| Message ID | 20250211073852.571625-1-aneesh.kumar@kernel.org (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | [kvmtool,1/2] cpu: vmexit: Handle KVM_EXIT_UNKNOWN exit reason correctly | expand |
On Tue, Feb 11, 2025 at 01:08:51PM +0530, Aneesh Kumar K.V (Arm) wrote: > The return value for the KVM_RUN ioctl is confusing and has led to > errors in different kernel exit handlers. A return value of 0 indicates > a return to the VMM, whereas a return value of 1 indicates resuming > execution in the guest. Some handlers mistakenly return 0 to force a > return to the guest. Oops. Did any of those broken handlers reach mainline? > This worked in kvmtool because the exit_reason defaulted to > 0 (KVM_EXIT_UNKNOWN), and kvmtool did not error out on an unknown exit > reason. However, forcing a KVM panic on an unknown exit reason would > help catch these bugs early. > > Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@kernel.org> > --- > kvm-cpu.c | 1 + > 1 file changed, 1 insertion(+) > > diff --git a/kvm-cpu.c b/kvm-cpu.c > index f66dcd07220c..66e30ba54e26 100644 > --- a/kvm-cpu.c > +++ b/kvm-cpu.c > @@ -170,6 +170,7 @@ int kvm_cpu__start(struct kvm_cpu *cpu) > > switch (cpu->kvm_run->exit_reason) { > case KVM_EXIT_UNKNOWN: > + goto panic_kvm; > break; The break is no longer needed. Will
Hi, On Tue, Feb 11, 2025 at 01:08:51PM +0530, Aneesh Kumar K.V (Arm) wrote: > The return value for the KVM_RUN ioctl is confusing and has led to > errors in different kernel exit handlers. A return value of 0 indicates > a return to the VMM, whereas a return value of 1 indicates resuming > execution in the guest. Some handlers mistakenly return 0 to force a > return to the guest. I find this paragraph confusing. KVM_RUN, as per the documentation, returns 0 or -1 (on error). As far as I can tell, at least on arm64, KVM_RUN can never return 1. Are you referring to the loop in kvm_arch_vcpu_ioctl_run() by any chance? That's the only place I found where a value of 1 from the handlers signifies return to the guest. > > This worked in kvmtool because the exit_reason defaulted to > 0 (KVM_EXIT_UNKNOWN), and kvmtool did not error out on an unknown exit > reason. However, forcing a KVM panic on an unknown exit reason would > help catch these bugs early. I would hope that a VMM cannot force KVM to panic at will, which will bring down the host. Are you referring to kvmtool exiting with an error? That's what the unfortunately named 'panic_kvm' label seems to be doing. Thanks, Alex > > Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@kernel.org> > --- > kvm-cpu.c | 1 + > 1 file changed, 1 insertion(+) > > diff --git a/kvm-cpu.c b/kvm-cpu.c > index f66dcd07220c..66e30ba54e26 100644 > --- a/kvm-cpu.c > +++ b/kvm-cpu.c > @@ -170,6 +170,7 @@ int kvm_cpu__start(struct kvm_cpu *cpu) > > switch (cpu->kvm_run->exit_reason) { > case KVM_EXIT_UNKNOWN: > + goto panic_kvm; > break; > case KVM_EXIT_DEBUG: > kvm_cpu__show_registers(cpu); > > base-commit: 6d754d01fe2cb366f3b636d8a530f46ccf3b10d8 > -- > 2.43.0 > >
