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Donenfeld" , Andrew Morton Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, kasan-dev@googlegroups.com, John Ogness , Alexander Potapenko , Dmitry Vyukov , Geert Uytterhoeven , Geert Uytterhoeven Subject: Re: [PATCH v2] mm/kfence: select random number before taking raw lock Message-ID: References: <20220609123319.17576-1-Jason@zx2c4.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220609123319.17576-1-Jason@zx2c4.com> User-Agent: Mutt/2.1.4 (2021-12-11) ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1654778525; a=rsa-sha256; cv=none; b=wHhWNWY1VJ3VjclW1wqQ+bNLnl5sReEdxf0DdkubIEuqJbInO5EU6cow9K7u1U5QKTQY1S tp/cif+rF3OA/K9TfT2+SKjw7vTmr4Z8+s3k/m3dU8Pv/3SiSNmCQq6kR0bWGvKx2FbJhX eSX44neuqLRbZpr8euX7ESXByy2tDgA= ARC-Authentication-Results: i=1; imf04.hostedemail.com; dkim=pass header.d=google.com header.s=20210112 header.b=gtgWlr41; spf=pass (imf04.hostedemail.com: domain of elver@google.com designates 209.85.221.48 as permitted sender) smtp.mailfrom=elver@google.com; dmarc=pass (policy=reject) header.from=google.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1654778525; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=arApoAXBepq4URi/Em9A+yWgPjBE5I/lG8/rZgaEm2s=; b=TcBAWtqpJMfBrXlhMM7kOqIOLHvy96QMU/CCym0l7vzcrG1CHeSXhHUsXzVCIrjRT0hH9h zT7n+eeWJBcx+7vQajNHl0esjw2xVztABwgDS2vBoOA5i4TM0IqfjNrNvXFpkTo47Yg/en 1j8x4FI5ATElfzLkJCdzf7hM+vSQg34= X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: AC4AF40068 X-Stat-Signature: keg78oaaydxbgdk3sur18byjx3cqp5r9 X-Rspam-User: Authentication-Results: imf04.hostedemail.com; dkim=pass header.d=google.com header.s=20210112 header.b=gtgWlr41; spf=pass (imf04.hostedemail.com: domain of elver@google.com designates 209.85.221.48 as permitted sender) smtp.mailfrom=elver@google.com; dmarc=pass (policy=reject) header.from=google.com X-HE-Tag: 1654778525-961195 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Thu, Jun 09, 2022 at 02:33PM +0200, Jason A. Donenfeld wrote: > The RNG uses vanilla spinlocks, not raw spinlocks, so kfence should pick > its random numbers before taking its raw spinlocks. This also has the > nice effect of doing less work inside the lock. It should fix a splat > that Geert saw with CONFIG_PROVE_RAW_LOCK_NESTING: > > dump_backtrace.part.0+0x98/0xc0 > show_stack+0x14/0x28 > dump_stack_lvl+0xac/0xec > dump_stack+0x14/0x2c > __lock_acquire+0x388/0x10a0 > lock_acquire+0x190/0x2c0 > _raw_spin_lock_irqsave+0x6c/0x94 > crng_make_state+0x148/0x1e4 > _get_random_bytes.part.0+0x4c/0xe8 > get_random_u32+0x4c/0x140 > __kfence_alloc+0x460/0x5c4 > kmem_cache_alloc_trace+0x194/0x1dc > __kthread_create_on_node+0x5c/0x1a8 > kthread_create_on_node+0x58/0x7c > printk_start_kthread.part.0+0x34/0xa8 > printk_activate_kthreads+0x4c/0x54 > do_one_initcall+0xec/0x278 > kernel_init_freeable+0x11c/0x214 > kernel_init+0x24/0x124 > ret_from_fork+0x10/0x20 > > Cc: John Ogness > Cc: Alexander Potapenko > Cc: Marco Elver > Cc: Dmitry Vyukov > Reported-by: Geert Uytterhoeven > Tested-by: Geert Uytterhoeven > Signed-off-by: Jason A. Donenfeld Reviewed-by: Marco Elver Thank you. > --- > Changes v1->v2: > - Make the bools const to help compiler elide branch when possible, > suggested by Marco. > > mm/kfence/core.c | 7 +++++-- > 1 file changed, 5 insertions(+), 2 deletions(-) > > diff --git a/mm/kfence/core.c b/mm/kfence/core.c > index 4e7cd4c8e687..4b5e5a3d3a63 100644 > --- a/mm/kfence/core.c > +++ b/mm/kfence/core.c > @@ -360,6 +360,9 @@ static void *kfence_guarded_alloc(struct kmem_cache *cache, size_t size, gfp_t g > unsigned long flags; > struct slab *slab; > void *addr; > + const bool random_right_allocate = prandom_u32_max(2); > + const bool random_fault = CONFIG_KFENCE_STRESS_TEST_FAULTS && > + !prandom_u32_max(CONFIG_KFENCE_STRESS_TEST_FAULTS); > > /* Try to obtain a free object. */ > raw_spin_lock_irqsave(&kfence_freelist_lock, flags); > @@ -404,7 +407,7 @@ static void *kfence_guarded_alloc(struct kmem_cache *cache, size_t size, gfp_t g > * is that the out-of-bounds accesses detected are deterministic for > * such allocations. > */ > - if (prandom_u32_max(2)) { > + if (random_right_allocate) { > /* Allocate on the "right" side, re-calculate address. */ > meta->addr += PAGE_SIZE - size; > meta->addr = ALIGN_DOWN(meta->addr, cache->align); > @@ -444,7 +447,7 @@ static void *kfence_guarded_alloc(struct kmem_cache *cache, size_t size, gfp_t g > if (cache->ctor) > cache->ctor(addr); > > - if (CONFIG_KFENCE_STRESS_TEST_FAULTS && !prandom_u32_max(CONFIG_KFENCE_STRESS_TEST_FAULTS)) > + if (random_fault) > kfence_protect(meta->addr); /* Random "faults" by protecting the object. */ > > atomic_long_inc(&counters[KFENCE_COUNTER_ALLOCATED]); > -- > 2.35.1 >