file system permissions regression affecting root

[Patrick Donnelly]

In newer kernels (at least 5.6), it appears root is not able to write
to files owned by other users in a sticky directory:

$ uname -r
5.6.11-arch1-1
$ stat -f /tmp
  File: "/tmp"
    ID: 0        Namelen: 255     Type: tmpfs
Block size: 4096       Fundamental block size: 4096
Blocks: Total: 2005160    Free: 2005092    Available: 2005092
Inodes: Total: 2005160    Free: 2005112
$ stat /tmp
  File: /tmp
  Size: 440             Blocks: 0          IO Block: 4096   directory
Device: 2fh/47d Inode: 21533       Links: 20
Access: (1777/drwxrwxrwt)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2020-05-11 11:50:52.780667565 -0700
Modify: 2020-05-13 07:40:19.617941285 -0700
Change: 2020-05-13 07:40:19.617941285 -0700
 Birth: -
$ touch /tmp/foo
$ stat /tmp/foo
  File: /tmp/foo
  Size: 0               Blocks: 0          IO Block: 4096   regular empty file
Device: 2fh/47d Inode: 3441684     Links: 1
Access: (0640/-rw-r-----)  Uid: ( 1000/pdonnell)   Gid: (  985/   users)
Access: 2020-05-13 07:40:29.218026785 -0700
Modify: 2020-05-13 07:40:29.218026785 -0700
Change: 2020-05-13 07:40:29.218026785 -0700
 Birth: -
$ sudo /bin/sh -c 'echo 1 > /tmp/foo'
/bin/sh: /tmp/foo: Permission denied
$ sudo strace -f -- /bin/sh -c 'echo 1 > /tmp/foo' |& grep foo
execve("/bin/sh", ["/bin/sh", "-c", "echo 1 > /tmp/foo"],
0x7fff92dec300 /* 15 vars */) = 0
openat(AT_FDCWD, "/tmp/foo", O_WRONLY|O_CREAT|O_TRUNC, 0666) = -1
EACCES (Permission denied)
write(2, "/bin/sh: /tmp/foo: Permission de"..., 37/bin/sh: /tmp/foo:
Permission denied


Compare to Linux 4.18:

$ uname -r
4.18.0-147.3.1.el8_1.x86_64
$ stat /dev/shm
  File: /dev/shm
  Size: 100             Blocks: 0          IO Block: 4096   directory
Device: 16h/22d Inode: 15466       Links: 2
Access: (1777/drwxrwxrwt)  Uid: (    0/    root)   Gid: (    0/    root)
Context: system_u:object_r:tmpfs_t:s0
Access: 2020-05-12 17:37:07.029131257 +0000
Modify: 2020-05-13 14:35:44.161036943 +0000
Change: 2020-05-13 14:35:44.161036943 +0000
 Birth: -
$ stat -f /dev/shm
  File: "/dev/shm"
    ID: 0        Namelen: 255     Type: tmpfs
Block size: 4096       Fundamental block size: 4096
Blocks: Total: 9243647    Free: 9243604    Available: 9243604
Inodes: Total: 9243647    Free: 9243643
$ touch /dev/shm/foo
$ stat /dev/shm/foo
  File: /dev/shm/foo
  Size: 0               Blocks: 0          IO Block: 4096   regular empty file
Device: 16h/22d Inode: 5616861     Links: 1
Access: (0640/-rw-r-----)  Uid: ( 1156/pdonnell)   Gid: ( 1156/pdonnell)
Context: unconfined_u:object_r:user_tmp_t:s0
Access: 2020-05-13 14:44:55.121908033 +0000
Modify: 2020-05-13 14:44:55.121908033 +0000
Change: 2020-05-13 14:44:55.121908033 +0000
 Birth: -
$ sudo -- /bin/sh -c 'echo 1 > /dev/shm/foo'
$

This seems to be related to the directory being owned by root; it does
not happen when the directory is owned by the user:

$ uname -r
5.6.11-arch1-1
$ sudo chown pdonnell:users gtmp
$ stat gtmp
  File: gtmp
  Size: 4096            Blocks: 8          IO Block: 4096   directory
Device: fe04h/65028d    Inode: 2819062     Links: 2
Access: (1777/drwxrwxrwt)  Uid: ( 1000/pdonnell)   Gid: (  985/   users)
Access: 2020-05-13 07:47:06.344892575 -0700
Modify: 2020-05-13 07:50:24.709987998 -0700
Change: 2020-05-13 07:52:52.137963637 -0700
 Birth: 2020-05-13 07:34:09.937974845 -0700
$ stat -f gtmp
  File: "gtmp"
    ID: f24a3528a175df48 Namelen: 255     Type: ext2/ext3
Block size: 4096       Fundamental block size: 4096
Blocks: Total: 36495867   Free: 8991668    Available: 7120360
Inodes: Total: 9338880    Free: 8147228
$ touch gtmp/foo
$ stat gtmp/foo
  File: gtmp/foo
  Size: 0               Blocks: 0          IO Block: 4096   regular empty file
Device: fe04h/65028d    Inode: 2754033     Links: 1
Access: (0640/-rw-r-----)  Uid: ( 1000/pdonnell)   Gid: (  985/   users)
Access: 2020-05-13 07:53:10.218124141 -0700
Modify: 2020-05-13 07:53:10.218124141 -0700
Change: 2020-05-13 07:53:10.218124141 -0700
 Birth: 2020-05-13 07:53:10.218124141 -0700
$ sudo strace -f -- /bin/sh -c 'echo 1 > gtmp/foo' |& grep foo
execve("/bin/sh", ["/bin/sh", "-c", "echo 1 > gtmp/foo"],
0x7ffe03362430 /* 15 vars */) = 0
openat(AT_FDCWD, "gtmp/foo", O_WRONLY|O_CREAT|O_TRUNC, 0666) = 3
$ rm gtmp/foo
$ sudo chown root:root gtmp
$ touch gtmp/foo
# stat gtmp/foo
  File: gtmp/foo
  Size: 0               Blocks: 0          IO Block: 4096   regular empty file
Device: fe04h/65028d    Inode: 2754033     Links: 1
Access: (0640/-rw-r-----)  Uid: ( 1000/pdonnell)   Gid: (  985/   users)
Access: 2020-05-13 07:55:18.892599600 -0700
Modify: 2020-05-13 07:55:18.892599600 -0700
Change: 2020-05-13 07:55:18.892599600 -0700
 Birth: 2020-05-13 07:55:18.892599600 -0700
$ sudo strace -f -- /bin/sh -c 'echo 1 > gtmp/foo' |& grep foo
execve("/bin/sh", ["/bin/sh", "-c", "echo 1 > gtmp/foo"],
0x7fff588732f0 /* 15 vars */) = 0
openat(AT_FDCWD, "gtmp/foo", O_WRONLY|O_CREAT|O_TRUNC, 0666) = -1
EACCES (Permission denied)
write(2, "/bin/sh: gtmp/foo: Permission de"..., 37/bin/sh: gtmp/foo:
Permission denied


-- 
Patrick Donnelly