Re: [syzbot] [mptcp?] general protection fault in proc_scheduler

Eric Dumazet
2025-01-05T08:32:48Z

On Sat, Jan 4, 2025 at 9:21 PM Al Viro <viro@zeniv.linux.org.uk> wrote:
>
> On Sat, Jan 04, 2025 at 08:11:49PM +0100, Matthieu Baerts wrote:
> > >> + if (S_ISREG(file_inode(file)->i_mode))
> ^^^^^^^^^
> > >> + return;
> > >
> > > ... won't help, since the file in question *is* a regular file. IOW, it's
> > > a wrong predicate here.
> >
> > On my side, it looks like I'm not able to reproduce the issue with this
> > patch. Without it, it is very easy to reproduce it. (But I don't know if
> > there are other consequences that would avoid the issue to happen: when
> > looking at the logs, with the patch, I don't have heaps of "Process
> > accounting resumed" messages that I had before.)
>
> Unsurprisingly so, since it rejects all regular files due to a typo;
> fix that and you'll see that the oops is still there.
>
> The real issue (and the one that affects more than just this scenario) is
> the use of current->nsproxy->net to get to the damn thing.

According to grep, we have many other places directly reading
current->nsproxy->net_ns
For instance in net/sctp/sysctl.c
Should we change them all ?

Perhaps an alternative would be to add a generic check in
proc_sys_call_handler()

diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
index 27a283d85a6e7df1a7edbfb513ce75832363e2e6..84968b10ce86e7fd88c6e3c43f52b601394b056f
100644
--- a/fs/proc/proc_sysctl.c
+++ b/fs/proc/proc_sysctl.c
@@ -576,6 +576,8 @@ static ssize_t proc_sys_call_handler(struct kiocb
*iocb, struct iov_iter *iter,
error = -EINVAL;
if (!table->proc_handler)
goto out;
+ if (unlikely(current->flags & PF_EXITING))
+ goto out;

/* don't even try if the size is too large */
error = -ENOMEM;

Thanks.