linux-kernel - Re: [PATCH] exitz syscall

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Date:   Mon, 13 Nov 2023 08:37:04 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Theodore Ts'o <tytso@....edu>, Willy Tarreau <w@....eu>
Cc:     York Jasper Niebuhr <yjnworkstation@...il.com>,
        akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        linux-api@...r.kernel.org, linux-security-module@...r.kernel.org,
        torvalds@...ux-foundation.org
Subject: Re: [PATCH] exitz syscall

On 11/12/23 05:52, Theodore Ts'o wrote:
> On Sat, Nov 11, 2023 at 02:24:31PM +0100, Willy Tarreau wrote:
>> Hello,
>> 
>> On Sat, Nov 11, 2023 at 01:51:26PM +0100, York Jasper Niebuhr wrote:
>> > Adds a system call to flag a process' resources to be cleared on
>> > exit (or, in the case of memory, on free). Currently, only zeroing
>> > memory is implemented.
>> (...)
>> 
>> IMHO it does not make sense to add a syscall for this, please have a
>> look at prctl(2) instead, which is already used for similar settings.
> 
> Another reason to use prctl() is there are other cases when you'd want
> to zero a process's memory.  For example, if the process gets killed
> to some kind of signal, or when it gets OOM killed (where there is no
> system call which forces the process to exit).  Also, if you want to
> zero memory when the process exits, you'd want to zero the process
> memory on an exec(2).

Probably also munmap() and maybe a number of other ways where the process
can give up its memory voluntarily. Then there are also involuntary ways
where the a copy of the data can end up leaking elsewhere than the pages the
process has mapped - e.g. swapout/swapin of pages, page migration...

So I'm not sure it's feasible to attempt making a whole process "sensitive"
and close all the holes. Instead what we have is to mark specific areas as
sensitive - things like mlock(), madvise(MADV_DONTDUMP / MADV_DONTFORK) and
ultimately memfd_secret().

> Cheers,
> 
> 						- Ted
>