From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-182.mta0.migadu.com (out-182.mta0.migadu.com [91.218.175.182]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id A53E93176EF for ; Wed, 10 Jun 2026 21:55:51 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.182 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781128555; cv=none; b=CZbPK0qT0dpKHv9SgIxgeg45laLIUniixXb8glkZFEINOnKiinmFGWz8BKvHZYRkDMn8kI/thNTsmy+D9wY5cuEis6BbKPDS9ki9aS1lVfYuulmUUE+rP3n1TGb3wJ5HyLih61ROn2UG0osFPZWi4285ie3M1FonYMGiia307Yk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781128555; c=relaxed/simple; bh=Xd+iV7Z7Ts9EpzVEc30Xw8zHpduDF5PdUnmNqKlYDRM=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=SnxJkjINl7o5m1UkrLYfYyRYghFTZaIJ/txyIG+TB4dJXE2E3xwFekCYbgnN6ofR35VyRxVw7tS2d61jYZCQwREzuY/Dn3UqnETG2Nv/SWA5vJ2RxfLJ/L+vohBUpipQwroabGto+dnlrBjHKvVsLNqHSi+aPgw5R5ceUcSaMW8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=NZx5kQWa; arc=none smtp.client-ip=91.218.175.182 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="NZx5kQWa" Message-ID: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1781128549; h=from:from: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:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=8Xq3T8jGSwlCoS9TBcLots22CwpjKVQ5tiqS1ynXAs0=; b=NZx5kQWaP3w/38qQC11nz4BQvD9L76TDbmaNJ5n5o5iZ5epV6H+HYWaKJLB+m6YcrolrNB pxs2I/GGTwF6LVABh2E04hBajrRJpfvEXIbfWaB/Qt1H9MgDMg5mbL4syBW/TsJew6bs+m 9M7v8eu16pO4qVkBmGyl78S+X4kc2No= Date: Wed, 10 Jun 2026 14:55:42 -0700 Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH v1] kasan: Fix false-positive wild-memory-access on x86 under 5-level paging To: Andrey Konovalov Cc: Alexei Starovoitov , Andrii Nakryiko , Daniel Borkmann , Eduard Zingerman , Kumar Kartikeya Dwivedi , Andrey Ryabinin , Andrew Morton , bpf@vger.kernel.org, kasan-dev@googlegroups.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20260610175651.647515-1-ihor.solodrai@linux.dev> Content-Language: en-US X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Ihor Solodrai In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT On 6/10/26 11:39 AM, Andrey Konovalov wrote: > On Wed, Jun 10, 2026 at 7:57 PM Ihor Solodrai wrote: >> >> On x86_64 with 5-level paging (LA57) and inline generic KASAN, the >> following flaky splat may be observed on boot: >> >> BUG: KASAN: wild-memory-access in do_raw_spin_lock+0xcf/0x260 >> Write of size 4 at addr ff110001000c90b8 by task swapper/0/0 >> >> CPU: 0 UID: 0 PID: 0 Comm: swapper/0 Not tainted 7.1.0-rc5-gcba33e0b2907 #1 PREEMPT(full) >> Hardware name: QEMU Ubuntu 24.04 PC v2 (i440FX + PIIX, arch_caps fix, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014 >> Call Trace: >> >> dump_stack_lvl+0x54/0x70 >> kasan_report+0x117/0x150 >> ? do_raw_spin_lock+0xcf/0x260 >> kasan_check_range+0x264/0x2c0 >> do_raw_spin_lock+0xcf/0x260 >> handle_edge_irq+0x35/0x770 >> ? do_raw_spin_unlock+0x51/0x2a0 >> __common_interrupt+0xae/0x120 >> common_interrupt+0x7c/0x90 >> >> >> asm_common_interrupt+0x26/0x40 >> RIP: 0010:identify_cpu+0x2b2/0x3460 >> Code: 00 41 c7 07 00 00 00 00 4d 89 e6 49 c1 ee 03 43 0f b6 04 06 84 c0 0f 85 a3 1c 00 00 41 c7 04 24 00 00 00 00 31 c0 31 c9 