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Message-ID: <15628c8a-9c71-5611-2edf-07087ad662b7@redhat.com>
Date: Thu, 12 Aug 2021 21:38:26 +0200
From: David Hildenbrand <david@...hat.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
"H. Peter Anvin" <hpa@...or.com>,
Alexander Viro <viro@...iv.linux.org.uk>,
Alexey Dobriyan <adobriyan@...il.com>,
Steven Rostedt <rostedt@...dmis.org>,
Peter Zijlstra <peterz@...radead.org>,
Arnaldo Carvalho de Melo <acme@...nel.org>,
Mark Rutland <mark.rutland@....com>,
Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
Jiri Olsa <jolsa@...hat.com>,
Namhyung Kim <namhyung@...nel.org>,
Petr Mladek <pmladek@...e.com>,
Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
Rasmus Villemoes <linux@...musvillemoes.dk>,
Kees Cook <keescook@...omium.org>,
"Eric W. Biederman" <ebiederm@...ssion.com>,
Greg Ungerer <gerg@...ux-m68k.org>,
Geert Uytterhoeven <geert@...ux-m68k.org>,
Mike Rapoport <rppt@...nel.org>,
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Chinwen Chang <chinwen.chang@...iatek.com>,
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Catalin Marinas <catalin.marinas@....com>,
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Suren Baghdasaryan <surenb@...gle.com>,
Shakeel Butt <shakeelb@...gle.com>,
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Daniel Jordan <daniel.m.jordan@...cle.com>,
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Michal Hocko <mhocko@...e.com>,
Miklos Szeredi <miklos@...redi.hu>,
Chengguang Xu <cgxu519@...ernel.net>,
Christian König <ckoenig.leichtzumerken@...il.com>,
linux-unionfs@...r.kernel.org,
Linux API <linux-api@...r.kernel.org>,
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Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH v1 3/7] kernel/fork: always deny write access to current
MM exe_file
On 12.08.21 18:51, Linus Torvalds wrote:
> On Wed, Aug 11, 2021 at 10:45 PM David Hildenbrand <david@...hat.com> wrote:
>>
>> /* No ordering required: file already has been exposed. */
>> - RCU_INIT_POINTER(mm->exe_file, get_mm_exe_file(oldmm));
>> + exe_file = get_mm_exe_file(oldmm);
>> + RCU_INIT_POINTER(mm->exe_file, exe_file);
>> + if (exe_file)
>> + deny_write_access(exe_file);
>
> Can we make a helper function for this, since it's done in two different places?
Sure, no compelling reason not to (except finding a suitable name, but
I'll think about that tomorrow).
>
>> - if (new_exe_file)
>> + if (new_exe_file) {
>> get_file(new_exe_file);
>> + /*
>> + * exec code is required to deny_write_access() successfully,
>> + * so this cannot fail
>> + */
>> + deny_write_access(new_exe_file);
>> + }
>> rcu_assign_pointer(mm->exe_file, new_exe_file);
>
> And the above looks positively wrong. The comment is also nonsensical,
> in that it basically says "we thought this cannot fail, so we'll just
> rely on it".
Well, it documents the expectation towards the caller, but in a
suboptimal way, I agree.
>
> If it truly cannot fail, then the comment should give the reason, not
> the "we depend on this not failing".
Right, "We depend on the caller already have done a deny_write_access()
successfully first such that this call cannot fail." combined with
if (deny_write_access(new_exe_file))
pr_warn("Unexpected failure of deny_write_access() in %s",
__func__);
suggestions welcome.
>
> And honestly, I don't see why it couldn't fail. And if it *does* fail,
> we cannot then RCU-assign the exe_file pointer with this, because
> you'll get a counter imbalance when you do the allow_write_access()
> later.
Anyone calling set_mm_exe_file() (-> begin_new_exec()) is expected to
successfully triggered a deny_write_access() upfront such that we won't
fail at that point.
Further, on the dup_mmap() path we are sure the previous oldmm exe_file
properly saw a successful deny_write_access() already, because that's
now guaranteed for any exe_file.
>
> Anyway, do_open_execat() does do deny_write_access() with proper error
> checking. I think that is the existing reference that you depend on -
> so that it doesn't fail. So the comment could possibly say that the
> only caller has done this, but can we not just use the reference
> deny_write_access() directly, and not do a new one here?
I think that might over-complicate the exec code where we would see a
allow_write_access() on error paths, but not on success paths. This here
looks cleaner to me, agreeing that the comment and the error check has
to be improved.
We handle all allow_write_access()/deny_write_access() regarding
exe_file completely in kernel/fork.c, which is IMHO quite nice.
>
> IOW, maybe there's an extraneous 'allow_write_access()' somewhere that
> should be dropped when we do the whole binprm dance in execve()?
fs/exec.c: free_bprm() and exec_binprm() to be precise.
Thanks!
--
Thanks,
David / dhildenb
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