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SubjectRe: > [PATCH] Security: Handle hidepid option correctly
Andrew's question makes me think if this fix is superficial. Actually
i have had same question. But when i saw a smilar patch in kernel-4.4
was already merged in 2012, i decided to submit this patch first.

Here is the call stack i got:
[ 0.003450] [<ffffff8bef2a0190>] proc_mount+0x2c/0x98
[ 0.003459] [<ffffff8bef22e560>] mount_fs+0x164/0x190
[ 0.003465] [<ffffff8bef24c138>] vfs_kern_mount+0x74/0x168
[ 0.003469] [<ffffff8bef24c244>] kern_mount_data+0x18/0x30
[ 0.003474] [<ffffff8bef2a0258>] pid_ns_prepare_proc+0x24/0x40
[ 0.003484] [<ffffff8bef0cd5ec>] alloc_pid+0x498/0x4b4
[ 0.003492] [<ffffff8bef0a9b94>] copy_process.isra.73.part.74+0xed0/0x1708
[ 0.003496] [<ffffff8bef0aa560>] _do_fork+0xdc/0x3f8
[ 0.003501] [<ffffff8bef0aa8c8>] kernel_thread+0x34/0x3c
[ 0.003511] [<ffffff8bf00cd498>] rest_init+0x20/0x80
[ 0.003522] [<ffffff8bf0c00c7c>] start_kernel+0x3e4/0x43c
[ 0.003527] [<ffffff8bf0c001e8>] __primary_switched+0x64/0x90
I notice only proc filesystem has function "pid_ns_prepare_proc".
there is no other "pid_ns_prepare_xxx" function in other filesystem.
Take the position of proc filesystem of kernel into consideration, the
answer of question "Other filesystems parse the options from
fill_super(). Is proc special in some fashion" could be "Yes, it is.
Because proc filesystem is special indeed. It's a filesystem kernel
will mount when it's booting".

But is it enough? Is anyone responsible for deinitialize sb->sroot?
Well actually i'm not an expert of filesystem, and don't unserstand
what does sb->s_root represent for. But i'm sure no one call
"pid_ns_release_proc" in the runtime(by add some logs). And even it is
called, it doesn't clean sb->s_root. Until now, i didn't see any
deeper issue. Maybe it's true that we should handle proc filesystem
specially.

If anyone who is sure about the functionality of sb->s_root and think
it should be handled in another way, feel free to correct me.


程洋 <chengyang@xiaomi.com> 于2018年11月30日周五 上午10:34写道:
>> Here is an article illustrates the details.
> https://medium.com/@topjohnwu/from-anime-game-to-android-system-security-vulnerability-9b955a182f20
>
> And There is a similar fix on kernel-4.4:
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=99663be772c827b8f5f594fe87eb4807be1994e5
>
> Q: Other filesystems parse the options from fill_super(). Is proc special in some fashion?
> A: According to my research, start_kernel will call proc_mount first, and initialize sb->s_root before any userspace process runs. If others want to mount it, all options will be ignored.
> AOSP change here: https://android-review.googlesource.com/c/platform/system/core/+/181345/4/init/init.cpp
> At first I though we should mount it with MS_REMOUNT flag. But kernel will crash if we did this.
>
> Q: Why is this considered to be security sensitive? I can guess, but I'd like to know your reasoning.
> A: See the article above. It's part of Android sanbox.
>
>
> > [PATCH] Security: Handle hidepid option correctly
>
> Why is this considered to be security sensitive? I can guess, but I'd like to know your reasoning.
>
> On Thu, 29 Nov 2018 19:08:21 +0800 mailto:d17103513@gmail.com wrote:
>
> > From: Cheng Yang <mailto:chengyang@xiaomi.com>
> >
> > The proc_parse_options() call from proc_mount() runs only once at boot
> > time. So on any later mount attempt, any mount options are ignored
> > because ->s_root is already initialized.
> > As a consequence, "mount -o <options>" will ignore the options. The
> > only way to change mount options is "mount -o remount,<options>".
> > To fix this, parse the mount options unconditionally.
> >
> > --- a/fs/proc/inode.c
> > +++ b/fs/proc/inode.c
> > @@ -493,13 +493,9 @@ struct inode *proc_get_inode(struct super_block
> > *sb, struct proc_dir_entry *de)
> >
> > int proc_fill_super(struct super_block *s, void *data, int silent) {
> > -struct pid_namespace *ns = get_pid_ns(s->s_fs_info);
> > struct inode *root_inode;
> > int ret;
> >
> > -if (!proc_parse_options(data, ns))
> > -return -EINVAL;
> > -
> > /* User space would break if executables or devices appear on proc */
> > s->s_iflags |= SB_I_USERNS_VISIBLE | SB_I_NOEXEC | SB_I_NODEV;
> > s->s_flags |= SB_NODIRATIME | SB_NOSUID | SB_NOEXEC; diff --git
> > a/fs/proc/root.c b/fs/proc/root.c index f4b1a9d..f5f3bf3 100644
> > --- a/fs/proc/root.c
> > +++ b/fs/proc/root.c
> > @@ -98,6 +98,9 @@ static struct dentry *proc_mount(struct file_system_type *fs_type,
> > ns = task_active_pid_ns(current);
> > }
> >
> > +if (!proc_parse_options(data, ns))
> > +return ERR_PTR(-EINVAL);
> > +
> > return mount_ns(fs_type, flags, data, ns, ns->user_ns,
> > proc_fill_super); }
>
> Other filesystems parse the options from fill_super(). Is proc special in some fashion?
>
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