Hi Christian,

We ran a quick test and confirm your suggestion would eliminate the
data race alert we observed. If the data race is benign (and it
appears to be), using WRITE_ONCE(cgrp_dfl_visible, true) instead of
cmpxchg in cgroup_get_tree() would probably also be ok.

Best,

Gabe

On Fri, Aug 19, 2022 at 3:23 AM Christian Brauner [off-list ref] wrote:

On Thu, Aug 18, 2022 at 07:24:00PM -0400, Abhishek Shah wrote:

quoted

Hi all,

We found the following data race involving the *cgrp_dfl_visible *variable.
We think it has security implications as the racing variable controls the
contents used in /proc/<pid>/cgroup which has been used in prior work
<https://urldefense.proofpoint.com/v2/url?u=https-3A__www.cyberark.com_resources_threat-2Dresearch-2Dblog_the-2Dstrange-2Dcase-2Dof-2Dhow-2Dwe-2Descaped-2Dthe-2Ddocker-2Ddefault-2Dcontainer&d=DwIBaQ&c=009klHSCxuh5AI1vNQzSO0KGjl4nbi2Q0M1QLJX9BeE&r=EyAJYRJu01oaAhhVVY3o8zKgZvacDAXd_PNRtaqACCo&m=oB43wXi5itVN6tAAOVg5q3rzeXp6QVvxICYqYL6p0wnMMhRB_HrHCwwt0dYa5x44&s=78sLv2vexAVEQwQPx_CuCJ90is9f3iixNbmbCp0Agpo&e=  >
in container escapes. Please let us know what you think. Thanks!

One straightforward fix might be to use
cmpxchg(&cgrp_dfl_visible, false, true) in cgroup_get_tree()
and READ_ONCE(cgrp_dfl_visible) in proc_cgroup_show() or sm like that.
I'm not sure this is an issue though but might still be nice to fix it.

quoted

*-----------------------------Report--------------------------------------*
*write* to 0xffffffff881d0344 of 1 bytes by task 6542 on cpu 0:
 cgroup_get_tree+0x30/0x1c0 kernel/cgroup/cgroup.c:2153
 vfs_get_tree+0x53/0x1b0 fs/super.c:1497
 do_new_mount+0x208/0x6a0 fs/namespace.c:3040
 path_mount+0x4a0/0xbd0 fs/namespace.c:3370
 do_mount fs/namespace.c:3383 [inline]
 __do_sys_mount fs/namespace.c:3591 [inline]
 __se_sys_mount+0x215/0x2d0 fs/namespace.c:3568
 __x64_sys_mount+0x67/0x80 fs/namespace.c:3568
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x3d/0x90 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae

*read* to 0xffffffff881d0344 of 1 bytes by task 6541 on cpu 1:
 proc_cgroup_show+0x1ec/0x4e0 kernel/cgroup/cgroup.c:6017
 proc_single_show+0x96/0x120 fs/proc/base.c:777
 seq_read_iter+0x2d2/0x8e0 fs/seq_file.c:230
 seq_read+0x1c9/0x210 fs/seq_file.c:162
 vfs_read+0x1b5/0x6e0 fs/read_write.c:480
 ksys_read+0xde/0x190 fs/read_write.c:620
 __do_sys_read fs/read_write.c:630 [inline]
 __se_sys_read fs/read_write.c:628 [inline]
 __x64_sys_read+0x43/0x50 fs/read_write.c:628
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x3d/0x90 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 6541 Comm: syz-executor2-n Not tainted 5.18.0-rc5+ #107
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1
04/01/2014


*Reproducing Inputs*
Input CPU 0:
r0 = fsopen(&(0x7f0000000000)='cgroup2\x00', 0x0)
fsconfig$FSCONFIG_CMD_CREATE(r0, 0x6, 0x0, 0x0, 0x0)
fsmount(r0, 0x0, 0x83)

Input CPU 1:
r0 = syz_open_procfs(0x0, &(0x7f0000000040)='cgroup\x00')
read$eventfd(r0, &(0x7f0000000080), 0x8)

-- 
Gabriel Ryan
PhD Candidate at Columbia University