path, loff_t length)
 	if (!error)
 		error = security_path_truncate(path);
 	if (!error)
-		error = do_truncate(path->dentry, length, 0, NULL);
+		error = do_truncate(user_ns, path->dentry, length, 0, NULL);
 
 put_write_and_out:
 	put_write_access(inode);
@@ -186,13 +188,16 @@ long do_sys_ftruncate(unsigned int fd, loff_t length, int small)
 	/* Check IS_APPEND on real upper inode */
 	if (IS_APPEND(file_inode(f.file)))
 		goto out_putf;
-
 	sb_start_write(inode->i_sb);
 	error = locks_verify_truncate(inode, f.file, length);
 	if (!error)
 		error = security_path_truncate(&f.file->f_path);
-	if (!error)
-		error = do_truncate(dentry, length, ATTR_MTIME|ATTR_CTIME, f.file);
+	if (!error) {
+		struct user_namespace *user_ns;
+
+		user_ns = mnt_user_ns(f.file->f_path.mnt);
+		error = do_truncate(user_ns, dentry, length, ATTR_MTIME | ATTR_CTIME, f.file);
+	}
 	sb_end_write(inode->i_sb);
 out_putf:
 	fdput(f);
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 7475d15ed209..78909e4071aa 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2581,8 +2581,8 @@ struct filename {
 static_assert(offsetof(struct filename, iname) % sizeof(long) == 0);
 
 extern long vfs_truncate(const struct path *, loff_t);
-extern int do_truncate(struct dentry *, loff_t start, unsigned int time_attrs,
-		       struct file *filp);
+extern int do_truncate(struct user_namespace *, struct dentry *, loff_t start,
+		       unsigned int time_attrs, struct file *filp);
 extern int vfs_fallocate(struct file *file, int mode, loff_t offset,
 			loff_t len);
 extern long do_sys_open(int dfd, const char __user *filename, int flags,
-- 
2.29.2