FAILED: patch "[PATCH] KVM: Replace guest-triggerable BUG_ON() in ioeventfd" failed to apply to 6.1-stable tree

PATCH New [email protected] 2 participants 2 messages Jul 2 – Jul 4

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Stable branch KVM

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[email protected]

author maintainer

2026-07-02 13:15 UTC #0 lore raw

The patch below does not apply to the 6.1-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.1.y
git checkout FETCH_HEAD
git cherry-pick -x f1edbed787ba67988ed34e0132ca128b052b6ce8
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026070241-manager-petted-4ccd@gregkh' --subject-prefix 'PATCH 6.1.y' 'HEAD^..'

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From f1edbed787ba67988ed34e0132ca128b052b6ce8 Mon Sep 17 00:00:00 2001
From: Sean Christopherson <[email protected]>
Date: Fri, 12 Jun 2026 15:52:41 -0700
Subject: [PATCH] KVM: Replace guest-triggerable BUG_ON() in ioeventfd
datamatch with get_unaligned()

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 [email protected]

Signed-off-by Sean Christopherson <[email protected]>

Message-ID: <[email protected]>

Signed-off-by Paolo Bonzini <[email protected]>

virt/kvm/eventfd.c +5 -7

index 0e8b8a2c5b79..93ad2ebc963f 100644
@@ -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;

Sasha Levin

maintainer

2026-07-04 11:49 UTC #1 lore raw

PATCH 1/1 KVM: Replace guest-triggerable BUG_ON() in ioeventfd datamatch with get_unaligned()

From: Sean Christopherson <[email protected]>

[ Upstream commit f1edbed787ba67988ed34e0132ca128b052b6ce8 ]