linux-kernel - [PATCH] fs: Make /proc/sys inodes be owned by global root.

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Message-Id: <20181126172607.125782-1-rburny@google.com>
Date:   Mon, 26 Nov 2018 18:26:07 +0100
From:   Radoslaw Burny <rburny@...gle.com>
To:     "Luis R. Rodriguez" <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Seth Forshee <seth.forshee@...onical.com>
Cc:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        jsperbeck@...gle.com, Radoslaw Burny <rburny@...gle.com>
Subject: [PATCH] fs: Make /proc/sys inodes be owned by global root.

Due to a recent commit (d151ddc00498 - fs: Update i_[ug]id_(read|write)
to translate relative to s_user_ns), inodes under /proc/sys have -1
written to their i_uid/i_gid members if a containing userns does not
have entries for root in the uid/gid_map.

This wouldn't normally matter, because these values are not used for
access checks. However, a later change (0bd23d09b874 - Don't modify
inodes with a uid or gid unknown to the vfs) changes the kernel to
prevent opens for write if the i_uid/i_gid field in the inode is -1,
even if the /proc/sys-specific access checks would otherwise pass.

This causes a problem: in a userns without root mapping, even the
namespace creator cannot write to e.g. /proc/sys/kernel/shmmax.
This change fixes the problem by overriding i_uid/i_gid back to
GLOBAL_ROOT_UID/GID.

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 uid/gid of 65534, with the change it's 0.

Signed-off-by: Radoslaw Burny <rburny@...gle.com>
---
 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 c5cbbdff3c3d..67379a389658 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;
+	}

 out:
 	return inode;
-- 
2.20.0.rc0.387.gc7a69e6b6c-goog