Re: [PATCH] proc: Fix uninitialized byte read in get_mm_cmdline()

[Alexey Dobriyan <adobriyan@gmail.com>]

On Fri, Jul 12, 2019 at 09:43:03PM +0300, Alexey Izbyshev wrote:
> On 7/12/19 8:46 PM, Alexey Dobriyan wrote:
> > On Fri, Jul 12, 2019 at 06:36:26PM +0200, Oleg Nesterov wrote:
> >> On 07/12, Alexey Izbyshev wrote:
> >>>
> >>> --- a/fs/proc/base.c
> >>> +++ b/fs/proc/base.c
> >>> @@ -275,6 +275,8 @@ static ssize_t get_mm_cmdline(struct mm_struct *mm, char __user *buf,
> >>>  		if (got <= offset)
> >>>  			break;
> >>>  		got -= offset;
> >>> +		if (got < size)
> >>> +			size = got;
> >>
> >> Acked-by: Oleg Nesterov <oleg@redhat.com>
> > 
> > The proper fix to all /proc/*/cmdline problems is to revert
> > 
> > 	f5b65348fd77839b50e79bc0a5e536832ea52d8d
> > 	proc: fix missing final NUL in get_mm_cmdline() rewrite
> > 
> > 	5ab8271899658042fabc5ae7e6a99066a210bc0e
> > 	fs/proc: simplify and clarify get_mm_cmdline() function
> > 
> Should this be interpreted as an actual suggestion to revert the patches,
> fix the conflicts, test and submit them, or is this more like thinking out
> loud?

Of course! Do you have a reproducer?

> In the former case, will it be OK for long term branches?

For everyone.

If a rewrite causes 1 bug, 1 user visible change and a infoleak, it is
called revert.

> get_mm_cmdline() does seem easier to read for me before 5ab8271899658042.
> But it also has different semantics in corner cases, for example:

All semantics changes are recent.

> - If there is no NUL at arg_end-1, it reads only the first string in
> the combined arg/env block, and doesn't terminate it with NUL.

That's because fixed-length /proc/*/cmdline did that.

> - If there is any problem with access_remote_vm() or copy_to_user(),
> it returns -EFAULT even if some data were copied to userspace.
> 
> On the other hand, 5ab8271899658042 was merged not too long ago (about a year),
> so it's possible that the current semantics isn't heavily relied upon.