From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 D46CC3A5448 for ; Sat, 4 Jul 2026 11:53:31 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783166013; cv=none; b=LX1En4BLX95m3JcdupC8lR+ZRsyECW1YnKlsYcd+RG1o8eF4JDs6Nytss8wMmqF8/btPwxSmrAG6tuGKojVPqDQNCP2xuGLRMohPVoKhDZRrRmEM1tS/8Hti7RZ1ttIe7mzxgbWohhxH010S/fiuXGfbfV1e9bKIcVGJn34iSvA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783166013; c=relaxed/simple; bh=RQ+vsecEE5yyCp7ZAdy8DeUQmsLKawQ9CKv7BR32V/A=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=A85+ALq2Hh+R3/RCMbOOOZVMP5FqoT4tqa1oVJ86vGAO8JpdjKvghMQYZtZ+B47kzyFOt+SaKGzox1/oPZPHvWH+RtimisxfU16i8Y/09phYLmgIlIYxKxF49TTuP410NZ2MEQXCepEvziHC/xk1kHAbEKWh2KhENUzRzqodvPU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=JtEZxIrs; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="JtEZxIrs" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 295711F000E9; Sat, 4 Jul 2026 11:53:31 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1783166011; bh=y9OqPCUBS6LD87XT6MmLVjidTTqrRy0t+vu1By8POKE=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=JtEZxIrs5hb80oqY1j90xPg0tjLRCdDkyQ6HcpZhpErSt3GqFcfoUU1z9YPb70p4U 66SCvgPnbYGWt50tVlhpXRL6LEedIZ+RnmdDyyqtSDqcD6Nlyd3WU/5KY48IQnppjl U6Zuf0IkS3Bn5KbaWFdw5TBj79XM7q0nErMp7d/eGypjV3I4fudULCjomcZdIvVLwQ 7NZ0GygLdbhYhrs1AzpNPD5nritGjDW3A2Qnba+I+R69CubHum0A4aA3RPl4mYouvD p5mldVYECD6zhJxmor4Q4/JryzEqWEhssTa0d3r64iblDhnIUkeSXLfARhaCY3U2bF paECOSUCECKUw== From: Sasha Levin To: stable@vger.kernel.org Cc: Sean Christopherson , Paolo Bonzini , Sasha Levin Subject: [PATCH 5.15.y] KVM: Replace guest-triggerable BUG_ON() in ioeventfd datamatch with get_unaligned() Date: Sat, 4 Jul 2026 07:53:29 -0400 Message-ID: <20260704115329.624939-1-sashal@kernel.org> X-Mailer: git-send-email 2.53.0 In-Reply-To: <2026070233-cytoplast-likewise-c9f6@gregkh> References: <2026070233-cytoplast-likewise-c9f6@gregkh> Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: Sean Christopherson [ Upstream commit f1edbed787ba67988ed34e0132ca128b052b6ce8 ] Drop a BUG_ON() that has been reachable since it was first added, way back in 2009, and instead use get_unaligned() to perform potentially-unaligned accesses. For a given store, KVM x86's emulator tracks the entire value in the destination operand, x86_emulate_ctxt.dst. If the destination is memory, and the target splits multiple pages and/or is emulated MMIO, then KVM handles each fragment independently. E.g. on a page split starting at page offset 0xffc, KVM writes 4 bytes to the first page, then the remaining bytes to the second page, using ctxt->dst as the source for both (with appropriate offsets). If the destination splits a page *and* hits emulated MMIO on the second page, then KVM will complete the write to the first page, then emulate the MMIO access to the second page. If there is a datamatch-enabled ioeventfd at offset 0 of the second page, then KVM will process the remainder of the store as a potential ioeventfd signal. Putting it all together, if the guest emits a store that splits a page starting at page offset N, and the second page has a datamatch-enabled ioeventfd at offset 0, then KVM will check for datamatch using &dst.valptr[N] as the source. Due to dst (and thus dst.valptr) being 32-byte aligned, if N is not aligned to @len, the BUG_ON() fires. E.g. with a 16-byte store at page offset 0xffc, to an ioeventfd of len 8, all initial checks in ioeventfd_in_range() will succeed, and the BUG_ON() fires due to @val being 4-byte aligned, but not 8-byte aligned. ------------[ cut here ]------------ kernel BUG at arch/x86/kvm/../../../virt/kvm/eventfd.c:783! Oops: invalid opcode: 0000 [#1] SMP CPU: 0 UID: 1000 PID: 615 Comm: repro Not tainted 7.1.0-rc2-ff238429d1ea #365 PREEMPT Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015 RIP: 0010:ioeventfd_write+0x6c/0x70 [kvm] Call Trace: __kvm_io_bus_write+0x85/0xb0 [kvm] kvm_io_bus_write+0x53/0x80 [kvm] vcpu_mmio_write+0x66/0xf0 [kvm] emulator_read_write_onepage+0x12a/0x540 [kvm] emulator_read_write+0x109/0x2b0 [kvm] x86_emulate_insn+0x4f8/0xfb0 [kvm] x86_emulate_instruction+0x181/0x790 [kvm] kvm_mmu_page_fault+0x313/0x630 [kvm] vmx_handle_exit+0x18a/0x590 [kvm_intel] kvm_arch_vcpu_ioctl_run+0xc81/0x1c90 [kvm] kvm_vcpu_ioctl+0x2d5/0x970 [kvm] __x64_sys_ioctl+0x8a/0xd0 do_syscall_64+0xb7/0x890 entry_SYSCALL_64_after_hwframe+0x4b/0x53 RIP: 0033:0x7f19c931a9bf Modules linked in: kvm_intel kvm irqbypass ---[ end trace 0000000000000000 ]--- In a perfect world, the fix would be to simply delete the BUG_ON(), as KVM x86 doesn't perform alignment checks on "normal" memory accesses at CPL0. Sadly, C99 ruins all the fun; while the x86 architecture plays nice, dereferencing an unaligned pointer directly is undefined behavior in C, e.g. triggers splats when running with CONFIG_UBSAN_ALIGNMENT=y. Fixes: d34e6b175e61 ("KVM: add ioeventfd support") Cc: stable@vger.kernel.org Signed-off-by: Sean Christopherson Message-ID: <20260612225241.678509-1-seanjc@google.com> Signed-off-by: Paolo Bonzini [ Adjusted context to use `#include ` since `linux/unaligned.h` doesn't exist in 6.6. ] Signed-off-by: Sasha Levin --- virt/kvm/eventfd.c | 12 +++++------- 1 file changed, 5 insertions(+), 7 deletions(-) diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c index c841f8bc8228a1..b78a82f6dbbbd0 100644 --- a/virt/kvm/eventfd.c +++ b/virt/kvm/eventfd.c @@ -24,6 +24,7 @@ #include #include #include +#include #include #include @@ -710,21 +711,18 @@ ioeventfd_in_range(struct _ioeventfd *p, gpa_t addr, int len, const void *val) return true; /* otherwise, we have to actually compare the data */ - - BUG_ON(!IS_ALIGNED((unsigned long)val, len)); - switch (len) { case 1: - _val = *(u8 *)val; + _val = get_unaligned((u8 *)val); break; case 2: - _val = *(u16 *)val; + _val = get_unaligned((u16 *)val); break; case 4: - _val = *(u32 *)val; + _val = get_unaligned((u32 *)val); break; case 8: - _val = *(u64 *)val; + _val = get_unaligned((u64 *)val); break; default: return false; -- 2.53.0