open: handle idmapped mounts in do_truncate()

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.

Link: https://lore.kernel.org/r/20210121131959.646623-16-christian.brauner@ubuntu.com
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
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

1 files changed, 7 insertions, 3 deletions

diff --git a/fs/coredump.c b/fs/coredump.c
index a2f6ecc8e345..ae778937a1ff 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 *mnt_userns;
 		struct inode *inode;
 		int open_flags = O_CREAT | O_RDWR | O_NOFOLLOW |
 				 O_LARGEFILE | O_EXCL;
@@ -780,13 +781,15 @@ void do_coredump(const kernel_siginfo_t *siginfo)
 		 * a process dumps core while its cwd is e.g. on a vfat
 		 * filesystem.
 		 */
-		if (!uid_eq(inode->i_uid, current_fsuid()))
+		mnt_userns = file_mnt_user_ns(cprm.file);
+		if (!uid_eq(i_uid_into_mnt(mnt_userns, inode), current_fsuid()))
 			goto close_fail;
 		if ((inode->i_mode & 0677) != 0600)
 			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))
+		if (do_truncate(mnt_userns, cprm.file->f_path.dentry,
+				0, 0, cprm.file))
 			goto close_fail;
 	}
 
@@ -931,7 +934,8 @@ 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);
+			do_truncate(file_mnt_user_ns(file), file->f_path.dentry,
+				    offset, 0, file);
 	}
 }
 EXPORT_SYMBOL(dump_truncate);