Re: [PATCH] fork: do not release lock that wasn't taken

[Christian Brauner <christian@brauner.io>]

On Fri, May 10, 2019 at 12:50:46PM +0200, Christian Brauner wrote:
> On Fri, May 10, 2019 at 12:49:13PM +0200, Christian Brauner wrote:
> > Avoid calling cgroup_threadgroup_change_end() without having called
> > cgroup_threadgroup_change_begin() first.
> > 
> > During process creation we need to check whether the cgroup we are in
> > allows us to fork. To perform this check the cgroup needs to guard itself
> > against threadgroup changes and takes a lock.
> > Prior to CLONE_PIDFD the cleanup target "bad_fork_free_pid" would also need
> > to call cgroup_threadgroup_change_end() because said lock had already been
> > taken.
> > However, this is not the case anymore with the addition of CLONE_PIDFD. We
> > are now allocating a pidfd before we check whether the cgroup we're in can
> > fork and thus prior to taking the lock. So when copy_process() fails at the
> > right step it would release a lock we haven't taken.
> > This bug is not even very subtle to be honest. It's just not very clear
> > from the naming of cgroup_threadgroup_change_{begin,end}() that a lock is
> > taken.
> > 
> > Here's the relevant splat:
> > 
> > entry_SYSENTER_compat+0x70/0x7f arch/x86/entry/entry_64_compat.S:139
> > RIP: 0023:0xf7fec849
> > Code: 85 d2 74 02 89 0a 5b 5d c3 8b 04 24 c3 8b 14 24 c3 8b 3c 24 c3 90 90
> > 90 90 90 90 90 90 90 90 90 90 51 52 55 89 e5 0f 34 cd 80 <5d> 5a 59 c3 90
> > 90 90 90 eb 0d 90 90 90 90 90 90 90 90 90 90 90 90
> > RSP: 002b:00000000ffed5a8c EFLAGS: 00000246 ORIG_RAX: 0000000000000078
> > RAX: ffffffffffffffda RBX: 0000000000003ffc RCX: 0000000000000000
> > RDX: 00000000200005c0 RSI: 0000000000000000 RDI: 0000000000000000
> > RBP: 0000000000000012 R08: 0000000000000000 R09: 0000000000000000
> > R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
> > R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
> > ------------[ cut here ]------------
> > DEBUG_LOCKS_WARN_ON(depth <= 0)
> > WARNING: CPU: 1 PID: 7744 at kernel/locking/lockdep.c:4052 __lock_release
> > kernel/locking/lockdep.c:4052 [inline]
> > WARNING: CPU: 1 PID: 7744 at kernel/locking/lockdep.c:4052
> > lock_release+0x667/0xa00 kernel/locking/lockdep.c:4321
> > Kernel panic - not syncing: panic_on_warn set ...
> > CPU: 1 PID: 7744 Comm: syz-executor007 Not tainted 5.1.0+ #4
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
> > Google 01/01/2011
> > Call Trace:
> >   __dump_stack lib/dump_stack.c:77 [inline]
> >   dump_stack+0x172/0x1f0 lib/dump_stack.c:113
> >   panic+0x2cb/0x65c kernel/panic.c:214
> >   __warn.cold+0x20/0x45 kernel/panic.c:566
> >   report_bug+0x263/0x2b0 lib/bug.c:186
> >   fixup_bug arch/x86/kernel/traps.c:179 [inline]
> >   fixup_bug arch/x86/kernel/traps.c:174 [inline]
> >   do_error_trap+0x11b/0x200 arch/x86/kernel/traps.c:272
> >   do_invalid_op+0x37/0x50 arch/x86/kernel/traps.c:291
> >   invalid_op+0x14/0x20 arch/x86/entry/entry_64.S:972
> > RIP: 0010:__lock_release kernel/locking/lockdep.c:4052 [inline]
> > RIP: 0010:lock_release+0x667/0xa00 kernel/locking/lockdep.c:4321
