Re: [PATCH v2] fs/coredump: Enable dynamic configuration of max file note size

[Kees Cook]

On Thu, May 02, 2024 at 01:03:52PM -0700, Allen wrote:
> On Thu, May 2, 2024 at 10:50 AM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> >
> > On Thu, May 02, 2024 at 02:59:20PM +0000, Allen Pais wrote:
> > > Introduce the capability to dynamically configure the maximum file
> > > note size for ELF core dumps via sysctl. This enhancement removes
> > > the previous static limit of 4MB, allowing system administrators to
> > > adjust the size based on system-specific requirements or constraints.
> > >
> > > - Remove hardcoded `MAX_FILE_NOTE_SIZE` from `fs/binfmt_elf.c`.
> > > - Define `max_file_note_size` in `fs/coredump.c` with an initial value
> > >   set to 4MB.
> > > - Declare `max_file_note_size` as an external variable in
> > >   `include/linux/coredump.h`.
> > > - Add a new sysctl entry in `kernel/sysctl.c` to manage this setting
> > >   at runtime.
> > >
> > > $ sysctl -a | grep max_file_note_size
> > > kernel.max_file_note_size = 4194304
> > >
> > > $ sysctl -n kernel.max_file_note_size
> > > 4194304
> > >
> > > $echo 519304 > /proc/sys/kernel/max_file_note_size
> > >
> > > $sysctl -n kernel.max_file_note_size
> > > 519304
> >
> > The names and paths in the commit log need a refresh here, since they've
> > changed.
> 
> Will fix it in v3.
> >
> > >
> > > Why is this being done?
> > > We have observed that during a crash when there are more than 65k mmaps
> > > in memory, the existing fixed limit on the size of the ELF notes section
> > > becomes a bottleneck. The notes section quickly reaches its capacity,
> > > leading to incomplete memory segment information in the resulting coredump.
> > > This truncation compromises the utility of the coredumps, as crucial
> > > information about the memory state at the time of the crash might be
> > > omitted.
> >
> > Thanks for adding this!
> >
> > >
> > > Signed-off-by: Vijay Nag <nagvijay@xxxxxxxxxxxxx>
> > > Signed-off-by: Allen Pais <apais@xxxxxxxxxxxxxxxxxxx>
> > >
> > > ---
> > > Changes in v2:
> > >    - Move new sysctl to fs/coredump.c [Luis & Kees]
> > >    - rename max_file_note_size to core_file_note_size_max [kees]
> > >    - Capture "why this is being done?" int he commit message [Luis & Kees]
> > > ---
> > >  fs/binfmt_elf.c          |  3 +--
> > >  fs/coredump.c            | 10 ++++++++++
> > >  include/linux/coredump.h |  1 +
> > >  3 files changed, 12 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
> > > index 5397b552fbeb..6aebd062b92b 100644
> > > --- a/fs/binfmt_elf.c
> > > +++ b/fs/binfmt_elf.c
> > > @@ -1564,7 +1564,6 @@ static void fill_siginfo_note(struct memelfnote *note, user_siginfo_t *csigdata,
> > >       fill_note(note, "CORE", NT_SIGINFO, sizeof(*csigdata), csigdata);
> > >  }
> > >
> > > -#define MAX_FILE_NOTE_SIZE (4*1024*1024)
> > >  /*
> > >   * Format of NT_FILE note:
> > >   *
> > > @@ -1592,7 +1591,7 @@ static int fill_files_note(struct memelfnote *note, struct coredump_params *cprm
> > >
> > >       names_ofs = (2 + 3 * count) * sizeof(data[0]);
> > >   alloc:
> > > -     if (size >= MAX_FILE_NOTE_SIZE) /* paranoia check */
> > > +     if (size >= core_file_note_size_max) /* paranoia check */
> > >               return -EINVAL;
> >
> > I wonder, given the purpose of this sysctl, if it would be a
> > discoverability improvement to include a pr_warn_once() before the
> > EINVAL? Like:
> >
> >         /* paranoia check */
> >         if (size >= core_file_note_size_max) {
> >                 pr_warn_once("coredump Note size too large: %zu (does kernel.core_file_note_size_max sysctl need adjustment?\n", size);
> >                 return -EINVAL;
> >         }
> >
> > What do folks think? (I can't imagine tracking down this problem
> > originally was much fun, for example.)
> 
>  I think this would really be helpful. I will go ahead and add this if
> there's no objection from anyone.
> 
> Also, I haven't received a reply from Luis, do you think we need to
> add a ceiling?
> 
> +#define MAX_FILE_NOTE_SIZE (4*1024*1024)
> +#define MAX_ALLOWED_NOTE_SIZE (16*1024*1024) // Define a reasonable max cap
> .....
> 
> +       {
> +               .procname       = "core_file_note_size_max",
> +               .data           = &core_file_note_size_max,
> +               .maxlen         = sizeof(unsigned int),
> +               .mode           = 0644,
> +               .proc_handler   = proc_core_file_note_size_max,
> +       },
>  };
> 
> +int proc_core_file_note_size_max(struct ctl_table *table, int write,
> void __user *buffer, size_t *lenp, loff_t *ppos) {
> +    int error = proc_douintvec(table, write, buffer, lenp, ppos);
> +    if (write && (core_file_note_size_max < MAX_FILE_NOTE_SIZE ||
> core_file_note_size_max > MAX_ALLOWED_NOTE_SIZE))
> +        core_file_note_size_max = MAX_FILE_NOTE_SIZE;  // Revert to
> default if out of bounds
> +    return error;
> +}
> 
> 
> Or, should we go ahead with the current patch(with the warning added)?

Let's add a ceiling just to avoid really pathological behavior. We got
this far with 4M, so having a new ceiling seems reasonable. And for
implementing it, see proc_douintvec_minmax.

-Kees

-- 
Kees Cook