linux-kernel - Re: + fs-binfmts-better-handling-of-binfmt-loops.patch added to -mm tree

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Message-ID: <20130801090223.Horde.UZGId5ir309R_oaPIezA7qA@imap.linux.ibm.com>
Date:	Thu, 01 Aug 2013 09:02:23 -0700
From:	Zach Levis <zml@...ux.vnet.ibm.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Zach Levis <zach@...hsthings.com>, Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: + fs-binfmts-better-handling-of-binfmt-loops.patch added to -mm
 tree


Quoting Oleg Nesterov <oleg@...hat.com>:

> On 07/31, Oleg Nesterov wrote:
>>
>> > From: Zach Levis <zml@...ux.vnet.ibm.com>
>> > 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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