linux-kernel - Re: [PATCH v1 0/7] Remove in-tree usage of MAP

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Date:   Thu, 12 Aug 2021 14:47:46 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Florian Weimer <fweimer@...hat.com>
Cc:     linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
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Subject: Re: [PATCH v1 0/7] Remove in-tree usage of MAP_DENYWRITE

On 12.08.21 14:20, Florian Weimer wrote:
> * David Hildenbrand:
> 
>> There are some (minor) user-visible changes with this series:
>> 1. We no longer deny write access to shared libaries loaded via legacy
>>     uselib(); this behavior matches modern user space e.g., via dlopen().
>> 2. We no longer deny write access to the elf interpreter after exec
>>     completed, treating it just like shared libraries (which it often is).
> 
> We have a persistent issue with people using cp (or similar tools) to
> replace system libraries.  Since the file is truncated first, all
> relocations and global data are replaced by file contents, result in
> difficult-to-diagnose crashes.  It would be nice if we had a way to
> prevent this mistake.  It doesn't have to be MAP_DENYWRITE or MAP_COPY.
> It could be something completely new, like an option that turns every
> future access beyond the truncation point into a signal (rather than
> getting bad data or bad code and crashing much later).
> 
> I don't know how many invalid copy operations are currently thwarted by
> the current program interpreter restriction.  I doubt that lifting the
> restriction matters.
> 
>> 3. We always deny write access to the file linked via /proc/pid/exe:
>>     sys_prctl(PR_SET_MM_EXE_FILE) will fail if write access to the file
>>     cannot be denied, and write access to the file will remain denied
>>     until the link is effectivel gone (exec, termination,
>>     PR_SET_MM_EXE_FILE) -- just as if exec'ing the file.
>>
>> I was wondering if we really care about permanently disabling write access
>> to the executable, or if it would be good enough to just disable write
>> access while loading the new executable during exec; but I don't know
>> the history of that -- and it somewhat makes sense to deny write access
>> at least to the main executable. With modern user space -- dlopen() -- we
>> can effectively modify the content of shared libraries while being used.
> 
> Is there a difference between ET_DYN and ET_EXEC executables?

No, I don't think so. When exec'ing, the main executable will see a 
deny_write_access(file); AFAIKT, that can either be ET_DYN or ET_EXEC.

-- 
Thanks,

David / dhildenb