Re: + fs-binfmts-better-handling-of-binfmt-loops.patch added to -mmtree

[Zach Levis]

Quoting Oleg Nesterov <oleg@xxxxxxxxxx>:

> On 07/31, Oleg Nesterov wrote:
> > > From: Zach Levis <zml@xxxxxxxxxxxxxxxxxx>
> > > Subject: fs/binfmts: better handling of binfmt loops
> > >
> > > With these changes, when a binfmt loop is encountered, the ELOOP will
> > > propagate back to the 0 depth.  At this point the argv and argc values
> > > will be reset to what they were originally and an attempt is made to
> > > continue with the following binfmt handlers.
> >
> > I must admit, I do not really understand why do we want to recover
> > after pr_err(). Perhaps the changelog could say a bit more.
>
> And still can't. Probably I missed something, but it seems that
> this tries to "fix" the wrong /proc/sys/fs/binfmt_misc/register...
So an example of what this would be used for (going into commit  
message of a v2 with your earlier suggestions):
A qemu is configured to run 64-bit ELFs on an otherwise 32-bit system.  
The system's owner switches to running with 64-bit executables, but  
forgets to disable the binfmt_misc option that redirects 64bit ELFs to  
qemu. Since the qemu executable is a 64-bit ELF now, binfmt_misc keeps  
on matching it with the qemu rule, preventing the execution of any  
64-bit binary.

With this patch, an error is printed and search_binary_handler()  
continues on to the next handler, allowing the original executable to  
run normally so the user can (hopefully) fix their misconfiguration  
more easily.

> > > +
> > > +				copy_strings(bprm->argc_orig, *((struct user_arg_ptr *)  
> > bprm->argv_orig), bprm);
> >
> > Perhaps it would be more clean to add "struct user_arg_ptr;"
> > into binfmts.h and avoid the typecast.

I was kinda trying to avoid exposing the struct, but yeah, that's better.
> > And I do not think we should ignore the possible error from
> > copy_strings(). Even if we know that it succeeded before, another
> > thread can, say, unmap this memory in between.
>
> And since we do copy_strings() again we probably need acct_arg_size()
> after remove_arg_zero() loop, although this is not that important.
I'm not sure if that's even necessary. It looks like there's  
copy_strings()->get_arg_page()->acct_arg_size() that's already called.
> And with this patch "depth == 0" check(s) look even worse, imho we
> need to cleanup this code first. And proc_exec_connector() looks
> simply wrong. I'll try to make a patch.
>
> But once again, I can be easily wrong, so please correct me.
>
> Oleg.


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