[PATCH v2] KVM: Replace guest-triggerable BUG_ON() in ioeventfd datamatch with get_unaligned()

[PATCH v2] KVM: Replace guest-triggerable BUG_ON() in ioeventfd datamatch with get_unaligned()

[Sean Christopherson]

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>
---

v2: Use get_unaligned() to make UBSAN happy. [Sashiko]

v1: https://lore.kernel.org/all/20260527002910.3931382-1-seanjc@google.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 0e8b8a2c5b79..93ad2ebc963f 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 <linux/unaligned.h>
 #include <trace/events/kvm.h>
 
 #include <kvm/iodev.h>
@@ -779,21 +780,18 @@ 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;

base-commit: c1f7303302927f9cbf4efedf70f0512cde168c65
-- 
2.54.0.1136.gdb2ca164c4-goog

Re: [PATCH v2] KVM: Replace guest-triggerable BUG_ON() in ioeventfd datamatch with get_unaligned()

[Paolo Bonzini]

Queued, thanks.

Paolo