0f a2 <89> c7 42 0f b6 44 05 00 84 c0 0f 85 ad 1c 00 00 41 89 3f 48 8b 44 >> RSP: 0000:ffffffff97807df0 EFLAGS: 00000246 >> RAX: 0000000000000020 RBX: 00000000756e6547 RCX: 000000006c65746e >> RDX: 0000000049656e69 RSI: 0000000000000000 RDI: ffffffff98632fd8 >> RBP: 1ffffffff30c65fc R08: dffffc0000000000 R09: 0000000000000004 >> R10: ffffffff98632fc4 R11: fffffbfff30c65fb R12: ffffffff98633050 >> R13: ffffffff98633048 R14: 1ffffffff30c660a R15: ffffffff98632fe0 >> identify_boot_cpu+0xd/0xd0 >> arch_cpu_finalize_init+0x24/0x1f0 >> start_kernel+0x31e/0x3e0 >> x86_64_start_reservations+0x24/0x30 >> x86_64_start_kernel+0x13a/0x140 >> common_startup_64+0x12c/0x137 >> >> >> It fires very early in boot. If kasan_multi_shot is set, the reports >> are non-fatal and keep repeating, and the boot CPU wedges before >> userspace is reached. The accessed addresses are valid 5-level kernel >> pointers, so the report is a false positive. >> >> The root cause is in generic KASAN not seeing >> cpu_feature_enabled(X86_FEATURE_LA57) set, because the bit is cleared >> in identify_cpu() when the offending interrupt happens [1]: >> >> memset(&c->x86_capability, 0, ...); /* clears X86_FEATURE_LA57 */ >> ... >> get_cpu_cap(c); /* re-reads CPUID, restores it */ >> >> addr_has_metadata() then uses the 4-level threshold, and 5-level >> kernel addresses fall below it, so kasan_check_range() reports them as >> wild-memory-access. >> >> Define USE_EARLY_PGTABLE_L5 in mm/kasan/generic.c so >> addr_has_metadata() uses the stable variable, as >> arch/x86/mm/kasan_init_64.c already does. > > It took me some digging to find which stable variable you refer to > here; I think this needs to be better explained. Hi Andrey, thank you for the quick review. Yeah, it's: unsigned int __section(".data") __pgtable_l5_enabled; I'll add more details in next revision. > >> >> Some context on how this was noticed and reproduced below. >> >> We started seeing flaky splats as above [2][3] on BPF CI runs after >> runner hardware has been upgraded. Specifically, new x86 runners are >> c7i.metal-24xl AWS EC2 instances, which are Intel Sapphire Rapids >> machines that support LA57 feature, and have it enabled. >> >> The splats can be reproduced with qemu on any x86_64 host with >> -cpu max -accel tcg >> >> Build a kernel with: >> CONFIG_KASAN=y >> CONFIG_KASAN_GENERIC=y >> CONFIG_KASAN_INLINE=y >> >> Boot it with kasan_multi_shot. The fault fires fast before userspace, >> so no rootfs is required. For example: >> >> qemu-system-x86_64 -display none -serial stdio -no-reboot \ >> -smp 4 -m 5G -cpu max -accel tcg \ >> -kernel arch/x86/boot/bzImage \ >> -append "console=ttyS0,115200 earlyprintk=serial,0,115200 panic=-1 kasan_multi_shot nokaslr" >> >> It's a timing race, so a single boot hits it only sometimes. >> However running several qemu instances in parallel on the same host >> significantly increases the hitrate. >> >> I confirmed the proposed fix eliminates the splats. >> >> [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/kernel/cpu/common.c?h=v7.1-rc7#n2001 >> [2] https://github.com/kernel-patches/bpf/actions/runs/27271262414/job/80542509369 >> [3] https://github.com/kernel-patches/bpf/actions/runs/27260143782/job/80505353689 >> >> Signed-off-by: Ihor Solodrai >> --- >> mm/kasan/generic.c | 7 +++++++ >> 1 file changed, 7 insertions(+) >> >> diff --git a/mm/kasan/generic.c b/mm/kasan/generic.c >> index 2b8e73f5f6a7..b5f430f2dbb6 100644 >> --- a/mm/kasan/generic.c >> +++ b/mm/kasan/generic.c >> @@ -9,6 +9,13 @@ >> * Andrey Konovalov >> */ >> >> +/* >> + * check_region_inline() and addr_has_metadata() can run very early. >> + * For example, in an interrupt taken while identify_cpu() has the CPU >> + * capability bits temporarily cleared. >> + */ >> +#define USE_EARLY_PGTABLE_L5 > > There are other users of addr_has_metadata(), besides ones in generic.c. > > Thus, should we place this definition into mm/kasan/kasan.h? And > likely guard it with ifdef KASAN_GENERIC || KASAN_SW_TAGS (HW_TAGS > should not be affected by this AFAIU). It appears setting USE_EARLY_PGTABLE_L5 in kasan.h will not work. USE_EARLY_PGTABLE_L5 needs to be set before arch/x86/include/asm/pgtable_64_types.h is included, which is very early. For example, it arrives to kasan/generic.c through a long path starting at linux/interrupt.h: In file included from ./arch/x86/include/asm/pgtable_types.h:277, from ./arch/x86/include/asm/processor.h:21, from ./arch/x86/include/asm/cpufeature.h:5, from ./arch/x86/include/asm/thread_info.h:59, from ./include/linux/thread_info.h:62, from ./include/linux/smp.h:119, from ./include/linux/alloc_tag.h:14, from ./include/linux/percpu.h:5, from ./include/linux/context_tracking_state.h:5, from ./include/linux/hardirq.h:5, from ./include/linux/interrupt.h:11, from mm/kasan/generic.c:13: So if we want to cover all users of addr_has_metadata(), we'll have to add #define USE_EARLY_PGTABLE_L5 in multiple files: generic.c, report.c, report_generic.c, maybe others? Any other ideas? I determined the include path like follows: cd linux # at 9716c086c8e8 torvalds/master make defconfig ./scripts/config -e KASAN -e KASAN_GENERIC -e KASAN_INLINE make olddefconfig make mm/kasan/generic.i # then modify pgtable_64_types.h diff --git a/arch/x86/include/asm/pgtable_64_types.h b/arch/x86/include/asm/pgtable_64_types.h index 7eb61ef6a185..c6b754143fb4 100644 --- a/arch/x86/include/asm/pgtable_64_types.h +++ b/arch/x86/include/asm/pgtable_64_types.h @@ -1,6 +1,7 @@ /* SPDX-License-Identifier: GPL-2.0 */ #ifndef _ASM_X86_PGTABLE_64_DEFS_H #define _ASM_X86_PGTABLE_64_DEFS_H +#error TRACE #include And then: eval "$(grep '^savedcmd_' mm/kasan/.generic.i.cmd | head -1 | sed 's/^savedcmd_[^ ]* := //')" > > And let's also extend the comment to explain better what's going on. I > like the way the issue is explained here: > > https://elixir.bootlin.com/linux/v7.0.12/source/arch/x86/boot/startup/sme.c#L36 ok > >> + >> #include >> #include >> #include >> -- >> 2.54.0 >> >> -- >> You received this message because you are subscribed to the Google Groups "kasan-dev" group. >> To unsubscribe from this group and stop receiving emails from it, send an email to kasan-dev+unsubscribe@googlegroups.com. >> To view this discussion visit https://groups.google.com/d/msgid/kasan-dev/20260610175651.647515-1-ihor.solodrai%40linux.dev.