From: Sean Christopherson <seanjc@google.com> stable inclusion from stable-v6.6.145 commit 92fc631b69deb1c7d56aec2663003600799dcd75 category: bugfix bugzilla: https://atomgit.com/src-openeuler/kernel/issues/16137 CVE: CVE-2026-63806 Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=... -------------------------------- [ 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: <TASK> __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 </TASK> 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 <seanjc@google.com> Message-ID: <20260612225241.678509-1-seanjc@google.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> [ Adjusted context to use `#include <asm/unaligned.h>` since `linux/unaligned.h` doesn't exist in 6.6. ] Signed-off-by: Sasha Levin <sashal@kernel.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Signed-off-by: Yi Yang <yiyang13@huawei.com> --- 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 1a32da5343e8..6ac7284ac5a2 100644 --- a/virt/kvm/eventfd.c +++ b/virt/kvm/eventfd.c @@ -24,6 +24,7 @@ #include <linux/slab.h> #include <linux/seqlock.h> #include <linux/irqbypass.h> +#include <asm/unaligned.h> #include <trace/events/kvm.h> #include <kvm/iodev.h> @@ -782,21 +783,18 @@ 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; -- 2.25.1