[PATCH v2] fs: Fix the default values of i_uid/i_gid on /proc/sys inodes.

[Radoslaw Burny <rburny@google.com>]

This also fixes a problem where, in a user namespace without root user
mapping, it is not possible to write to /proc/sys/kernel/shmmax.

The problem was introduced by the combination of the two commits:
* 81754357770ebd900801231e7bc8d151ddc00498: fs: Update
  i_[ug]id_(read|write) to translate relative to s_user_ns
    - this caused the kernel to write INVALID_[UG]ID to i_uid/i_gid
    members of /proc/sys inodes if a containing userns does not have
    entries for root in the uid/gid_map.
* 0bd23d09b874e53bd1a2fe2296030aa2720d7b08: vfs: Don't modify inodes
  with a uid or gid unknown to the vfs
    - changed the kernel to prevent opens for write if the i_uid/i_gid
    field in the inode is invalid

This commit fixes the issue by defaulting i_uid/i_gid to
GLOBAL_ROOT_UID/GID. Note that these values are not used for /proc/sys
access checks, so the change does not otherwise affect /proc semantics.

Tested: Used a repro program that creates a user namespace without any
mapping and stat'ed /proc/$PID/root/proc/sys/kernel/shmmax from outside.
Before the change, it shows the overflow uid, with the change it's 0.

Signed-off-by: Radoslaw Burny <rburny@google.com>
---
Changelog since v1:
- Updated the commit title and description.

 fs/proc/proc_sysctl.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
index c74570736b24..36ad1b0d6259 100644
--- a/fs/proc/proc_sysctl.c
+++ b/fs/proc/proc_sysctl.c
@@ -499,6 +499,10 @@ static struct inode *proc_sys_make_inode(struct super_block *sb,
 
 	if (root->set_ownership)
 		root->set_ownership(head, table, &inode->i_uid, &inode->i_gid);
+	else {
+		inode->i_uid = GLOBAL_ROOT_UID;
+		inode->i_gid = GLOBAL_ROOT_GID;
+	}
 
 	return inode;
 }
-- 
2.22.0.410.gd8fdbe21b5-goog