github-mirror/linux
1
0
Fork 0
mirror of https://github.com/torvalds/linux.git synced 2026-05-30 18:13:41 +02:00
Code Issues Actions Packages Projects Releases Wiki Activity Graph

KVM: selftests: Add a shameful hack to preserve/clobber GPRs across ucall

Browse Source 
Preserve or clobber all GPRs (except RIP and RSP, as they're saved and
restored via the VMCS) when performing a ucall on x86 to fudge around a
horrific long-standing bug in selftests' nested VMX support where L2's
GPRs are not preserved across a nested VM-Exit.  I.e. if a test triggers a
nested VM-Exit to L1 in response to a ucall, e.g. GUEST_SYNC(), then L2's
GPR state can be corrupted.

The issues manifests as an unexpected #GP in clear_bit() when running the
hyperv_evmcs test due to RBX being used to track the ucall object, and RBX
being clobbered by the nested VM-Exit.  The problematic hyperv_evmcs
testcase is where L0 (test's host userspace) injects an NMI in response to
GUEST_SYNC(8) from L2, but the bug could "randomly" manifest in any test
that induces a nested VM-Exit from L0.  The bug hasn't caused failures in
the past due to sheer dumb luck.

The obvious fix is to rework the nVMX helpers to save/restore L2 GPRs
across VM-Exit and VM-Enter, but that is a much bigger task and carries
its own risks, e.g. nSVM does save/restore GPRs, but not in a thread-safe
manner, and there is a _lot_ of cleanup that can be done to unify code
for doing VM-Enter on nVMX, nSVM, and eVMCS.

Link: https://lore.kernel.org/r/20230729003643.1053367-4-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
This commit is contained in:
Sean Christopherson 2023-07-28 17:36:12 -07:00
parent b145c58d95
commit 6783ca4105
1 changed files with 30 additions and 2 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

32
tools/testing/selftests/kvm/lib/x86_64/ucall.c
View File

@ -14,8 +14,36 @@ void ucall_arch_init(struct kvm_vm *vm, vm_paddr_t mmio_gpa)
void ucall_arch_do_ucall(vm_vaddr_t uc)
{
asm volatile("in %[port], %%al"
: : [port] "d" (UCALL_PIO_PORT), "D" (uc) : "rax", "memory");
/*
* FIXME: Revert this hack (the entire commit that added it) once nVMX
* preserves L2 GPRs across a nested VM-Exit. If a ucall from L2, e.g.
* to do a GUEST_SYNC(), lands the vCPU in L1, any and all GPRs can be
* clobbered by L1. Save and restore non-volatile GPRs (clobbering RBP
* in particular is problematic) along with RDX and RDI (which are
* inputs), and clobber volatile GPRs. *sigh*
*/
#define HORRIFIC_L2_UCALL_CLOBBER_HACK \
"rcx", "rsi", "r8", "r9", "r10", "r11"
asm volatile("push %%rbp\n\t"
"push %%r15\n\t"
"push %%r14\n\t"
"push %%r13\n\t"
"push %%r12\n\t"
"push %%rbx\n\t"
"push %%rdx\n\t"
"push %%rdi\n\t"
"in %[port], %%al\n\t"
"pop %%rdi\n\t"
"pop %%rdx\n\t"
"pop %%rbx\n\t"
"pop %%r12\n\t"
"pop %%r13\n\t"
"pop %%r14\n\t"
"pop %%r15\n\t"
"pop %%rbp\n\t"
: : [port] "d" (UCALL_PIO_PORT), "D" (uc) : "rax", "memory",
HORRIFIC_L2_UCALL_CLOBBER_HACK);
}
void *ucall_arch_get_ucall(struct kvm_vcpu *vcpu)