Re: [PATCH] procfs: block chmod on /proc/thread-self/comm

[Christian Brauner]

On Thu, Jul 13, 2023 at 03:01:24PM +0200, Thomas Weißschuh wrote:
> On 2023-07-13 22:19:04+1000, Aleksa Sarai wrote:
> > Due to an oversight in commit 1b3044e39a89 ("procfs: fix pthread
> > cross-thread naming if !PR_DUMPABLE") in switching from REG to NOD,
> > chmod operations on /proc/thread-self/comm were no longer blocked as
> > they are on almost all other procfs files.
> > 
> > A very similar situation with /proc/self/environ was used to as a root
> > exploit a long time ago, but procfs has SB_I_NOEXEC so this is simply a
> > correctness issue.
> > 
> > Ref: https://lwn.net/Articles/191954/
> > Ref: 6d76fa58b050 ("Don't allow chmod() on the /proc/<pid>/ files")
> > Fixes: 1b3044e39a89 ("procfs: fix pthread cross-thread naming if !PR_DUMPABLE")
> > Cc: stable@xxxxxxxxxxxxxxx # v4.7+
> > Signed-off-by: Aleksa Sarai <cyphar@xxxxxxxxxx>
> > ---
> >  fs/proc/base.c                               | 3 ++-
> >  tools/testing/selftests/nolibc/nolibc-test.c | 4 ++++
> >  2 files changed, 6 insertions(+), 1 deletion(-)
> > 
> > diff --git a/fs/proc/base.c b/fs/proc/base.c
> > index 05452c3b9872..7394229816f3 100644
> > --- a/fs/proc/base.c
> > +++ b/fs/proc/base.c
> > @@ -3583,7 +3583,8 @@ static int proc_tid_comm_permission(struct mnt_idmap *idmap,
> >  }
> >  
> >  static const struct inode_operations proc_tid_comm_inode_operations = {
> > -		.permission = proc_tid_comm_permission,
> > +		.setattr	= proc_setattr,
> > +		.permission	= proc_tid_comm_permission,
> >  };
> 
> Given that this seems to be a recurring theme a more systematic
> aproach would help.
> 
> Something like the following (untested) patch:
> 
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index 05452c3b9872..b90f2e9cda66 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -2649,6 +2649,7 @@ static struct dentry *proc_pident_instantiate(struct dentry *dentry,
>  		set_nlink(inode, 2);	/* Use getattr to fix if necessary */
>  	if (p->iop)
>  		inode->i_op = p->iop;
> +	WARN_ON(!inode->i_op->setattr);

Hm, no. This is hacky.

To fix this properly we will need to wean off notify_change() from
falling back to simple_setattr() when no i_op->setattr() method is
defined. To do that we will have to go through every filesystem and port
all that rely on this fallback to set simple_setattr() explicitly as
their i_op->setattr() method.

Christoph and I just discussed this in relation to another patch.

This is a bugfix so it should be as minimal as possible for easy
backport.