KVM: x86: Add SRCU protection for reading PDPTRs in __get_sregs2()

Overview

In the Linux kernel, the following vulnerability has been resolved:

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.

Affected

• Linux >= 6dba940352038b56db9b591b172fb2ec76a5fd5e < f621ca24f9f489e226e22560761b04884984133b
• Linux >= 6dba940352038b56db9b591b172fb2ec76a5fd5e < 708e20c66b2761d878a2bc3c7534e7f814e4dec5
• Linux >= 6dba940352038b56db9b591b172fb2ec76a5fd5e < 9f2bfea51151dfbb24b52f452eb3d5f5fe0e506e
• Linux >= 6dba940352038b56db9b591b172fb2ec76a5fd5e < 57536ff0a6bd69a5808d682925202babdb5ddc13
• Linux >= 6dba940352038b56db9b591b172fb2ec76a5fd5e < b33f8d816950b10e7879cd8ffd7ae4b649ada4db
• Linux >= 6dba940352038b56db9b591b172fb2ec76a5fd5e < 95d848dc7e639988dbb385a8cba9b484607cf98c
• Linux 5.14

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.