Will Deacon <will@kernel.org> writes: > On Tue, Feb 11, 2025 at 01:08:51PM +0530, Aneesh Kumar K.V (Arm) wrote: >> The return value for the KVM_RUN ioctl is confusing and has led to >> errors in different kernel exit handlers. A return value of 0 indicates >> a return to the VMM, whereas a return value of 1 indicates resuming >> execution in the guest. Some handlers mistakenly return 0 to force a >> return to the guest. > > Oops. Did any of those broken handlers reach mainline? > Not that I noticed. We do have patches in review. https://lore.kernel.org/all/20241212155610.76522-18-steven.price@arm.com >> This worked in kvmtool because the exit_reason defaulted to >> 0 (KVM_EXIT_UNKNOWN), and kvmtool did not error out on an unknown exit >> reason. However, forcing a KVM panic on an unknown exit reason would >> help catch these bugs early. >> >> Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@kernel.org> >> --- >> kvm-cpu.c | 1 + >> 1 file changed, 1 insertion(+) >> >> diff --git a/kvm-cpu.c b/kvm-cpu.c >> index f66dcd07220c..66e30ba54e26 100644 >> --- a/kvm-cpu.c >> +++ b/kvm-cpu.c >> @@ -170,6 +170,7 @@ int kvm_cpu__start(struct kvm_cpu *cpu) >> >> switch (cpu->kvm_run->exit_reason) { >> case KVM_EXIT_UNKNOWN: >> + goto panic_kvm; >> break; > > The break is no longer needed. > ok. -aneesh
Alexandru Elisei <alexandru.elisei@arm.com> writes: > Hi, > > On Tue, Feb 11, 2025 at 01:08:51PM +0530, Aneesh Kumar K.V (Arm) wrote: >> The return value for the KVM_RUN ioctl is confusing and has led to >> errors in different kernel exit handlers. A return value of 0 indicates >> a return to the VMM, whereas a return value of 1 indicates resuming >> execution in the guest. Some handlers mistakenly return 0 to force a >> return to the guest. > > I find this paragraph confusing. KVM_RUN, as per the documentation, returns 0 or > -1 (on error). As far as I can tell, at least on arm64, KVM_RUN can never return > 1. > > Are you referring to the loop in kvm_arch_vcpu_ioctl_run() by any chance? That's > the only place I found where a value of 1 from the handlers signifies return to > the guest. > Yes. I will update the commit message to reflect that. It is the exit handler return value rather than KVM_RUN ioctl return value. > >> >> This worked in kvmtool because the exit_reason defaulted to >> 0 (KVM_EXIT_UNKNOWN), and kvmtool did not error out on an unknown exit >> reason. However, forcing a KVM panic on an unknown exit reason would >> help catch these bugs early. > > I would hope that a VMM cannot force KVM to panic at will, which will bring down > the host. Are you referring to kvmtool exiting with an error? That's what the > unfortunately named 'panic_kvm' label seems to be doing. > yes. I will update the commit message to indicate that for KVM_EXIT_UNKNOWN exit reason, kvmtool will now exit with an error instead of returning to the guest." -aneesh
diff --git a/kvm-cpu.c b/kvm-cpu.c index f66dcd07220c..66e30ba54e26 100644 --- a/kvm-cpu.c +++ b/kvm-cpu.c @@ -170,6 +170,7 @@ int kvm_cpu__start(struct kvm_cpu *cpu) switch (cpu->kvm_run->exit_reason) { case KVM_EXIT_UNKNOWN: + goto panic_kvm; break; case KVM_EXIT_DEBUG: kvm_cpu__show_registers(cpu);
The return value for the KVM_RUN ioctl is confusing and has led to errors in different kernel exit handlers. A return value of 0 indicates a return to the VMM, whereas a return value of 1 indicates resuming execution in the guest. Some handlers mistakenly return 0 to force a return to the guest. This worked in kvmtool because the exit_reason defaulted to 0 (KVM_EXIT_UNKNOWN), and kvmtool did not error out on an unknown exit reason. However, forcing a KVM panic on an unknown exit reason would help catch these bugs early. Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@kernel.org> --- kvm-cpu.c | 1 + 1 file changed, 1 insertion(+) base-commit: 6d754d01fe2cb366f3b636d8a530f46ccf3b10d8