linux-kernel - Re: [PATCH v2 06/17] vdso: Change getrandom's generation to unsigned long

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Message-ID: <ZsxPUXq0qo4MPDbW@zx2c4.com>
Date: Mon, 26 Aug 2024 11:48:01 +0200
From: "Jason A. Donenfeld" <Jason@...c4.com>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: Christophe Leroy <christophe.leroy@...roup.eu>,
	Michael Ellerman <mpe@...erman.id.au>,
	Nicholas Piggin <npiggin@...il.com>,
	Naveen N Rao <naveen@...nel.org>, Andy Lutomirski <luto@...nel.org>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>, Theodore Ts'o <tytso@....edu>,
	Arnd Bergmann <arnd@...db.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Vincenzo Frascino <vincenzo.frascino@....com>,
	Shuah Khan <shuah@...nel.org>, linuxppc-dev@...ts.ozlabs.org,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-arch@...r.kernel.org, linux-mm@...ck.org,
	linux-trace-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v2 06/17] vdso: Change getrandom's generation to unsigned
 long

On Mon, Aug 26, 2024 at 11:43:39AM +0200, Thomas Gleixner wrote:
> As explained before, there is no problem with store or load tearing on
> 32bit systems because the generation counter is only 32bit wide. So the
> obvious solution is to only update 32 bits on a 32bit kernel:
> 
> --- a/drivers/char/random.c
> +++ b/drivers/char/random.c
> @@ -282,7 +282,7 @@ static void crng_reseed(struct work_stru
>  	 * is ordered with the write above to base_crng.generation. Pairs with
>  	 * the smp_rmb() before the syscall in the vDSO code.
>  	 */
> -	smp_store_release(&_vdso_rng_data.generation, next_gen + 1);
> +	smp_store_release((unsigned long *)&_vdso_rng_data.generation, next_gen + 1);
>  #endif
>  	if (!static_branch_likely(&crng_is_ready))
>  		crng_init = CRNG_READY;

That seems like a pretty clean fix.

> But that's a trivial fix compared to making VM_DROPPABLE work on 32-bit
> correclty. :)

My initial response too, and then I noticed he posted this:
https://lore.kernel.org/all/315e3a268b165b6edad7dcb723b0d8a506a56c4e.1724309198.git.christophe.leroy@csgroup.eu/
If that's correct, maybe it's not so bad, at least here. I haven't yet
looked into the details of it.

Jason