linux-kernel - Re: [PATCH] ksys_mount: check for permissions before resource allocation

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Date:   Tue, 05 Jun 2018 14:56:31 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Ilya Matveychikov <matvejchikov@...il.com>
Cc:     linux-kernel@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] ksys_mount: check for permissions before resource allocation

Ilya Matveychikov <matvejchikov@...il.com> writes:

> Just CC’ed to some of maintainers.
>
> $ perl scripts/get_maintainer.pl fs/0001-ksys_mount-check-for-permissions-before-resource-all.patch
> Alexander Viro <viro@...iv.linux.org.uk> (maintainer:FILESYSTEMS (VFS and infrastructure))
> linux-fsdevel@...r.kernel.org (open list:FILESYSTEMS (VFS and infrastructure))
> linux-kernel@...r.kernel.org (open list)
>
>> On Jun 5, 2018, at 6:00 AM, Ilya Matveychikov <matvejchikov@...il.com> wrote:
>> 
>> Early check for mount permissions prevents possible allocation of 3
>> pages from kmalloc() pool by unpriveledged user which can be used for
>> spraying the kernel heap.

*Snort*

You clearly have not read may_mount.  Your modified code still
let's unprivileged users in.  So even if all of Al's good objections
were not applicable this change would still be buggy and wrong.

Nacked-by: "Eric W. Biederman" <ebiederm@...ssion.com>

>> Signed-off-by: Ilya V. Matveychikov <matvejchikov@...il.com>
>> ---
>> fs/namespace.c | 3 +++
>> 1 file changed, 3 insertions(+)
>> 
>> diff --git a/fs/namespace.c b/fs/namespace.c
>> index 5f75969adff1..1ef8feb2de2a 100644
>> --- a/fs/namespace.c
>> +++ b/fs/namespace.c
>> @@ -3046,6 +3046,9 @@ int ksys_mount(char __user *dev_name, char __user *dir_name, char __user *type,
>> 	char *kernel_dev;
>> 	void *options;
>> 
>> +	if (!may_mount())
>> +		return -EPERM;
>> +
>> 	kernel_type = copy_mount_string(type);
>> 	ret = PTR_ERR(kernel_type);
>> 	if (IS_ERR(kernel_type))
>> --
>> 2.17.0
>>