linux-kernel - Re: [PATCH v2 3/5] mm: Make PR_MDWE_REFUSE_EXEC

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Message-ID: <ZGzJNvKu8h5nDXsa@arm.com>
Date:   Tue, 23 May 2023 15:09:58 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Alexey Izbyshev <izbyshev@...ras.ru>
Cc:     David Hildenbrand <david@...hat.com>,
        Florent Revest <revest@...omium.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org, anshuman.khandual@....com,
        joey.gouly@....com, mhocko@...e.com, keescook@...omium.org,
        peterx@...hat.com, broonie@...nel.org, szabolcs.nagy@....com,
        kpsingh@...nel.org, gthelen@...gle.com, toiwoton@...il.com
Subject: Re: [PATCH v2 3/5] mm: Make PR_MDWE_REFUSE_EXEC_GAIN an unsigned long

On Tue, May 23, 2023 at 04:25:45PM +0300, Alexey Izbyshev wrote:
> On 2023-05-23 16:07, Catalin Marinas wrote:
> > On Tue, May 23, 2023 at 11:12:37AM +0200, David Hildenbrand wrote:
> > > Also, how is passing "0"s to e.g., PR_GET_THP_DISABLE reliable? We
> > > need arg2
> > > -> arg5 to be 0. But wouldn't the following also just pass a 0 "int" ?
> > > 
> > > prctl(PR_GET_THP_DISABLE, 0, 0, 0, 0)
> > > 
> > > I'm easily confused by such (va_args) things, so sorry for the dummy
> > > questions.
> > 
> > Isn't the prctl() prototype in the user headers defined with the first
> > argument as int while the rest as unsigned long? At least from the man
> > page:
> > 
> > int prctl(int option, unsigned long arg2, unsigned long arg3,
> > 	  unsigned long arg4, unsigned long arg5);
> > 
> > So there are no va_args tricks (which confuse me as well).
> > 
> I have explicitly mentioned the problem with man pages in my response to
> David[1]. Quoting myself:
> 
> > This stuff *is* confusing, and note that Linux man pages don't even tell
> that prctl() is actually declared as a variadic function (and for
> ptrace() this is mentioned only in the notes, but not in its signature).

Ah, thanks for the clarification (I somehow missed your reply).

> The reality:
> 
> * glibc: https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/sys/prctl.h;h=821aeefc1339b35210e8918ecfe9833ed2792626;hb=glibc-2.37#l42
> 
> * musl:
> https://git.musl-libc.org/cgit/musl/tree/include/sys/prctl.h?h=v1.2.4#n180
> 
> Though there is a test in the kernel that does define its own prototype,
> avoiding the issue: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/testing/selftests/sched/cs_prctl_test.c?h=v6.3#n77

At least for glibc, it seems that there is a conversion to unsigned
long:

https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/prctl.c#l28

unsigned long int arg2 = va_arg (arg, unsigned long int);

(does va_arg expand to an actual cast?)

If the libc passes a 32-bit to a kernel ABI that expects 64-bit, I think
it's a user-space bug and not a kernel ABI issue.

-- 
Catalin