Re: [PATCH] coredump: Limit what can interrupt coredumps

[Olivier Langlois]

On Tue, 2021-06-15 at 17:08 -0500, Eric W. Biederman wrote:
> Oleg Nesterov <oleg@xxxxxxxxxx> writes:
> 
> > > --- a/fs/coredump.c
> > > +++ b/fs/coredump.c
> > > @@ -519,7 +519,7 @@ static bool dump_interrupted(void)
> > >          * but then we need to teach dump_write() to restart and
> > > clear
> > >          * TIF_SIGPENDING.
> > >          */
> > > -       return signal_pending(current);
> > > +       return fatal_signal_pending(current) || freezing(current);
> > >  }
> > 
> > 
> > Well yes, this is what the comment says.
> > 
> > But note that there is another reason why dump_interrupted() returns
> > true
> > if signal_pending(), it assumes thagt __dump_emit()->__kernel_write()
> > may
> > fail anyway if signal_pending() is true. Say, pipe_write(), or iirc
> > nfs,
> > perhaps something else...
> > 
> > That is why zap_threads() clears TIF_SIGPENDING. Perhaps it should
> > clear
> > TIF_NOTIFY_SIGNAL as well and we should change io-uring to not abuse
> > the
> > dumping threads?
> > 
> > Or perhaps we should change __dump_emit() to clear signal_pending()
> > and
> > restart __kernel_write() if it fails or returns a short write.
> > 
> > Otherwise the change above doesn't look like a full fix to me.
> 
> Agreed.  The coredump to a pipe will still be short.  That needs
> something additional.
> 
> The problem Olivier Langlois <olivier@xxxxxxxxxxxxxx> reported was
> core dumps coming up short because TIF_NOTIFY_SIGNAL was being
> set during a core dump.
> 
> We can see this with pipe_write returning -ERESTARTSYS
> on a full pipe if signal_pending which includes TIF_NOTIFY_SIGNAL
> is true.
> 
> Looking further if the thread that is core dumping initiated
> any io_uring work then io_ring_exit_work will use task_work_add
> to request that thread clean up it's io_uring state.
> 
> Perhaps we can put a big comment in dump_emit and if we
> get back -ERESTARTSYS run tracework_notify_signal.  I am not
> seeing any locks held at that point in the coredump, so it
> should be safe.  The coredump is run inside of file_start_write
> which is the only potential complication.
> 
> 
> 
> The code flow is complicated but it looks like the entire
> point of the exercise is to call io_uring_del_task_file
> on the originating thread.  I suppose that keeps the
> locking of the xarray in io_uring_task simple.
> 
> 
> Hmm.   All of this comes from io_uring_release.
> How do we get to io_uring_release?  The coredump should
> be catching everything in exit_mm before exit_files?
> 
> Confused and hopeful someone can explain to me what is going on,
> and perhaps simplify it.
> 
> Eric

Hi all,

I didn't forgot about this remaining issue and I have kept thinking
about it on and off.

I did try the following on 5.12.19:

diff --git a/fs/coredump.c b/fs/coredump.c
index 07afb5ddb1c4..614fe7a54c1a 100644
--- a/fs/coredump.c
+++ b/fs/coredump.c
@@ -41,6 +41,7 @@
 #include <linux/fs.h>
 #include <linux/path.h>
 #include <linux/timekeeping.h>
+#include <linux/io_uring.h>
 
 #include <linux/uaccess.h>
 #include <asm/mmu_context.h>
@@ -625,6 +626,8 @@ void do_coredump(const kernel_siginfo_t *siginfo)
 		need_suid_safe = true;
 	}
 
+	io_uring_files_cancel(current->files);
+
 	retval = coredump_wait(siginfo->si_signo, &core_state);
 	if (retval < 0)
 		goto fail_creds;
-- 
2.32.0

with my current understanding, io_uring_files_cancel is supposed to
cancel everything that might set the TIF_NOTIFY_SIGNAL.

I must report that in my testing with generating a core dump through a
pipe with the modif above, I still get truncated core dumps.

systemd is having a weird error:
[ 2577.870742] systemd-coredump[4056]: Failed to get COMM: No such
process

and nothing is captured

so I have replaced it with a very simple shell:
$ cat /proc/sys/kernel/core_pattern 
|/home/lano1106/bin/pipe_core.sh %e %p

~/bin $ cat pipe_core.sh 
#!/bin/sh

cat > /home/lano1106/core/core.$1.$2

BFD: warning: /home/lano1106/core/core.test.10886 is truncated:
expected core file size >= 24129536, found: 61440

I conclude from my attempt that maybe io_uring_files_cancel is not 100%
cleaning everything that it should clean.