• Tejun Heo's avatar
    cgroup: fix css_task_iter crash on CSS_TASK_ITER_PROC · 74d0833c
    Tejun Heo authored
    While teaching css_task_iter to handle skipping over tasks which
    aren't group leaders, bc2fb7ed ("cgroup: add @flags to
    css_task_iter_start() and implement CSS_TASK_ITER_PROCS") introduced a
    silly bug.
    
    CSS_TASK_ITER_PROCS is implemented by repeating
    css_task_iter_advance() while the advanced cursor is pointing to a
    non-leader thread.  However, the cursor variable, @l, wasn't updated
    when the iteration has to advance to the next css_set and the
    following repetition would operate on the terminal @l from the
    previous iteration which isn't pointing to a valid task leading to
    oopses like the following or infinite looping.
    
      BUG: unable to handle kernel NULL pointer dereference at 0000000000000254
      IP: __task_pid_nr_ns+0xc7/0xf0
      PGD 0 P4D 0
      Oops: 0000 [#1] SMP
      ...
      CPU: 2 PID: 1 Comm: systemd Not tainted 4.14.4-200.fc26.x86_64 #1
      Hardware name: System manufacturer System Product Name/PRIME B350M-A, BIOS 3203 11/09/2017
      task: ffff88c4baee8000 task.stack: ffff96d5c3158000
      RIP: 0010:__task_pid_nr_ns+0xc7/0xf0
      RSP: 0018:ffff96d5c315bd50 EFLAGS: 00010206
      RAX: 0000000000000000 RBX: ffff88c4b68c6000 RCX: 0000000000000250
      RDX: ffffffffa5e47960 RSI: 0000000000000000 RDI: ffff88c490f6ab00
      RBP: ffff96d5c315bd50 R08: 0000000000001000 R09: 0000000000000005
      R10: ffff88c4be006b80 R11: ffff88c42f1b8004 R12: ffff96d5c315bf18
      R13: ffff88c42d7dd200 R14: ffff88c490f6a510 R15: ffff88c4b68c6000
      FS:  00007f9446f8ea00(0000) GS:ffff88c4be680000(0000) knlGS:0000000000000000
      CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
      CR2: 0000000000000254 CR3: 00000007f956f000 CR4: 00000000003406e0
      Call Trace:
       cgroup_procs_show+0x19/0x30
       cgroup_seqfile_show+0x4c/0xb0
       kernfs_seq_show+0x21/0x30
       seq_read+0x2ec/0x3f0
       kernfs_fop_read+0x134/0x180
       __vfs_read+0x37/0x160
       ? security_file_permission+0x9b/0xc0
       vfs_read+0x8e/0x130
       SyS_read+0x55/0xc0
       entry_SYSCALL_64_fastpath+0x1a/0xa5
      RIP: 0033:0x7f94455f942d
      RSP: 002b:00007ffe81ba2d00 EFLAGS: 00000293 ORIG_RAX: 0000000000000000
      RAX: ffffffffffffffda RBX: 00005574e2233f00 RCX: 00007f94455f942d
      RDX: 0000000000001000 RSI: 00005574e2321a90 RDI: 000000000000002b
      RBP: 0000000000000000 R08: 00005574e2321a90 R09: 00005574e231de60
      R10: 00007f94458c8b38 R11: 0000000000000293 R12: 00007f94458c8ae0
      R13: 00007ffe81ba3800 R14: 0000000000000000 R15: 00005574e2116560
      Code: 04 74 0e 89 f6 48 8d 04 76 48 8d 04 c5 f0 05 00 00 48 8b bf b8 05 00 00 48 01 c7 31 c0 48 8b 0f 48 85 c9 74 18 8b b2 30 08 00 00 <3b> 71 04 77 0d 48 c1 e6 05 48 01 f1 48 3b 51 38 74 09 5d c3 8b
      RIP: __task_pid_nr_ns+0xc7/0xf0 RSP: ffff96d5c315bd50
    
    Fix it by moving the initialization of the cursor below the repeat
    label.  While at it, rename it to @next for readability.
    Signed-off-by: default avatarTejun Heo <tj@kernel.org>
    Fixes: bc2fb7ed ("cgroup: add @flags to css_task_iter_start() and implement CSS_TASK_ITER_PROCS")
    Cc: stable@vger.kernel.org # v4.14+
    Reported-by: default avatarLaura Abbott <labbott@redhat.com>
    Reported-by: default avatarBronek Kozicki <brok@incorrekt.com>
    Reported-by: default avatarGeorge Amanakis <gamanakis@gmail.com>
    Signed-off-by: default avatarTejun Heo <tj@kernel.org>
    74d0833c
cgroup.c 156 KB