FAILED: Patch "KVM: x86: Add SRCU protection for reading PDPTRs in __get_sregs2()" failed to apply to 5.15-stable tree

[Sasha Levin <sashal@kernel.org>]

The patch below does not apply to the 5.15-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 <stable@vger.kernel.org>.

Thanks,
Sasha

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

From 95d848dc7e639988dbb385a8cba9b484607cf98c Mon Sep 17 00:00:00 2001
From: Vasiliy Kovalev <kovalev@altlinux.org>
Date: Sat, 24 Jan 2026 01:28:01 +0300
Subject: [PATCH] KVM: x86: Add SRCU protection for reading PDPTRs in
 __get_sregs2()

Add SRCU read-side protection when reading PDPTR registers in
__get_sregs2().

Reading PDPTRs may trigger access to guest memory:
kvm_pdptr_read() -> svm_cache_reg() -> load_pdptrs() ->
kvm_vcpu_read_guest_page() -> kvm_vcpu_gfn_to_memslot()

kvm_vcpu_gfn_to_memslot() dereferences memslots via __kvm_memslots(),
which uses srcu_dereference_check() and requires either kvm->srcu or
kvm->slots_lock to be held. Currently only vcpu->mutex is held,
triggering lockdep warning:

=============================
WARNING: suspicious RCU usage in kvm_vcpu_gfn_to_memslot
6.12.59+ #3 Not tainted

include/linux/kvm_host.h:1062 suspicious rcu_dereference_check() usage!

other info that might help us debug this:

rcu_scheduler_active = 2, debug_locks = 1
1 lock held by syz.5.1717/15100:
 #0: ff1100002f4b00b0 (&vcpu->mutex){+.+.}-{3:3}, at: kvm_vcpu_ioctl+0x1d5/0x1590

Call Trace:
 <TASK>
 __dump_stack lib/dump_stack.c:94 [inline]
 dump_stack_lvl+0xf0/0x120 lib/dump_stack.c:120
 lockdep_rcu_suspicious+0x1e3/0x270 kernel/locking/lockdep.c:6824
 __kvm_memslots include/linux/kvm_host.h:1062 [inline]
 __kvm_memslots include/linux/kvm_host.h:1059 [inline]
 kvm_vcpu_memslots include/linux/kvm_host.h:1076 [inline]
 kvm_vcpu_gfn_to_memslot+0x518/0x5e0 virt/kvm/kvm_main.c:2617
 kvm_vcpu_read_guest_page+0x27/0x50 virt/kvm/kvm_main.c:3302
 load_pdptrs+0xff/0x4b0 arch/x86/kvm/x86.c:1065
 svm_cache_reg+0x1c9/0x230 arch/x86/kvm/svm/svm.c:1688
 kvm_pdptr_read arch/x86/kvm/kvm_cache_regs.h:141 [inline]
 __get_sregs2 arch/x86/kvm/x86.c:11784 [inline]
 kvm_arch_vcpu_ioctl+0x3e20/0x4aa0 arch/x86/kvm/x86.c:6279
 kvm_vcpu_ioctl+0x856/0x1590 virt/kvm/kvm_main.c:4663
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:907 [inline]
 __se_sys_ioctl fs/ioctl.c:893 [inline]
 __x64_sys_ioctl+0x18b/0x210 fs/ioctl.c:893
 do_syscall_x64 arch/x86/entry/common.c:52 [inline]
 do_syscall_64+0xbd/0x1d0 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Found by Linux Verification Center (linuxtesting.org) with Syzkaller.

Suggested-by: Sean Christopherson <seanjc@google.com>
Cc: stable@vger.kernel.org
Fixes: 6dba94035203 ("KVM: x86: Introduce KVM_GET_SREGS2 / KVM_SET_SREGS2")
Signed-off-by: Vasiliy Kovalev <kovalev@altlinux.org>
Link: https://patch.msgid.link/20260123222801.646123-1-kovalev@altlinux.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/x86.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 386cdb775fd48..1ea94f4a3dcbc 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -12145,9 +12145,11 @@ static void __get_sregs2(struct kvm_vcpu *vcpu, struct kvm_sregs2 *sregs2)
 		return;
 
 	if (is_pae_paging(vcpu)) {
+		kvm_vcpu_srcu_read_lock(vcpu);
 		for (i = 0 ; i < 4 ; i++)
 			sregs2->pdptrs[i] = kvm_pdptr_read(vcpu, i);
 		sregs2->flags |= KVM_SREGS2_FLAGS_PDPTRS_VALID;
+		kvm_vcpu_srcu_read_unlock(vcpu);
 	}
 }
 
-- 
2.51.0