[CentOS] Trouble removing files in chrooted sftp
Myyrä, Timo
timo.myyra at edita.fi
Wed Jan 11 11:07:45 UTC 2017
I just did a bit of testing on OpenBSD and there the above setup seems to
work and I can remove the files just fine over sftp.
So this thing should work but there's still something causing it to fail on
CentOS's side.
One difference between our CentOS and OpenBSD is that OpenBSD uses newer
openssh server. I looked through the release notes and didn't see any
changes related to internal-sftp.
I fixed the /home/chroot-user/etc/localtime permissions to 0644 and run
strace on internal-sftp process and got following output:
read(0, "\0\0\0\23\v\0\0\0009\0\0\0\n/intranet/", 16384) = 23
openat(AT_FDCWD, "/intranet/", O_RDONLY|O_NONBLOCK|O_DIRECTORY|O_CLOEXEC) =
3
select(2, [0], [1], NULL, NULL) = 1 (out [1])
write(1, "\0\0\0\rf\0\0\0009\0\0\0\4\0\0\0\0", 17) = 17
select(2, [0], [], NULL, NULL) = 1 (in [0])
read(0, "\0\0\0\r\f\0\0\0:\0\0\0\4\0\0\0\0", 16384) = 17
getdents(3, /* 7 entries */, 32768) = 216
lstat("/intranet//.", {st_mode=S_IFDIR|0775, st_size=98, ...}) = 0
stat("/etc/localtime", {st_mode=S_IFREG|0644, st_size=1883, ...}) = 0
lstat("/intranet//..", {st_mode=S_IFDIR|0755, st_size=56, ...}) = 0
stat("/etc/localtime", {st_mode=S_IFREG|0644, st_size=1883, ...}) = 0
lstat("/intranet//120007f.dotx", {st_mode=S_IFREG|0664, st_size=63788,
...}) = 0
stat("/etc/localtime", {st_mode=S_IFREG|0644, st_size=1883, ...}) = 0
lstat("/intranet//120007f.xml.orig", {st_mode=S_IFREG|0640, st_size=607,
...}) = 0
stat("/etc/localtime", {st_mode=S_IFREG|0644, st_size=1883, ...}) = 0
lstat("/intranet//120007f.xml", {st_mode=S_IFREG|0664, st_size=607, ...}) =
0
stat("/etc/localtime", {st_mode=S_IFREG|0644, st_size=1883, ...}) = 0
lstat("/intranet//test1.txt", {st_mode=S_IFREG|0664, st_size=0, ...}) = 0
stat("/etc/localtime", {st_mode=S_IFREG|0644, st_size=1883, ...}) = 0
lstat("/intranet//test2.txt", {st_mode=S_IFREG|0664, st_size=0, ...}) = 0
stat("/etc/localtime", {st_mode=S_IFREG|0644, st_size=1883, ...}) = 0
getdents(3, /* 0 entries */, 32768) = 0
select(2, [0], [1], NULL, NULL) = 1 (out [1])
write(1, "\0\0\3/h\0\0\0:\0\0\0\7\0\0\0\1.\0\0\0;drwxrwxr-x"..., 819) = 819
select(2, [0], [], NULL, NULL) = 1 (in [0])
read(0, "\0\0\0\r\f\0\0\0;\0\0\0\4\0\0\0\0", 16384) = 17
getdents(3, /* 0 entries */, 32768) = 0
select(2, [0], [1], NULL, NULL) = 1 (out [1])
write(1, "\0\0\0\34e\0\0\0;\0\0\0\1\0\0\0\vEnd of file\0\0\0\0", 32) = 32
select(2, [0], [], NULL, NULL) = 1 (in [0])
read(0, "\0\0\0\r\4\0\0\0<\0\0\0\4\0\0\0\0", 16384) = 17
close(3) = 0
select(2, [0], [1], NULL, NULL) = 1 (out [1])
write(1, "\0\0\0\30e\0\0\0<\0\0\0\0\0\0\0\7Success\0\0\0\0", 28) = 28
select(2, [0], [], NULL, NULL) = 1 (in [0])
read(0, "\0\0\0\34\7\0\0\0=\0\0\0\23/intranet/test2.txt", 16384) = 32
lstat("/intranet/test2.txt", {st_mode=S_IFREG|0664, st_size=0, ...}) = 0
select(2, [0], [1], NULL, NULL) = 1 (out [1])
write(1,
"\0\0\0%i\0\0\0=\0\0\0\17\0\0\0\0\0\0\0\0\0\0\23\234\0\0\23\234\0\0\201"...,
41) = 41
select(2, [0], [], NULL, NULL) = 1 (in [0])
read(0, "\0\0\0\34\7\0\0\0>\0\0\0\23/intranet/test2.txt", 16384) = 32
lstat("/intranet/test2.txt", {st_mode=S_IFREG|0664, st_size=0, ...}) = 0