> > Code: 0f 85 a0 03 00 00 8b 35 77 66 08 08 85 f6 75 23 48 c7 c6 a0 55 6b 87
> > 48 c7 c7 40 25 6b 87 4c 89 85 70 ff ff ff e8 b7 a9 eb ff <0f> 0b 4c 8b 85
> > 70 ff ff ff 4c 89 ea 4c 89 e6 4c 89 c7 e8 52 63 ff
> > RSP: 0018:ffff888094117b48 EFLAGS: 00010086
> > RAX: 0000000000000000 RBX: 1ffff11012822f6f RCX: 0000000000000000
> > RDX: 0000000000000000 RSI: ffffffff815af236 RDI: ffffed1012822f5b
> > RBP: ffff888094117c00 R08: ffff888092bfc400 R09: fffffbfff113301d
> > R10: fffffbfff113301c R11: ffffffff889980e3 R12: ffffffff8a451df8
> > R13: ffffffff8142e71f R14: ffffffff8a44cc80 R15: ffff888094117bd8
> >   percpu_up_read.constprop.0+0xcb/0x110 include/linux/percpu-rwsem.h:92
> >   cgroup_threadgroup_change_end include/linux/cgroup-defs.h:712 [inline]
> >   copy_process.part.0+0x47ff/0x6710 kernel/fork.c:2222
> >   copy_process kernel/fork.c:1772 [inline]
> >   _do_fork+0x25d/0xfd0 kernel/fork.c:2338
> >   __do_compat_sys_x86_clone arch/x86/ia32/sys_ia32.c:240 [inline]
> >   __se_compat_sys_x86_clone arch/x86/ia32/sys_ia32.c:236 [inline]
> >   __ia32_compat_sys_x86_clone+0xbc/0x140 arch/x86/ia32/sys_ia32.c:236
> >   do_syscall_32_irqs_on arch/x86/entry/common.c:334 [inline]
> >   do_fast_syscall_32+0x281/0xd54 arch/x86/entry/common.c:405
> >   entry_SYSENTER_compat+0x70/0x7f arch/x86/entry/entry_64_compat.S:139
> > RIP: 0023:0xf7fec849
> > Code: 85 d2 74 02 89 0a 5b 5d c3 8b 04 24 c3 8b 14 24 c3 8b 3c 24 c3 90 90
> > 90 90 90 90 90 90 90 90 90 90 51 52 55 89 e5 0f 34 cd 80 <5d> 5a 59 c3 90
> > 90 90 90 eb 0d 90 90 90 90 90 90 90 90 90 90 90 90
> > RSP: 002b:00000000ffed5a8c EFLAGS: 00000246 ORIG_RAX: 0000000000000078
> > RAX: ffffffffffffffda RBX: 0000000000003ffc RCX: 0000000000000000
> > RDX: 00000000200005c0 RSI: 0000000000000000 RDI: 0000000000000000
> > RBP: 0000000000000012 R08: 0000000000000000 R09: 0000000000000000
> > R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
> > R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
> > Kernel Offset: disabled
> > Rebooting in 86400 seconds..
> > 
> > Reported-by: syzbot+3286e58549edc479faae@syzkaller.appspotmail.com
> > Fixes: b3e583825266 ("clone: add CLONE_PIDFD")
> > Signed-off-by: Christian Brauner <christian@brauner.io>
> 
> Assuming we agree that this is correct I bundle this into my pidfd tree
> together with the pidfd-metadata fix for .gitignore that Linus pointed
> out beginning of this week and send pr later today.

With this patch applied, syzkaller did not reproduce this bug. So I'm
sending a PR in a little while. This should go in before rc1 is out.
Here's what syzkaller had to say:

Hello,

syzbot has tested the proposed patch and the reproducer did not trigger  
crash:

Reported-and-tested-by:  
syzbot+3286e58549edc479faae@syzkaller.appspotmail.com

Tested on:

commit:         f81f17b6 fork: do not release lock that wasn't taken
git tree:        
git://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux.git pidfd
kernel config:  https://syzkaller.appspot.com/x/.config?x=4005028a9d5ddac8
compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
userspace arch: i386

Note: testing is done by a robot and is best-effort only.