Re: [PATCH] proc: Check /proc/$pid/attr/ writes against file opener

[Eric W. Biederman]

Jann Horn <jannh@xxxxxxxxxx> writes:

> On Tue, May 25, 2021 at 9:37 PM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>> Fix another "confused deputy" weakness[1]. Writes to /proc/$pid/attr/
>> files need to check the opener credentials, since these fds do not
>> transition state across execve(). Without this, it is possible to
>> trick another process (which may have different credentials) to write
>> to its own /proc/$pid/attr/ files, leading to unexpected and possibly
>> exploitable behaviors.
>>
>> [1] https://www.kernel.org/doc/html/latest/security/credentials.html?highlight=confused#open-file-credentials
>>
>> Fixes: 1da177e4c3f41 ("Linux-2.6.12-rc2")
>> Cc: stable@xxxxxxxxxxxxxxx
>> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
>> ---
>>  fs/proc/base.c | 4 ++++
>>  1 file changed, 4 insertions(+)
>>
>> diff --git a/fs/proc/base.c b/fs/proc/base.c
>> index 3851bfcdba56..58bbf334265b 100644
>> --- a/fs/proc/base.c
>> +++ b/fs/proc/base.c
>> @@ -2703,6 +2703,10 @@ static ssize_t proc_pid_attr_write(struct file * file, const char __user * buf,
>>         void *page;
>>         int rv;
>>
>> +       /* A task may only write when it was the opener. */
>> +       if (file->f_cred != current_real_cred())
>> +               return -EPERM;
>
> With this, if a task forks, the child will then still be able to open
> its parent's /proc/$pid/attr/current and trick the parent into writing
> to that, right? Is that acceptable? If not, the ->open handler should
> probably also check for "current->thread_pid == proc_pid(inode)", or
> something like that?

Currently exec always allocates a new cred.  So you can only ``trick''
another process that was forked from you.  I don't think it counts as
tricking or any kind of danger if you are simply confusing yourself.

Eric