linux-kernel - [PATCH 5.12 017/292] cgroup: verify that source is a string

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Message-Id: <20210719144943.092843909@linuxfoundation.org>
Date:   Mon, 19 Jul 2021 16:51:19 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        syzbot+283ce5a46486d6acdbaf@...kaller.appspotmail.com,
        Christoph Hellwig <hch@....de>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Dmitry Vyukov <dvyukov@...gle.com>, stable@...nel.org,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH 5.12 017/292] cgroup: verify that source is a string

From: Christian Brauner <christian.brauner@...ntu.com>

commit 3b0462726e7ef281c35a7a4ae33e93ee2bc9975b upstream.

The following sequence can be used to trigger a UAF:

    int fscontext_fd = fsopen("cgroup");
    int fd_null = open("/dev/null, O_RDONLY);
    int fsconfig(fscontext_fd, FSCONFIG_SET_FD, "source", fd_null);
    close_range(3, ~0U, 0);

The cgroup v1 specific fs parser expects a string for the "source"
parameter.  However, it is perfectly legitimate to e.g.  specify a file
descriptor for the "source" parameter.  The fs parser doesn't know what
a filesystem allows there.  So it's a bug to assume that "source" is
always of type fs_value_is_string when it can reasonably also be
fs_value_is_file.

This assumption in the cgroup code causes a UAF because struct
fs_parameter uses a union for the actual value.  Access to that union is
guarded by the param->type member.  Since the cgroup paramter parser
didn't check param->type but unconditionally moved param->string into
fc->source a close on the fscontext_fd would trigger a UAF during
put_fs_context() which frees fc->source thereby freeing the file stashed
in param->file causing a UAF during a close of the fd_null.

Fix this by verifying that param->type is actually a string and report
an error if not.

In follow up patches I'll add a new generic helper that can be used here
and by other filesystems instead of this error-prone copy-pasta fix.
But fixing it in here first makes backporting a it to stable a lot
easier.

Fixes: 8d2451f4994f ("cgroup1: switch to option-by-option parsing")
Reported-by: syzbot+283ce5a46486d6acdbaf@...kaller.appspotmail.com
Cc: Christoph Hellwig <hch@....de>
Cc: Alexander Viro <viro@...iv.linux.org.uk>
Cc: Dmitry Vyukov <dvyukov@...gle.com>
Cc: <stable@...nel.org>
Cc: syzkaller-bugs <syzkaller-bugs@...glegroups.com>
Signed-off-by: Christian Brauner <christian.brauner@...ntu.com>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 kernel/cgroup/cgroup-v1.c |    2 ++
 1 file changed, 2 insertions(+)

--- a/kernel/cgroup/cgroup-v1.c
+++ b/kernel/cgroup/cgroup-v1.c
@@ -912,6 +912,8 @@ int cgroup1_parse_param(struct fs_contex
 	opt = fs_parse(fc, cgroup1_fs_parameters, param, &result);
 	if (opt == -ENOPARAM) {
 		if (strcmp(param->key, "source") == 0) {
+			if (param->type != fs_value_is_string)
+				return invalf(fc, "Non-string source");
 			if (fc->source)
 				return invalf(fc, "Multiple sources not supported");
 			fc->source = param->string;