select(2, [0], [1], NULL, NULL) = 1 (out [1])
write(1,
"\0\0\0%i\0\0\0>\0\0\0\17\0\0\0\0\0\0\0\0\0\0\23\234\0\0\23\234\0\0\201"...,
41) = 41
select(2, [0], [], NULL, NULL) = 1 (in [0])
read(0, "\0\0\0\34\r\0\0\0?\0\0\0\23/intranet/test2.txt", 16384) = 32
unlink("/intranet/test2.txt") = -1 EACCES (Permission denied)
select(2, [0], [1], NULL, NULL) = 1 (out [1])
write(1, "\0\0\0\"e\0\0\0?\0\0\0\3\0\0\0\21Permission deni"..., 38) = 38
select(2, [0], [], NULL, NULL
The relevant sections are probably:
stat("/intranet//.", {st_mode=S_IFDIR|0775, st_size=98, ...}) = 0
...
unlink("/intranet/test2.txt") = -1 EACCES (Permission denied)
So the /intranet has 0775 permissions which should allow chroot-user to
remove files if it belongs to web-user group.
Removal works outside chroot.
How can I verify which groups the user has in sftp chroot?
Timo
2017-01-09 12:31 GMT+02:00 Myyrä, Timo <timo.myyra at edita.fi>:
> Hi,
>
> I have trouble setting up chrooted SFTP for our user.
> I got the basic SFTP chroot working, user is chrooted to its home
> directory, I've added /home/userb/etc directory with dummy passwd, group
> and localtime files.
>
> The problem is that instead of only accessing its own files, I need the
> user to be able to remove another users files.
> I have web application which runs as different user, the sftp user is
> member of the web users group. I then setup bind mounts from web users data
> dir to chrooted users home directory.
>
> The chrooted user can't remove any files in the directory owned by another
> user. The directory permissions are 0775 so they should allow the access.
> If I don't use chroot I can remove the file just fine so it seems to be
> something on the chroot that limits the access.
>
> I tried to add simple directory, checked the permissions and tried to
> delete file and it failed as well so the bind mount should be the issue.
>
> Here's sample output:
> # cat etc/{passwd,group}
> webapp-user:x:5020:5020::/home/webapp-user:/sbin/nologin
> chroot-user:x:5029:10000::/home/chroot-user:/sbin/nologin
> root:x:0:0:not really root:::
> sftp-chroot:x:10000:chroot-user
> webapp-user:x:5020:chroot-user
> # ls -lR
> .:
> total 0
> drwx--x--- 2 root sftp-chroot 61 Jan 5 07:34 etc
> drwxr-x--- 2 chroot-user sftp-chroot 6 Nov 20 2015 failed
> drwxr-x--- 2 chroot-user sftp-chroot 16 Jan 5 07:46 input
> drwxrwxr-x 2 webapp-user webapp-user 98 Jan 5 07:18 intranet
>
> ./etc:
> total 16
> -rw-r--r-- 1 root root 55 Jan 5 07:34 group
> -rw-r----- 1 root root 1883 Jan 5 06:45 localtime
> -rw-r--r-- 1 root root 172 Jan 5 06:51 passwd
> -rw-r--r-- 1 root root 105 Jan 5 06:46 passwd~
>
> ./failed:
> total 0
>
> ./input:
> total 0
>
> ./intranet:
> total 72
> -rw-rw-r-- 1 chroot-user sftp-chroot 0 Jan 5 07:02 test1.txt
> -rw-rw-r-- 1 webapp-user webapp-user 0 Jan 5 07:02 test2.txt
>
> So why can't I remove the intranet/test* files inside sftp chroot even if
> the chroot-user is member of webapp-user group and the directory itself has
> group permissions?
>
> Br,
> Timo M
>
--
Parhain terveisin,
Timo Myyrä
Palvelinylläpitäjä
Edita Prima Oy, Kehitysyksikkö
Vilhonvuorenkatu 12
PL 510
00043 NORDIC MORNING
+358 40 860 2103
timo.myyra at edita.fi
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