linux-kernel - Re: [PATCH v6 1/3] random: add vgetrandom

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Message-Id: <7928bec7-4659-4b32-aad7-2300409ea670@app.fastmail.com>
Date:   Thu, 24 Nov 2022 14:18:03 +0100
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "Jason A . Donenfeld" <Jason@...c4.com>,
        "Florian Weimer" <fweimer@...hat.com>
Cc:     linux-kernel@...r.kernel.org, patches@...ts.linux.dev,
        "Thomas Gleixner" <tglx@...utronix.de>,
        linux-crypto@...r.kernel.org, x86@...nel.org,
        "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
        "Adhemerval Zanella Netto" <adhemerval.zanella@...aro.org>,
        "Carlos O'Donell" <carlos@...hat.com>, linux-api@...r.kernel.org
Subject: Re: [PATCH v6 1/3] random: add vgetrandom_alloc() syscall

On Thu, Nov 24, 2022, at 13:48, Jason A. Donenfeld wrote:
> On Thu, Nov 24, 2022 at 01:24:42PM +0100, Jason A. Donenfeld wrote:

> Looks like set_mempolicy, get_mempoliy, and migrate_pages pass an
> unsigned long pointer and I don't see any compat stuff around it:
>
>     SYSCALL_DEFINE3(set_mempolicy, int, mode, const unsigned long 
> __user *, nmask,
>                     unsigned long, maxnode)
>    
>     SYSCALL_DEFINE5(get_mempolicy, int __user *, policy,
>                     unsigned long __user *, nmask, unsigned long, maxnode,
>                     unsigned long, addr, unsigned long, flags)
>
>     SYSCALL_DEFINE4(migrate_pages, pid_t, pid, unsigned long, maxnode,
>                     const unsigned long __user *, old_nodes,
>                     const unsigned long __user *, new_nodes)

Compat handling for these is done all the way down in the
pointer access:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/mm/mempolicy.c#n1368

This works here because it's a special bitmap but is not the
best approach if you just have a pointer to a single value.

       Arnd