Re: Should unprivileged linkat(..., AT_EMPTY_PATH) work on O_TMPFILE files?

[Andy Lutomirski]

On Sun, Aug 11, 2013 at 9:25 PM, Aneesh Kumar K.V
<aneesh.kumar@xxxxxxxxxxxxxxxxxx> wrote:
> "Aneesh Kumar K.V" <aneesh.kumar@xxxxxxxxxxxxxxxxxx> writes:
>
>> Andy Lutomirski <luto@xxxxxxxxxxxxxx> writes:
>>
>>> On Sun, Aug 11, 2013 at 9:45 AM, Aneesh Kumar K.V
>>> <aneesh.kumar@xxxxxxxxxxxxxxxxxx> wrote:
>>>> Andy Lutomirski <luto@xxxxxxxxxxxxxx> writes:
>>>>
>>>>> The change:
>>>>>
>>>>> commit f4e0c30c191f87851c4a53454abb55ee276f4a7e
>>>>> Author: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
>>>>> Date:   Tue Jun 11 08:34:36 2013 +0400
>>>>>
>>>>>     allow the temp files created by open() to be linked to
>>>>>
>>>>>     O_TMPFILE | O_CREAT => linkat() with AT_SYMLINK_FOLLOW and /proc/self/fd/<n>
>>>>>     as oldpath (i.e. flink()) will create a link
>>>>>     O_TMPFILE | O_CREAT | O_EXCL => ENOENT on attempt to link those guys
>>>>>
>>>>>     Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
>>>>>
>>>>> makes it possible to hardlink an O_TMPFILE file using procfs.  Should
>>>>> linkat(fd, "", newdirfd, newpath, AT_EMPTY_PATH) also work?
>>>>>
>>>>> AFAICS it currently requires CAP_DAC_READ_SEARCH, but I'm a bit
>>>>> confused as to why linkat(..., AT_EMPTY_PATH) should have a stricter
>>>>> check than linkat(AT_FDCWD, "/proc/self/fd/n", ...,
>>>>> AT_SYMLINK_FOLLOW),  (The relevant change is
>>>>> 11a7b371b64ef39fc5fb1b6f2218eef7c4d035e3.)
>>>>>
>>>>> FWIW, this program works on Linux 3.9, which makes me doubt that the
>>>>> security restriction on linkat is doing any good:
>>>>>
>>>>> #include <stdio.h>
>>>>> #include <err.h>
>>>>> #include <fcntl.h>
>>>>> #include <unistd.h>
>>>>>
>>>>> int main(int argc, char **argv)
>>>>> {
>>>>>   char buf[128];
>>>>>
>>>>>   if (argc != 3)
>>>>>     errx(1, "Usage: flink FD PATH");
>>>>>
>>>>>   sprintf(buf, "/proc/self/fd/%d", atoi(argv[1]));
>>>>>   if (linkat(AT_FDCWD, buf, AT_FDCWD, argv[2], AT_SYMLINK_FOLLOW) != 0)
>>>>>     err(1, "linkat");
>>>>>   return 0;
>>>>> }
>>>>>
>>>>>
>>>>> Removing the check from the AT_EMPTY_PATH case would simplify code
>>>>> that wants to write a file, fsync it, and then flink it in.
>>>>
>>>> I understand that this got merged upstream. But in case of the above
>>>> test we would walk the path pointed by /proc/self/fd/<x> right ?
>>>>
>>>> ie,
>>>>
>>>> 20 -> /home/no-access/k
>>>>
>>>> will the above test work ? Now if i pass the '20' to another application
>>>> I can affectively create a hardlink to that outside no-access and if k
>>>> happens to have 'r' for others, then everybody will be able to read
>>>> right ?. I understand that limitting the read access based on directory
>>>> permission is not a good idea. But aren't we expected to keep that ?
>>>
>>> The symlinks in /proc/self/fd are rather magical and don't actually
>>> walk the path.  Give it a try :)
>>>
>>
>> How about fd passed from one application to another(say from a1 to
>> a2). a2 won't have read permission on /proc/a1/fd/ and also don't know
>> the value of file descriptor he should use right ? Will the /proc/self/fd
>> method work in such case ? IIUC with AT_EMPTY_PATH a2 can create the
>> link in the above case right ? So if /proc/self/fd doesn't work should
>> we allow that ?
>
> Hmm I guess a2 will be able to use /proc/a2/fd/<received_fd> to create a
> link ?

Yes.

-- 
Andy Lutomirski
AMA Capital Management, LLC
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