Re: [PATCH] KVM: s390: Fix potential spectre warnings

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On 4/17/19 3:49 AM, David Hildenbrand wrote:
On 17.04.19 02:54, Eric Farman wrote:
Fix some warnings from smatch:

arch/s390/kvm/interrupt.c:2310 get_io_adapter() warn: potential spectre issue 'kvm->arch.adapters' [r] (local cap)
arch/s390/kvm/interrupt.c:2341 register_io_adapter() warn: potential spectre issue 'dev->kvm->arch.adapters' [w]

Signed-off-by: Eric Farman <farman@xxxxxxxxxxxxx>
---
A recent patch from Paolo [1] acted as a reminder (thanks, Christian!)
that I had one for the s390 KVM code after some code reviews [2].
Let's clean that up.

[1] https://patchwork.kernel.org/patch/10895463/
[2] https://patchwork.kernel.org/patch/10788565/#22484223
---
  arch/s390/kvm/interrupt.c | 11 +++++++++--
  1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/arch/s390/kvm/interrupt.c b/arch/s390/kvm/interrupt.c
index 82162867f378..bfd55ad34a3e 100644
--- a/arch/s390/kvm/interrupt.c
+++ b/arch/s390/kvm/interrupt.c
@@ -14,6 +14,7 @@
  #include <linux/kvm_host.h>
  #include <linux/hrtimer.h>
  #include <linux/mmu_context.h>
+#include <linux/nospec.h>
  #include <linux/signal.h>
  #include <linux/slab.h>
  #include <linux/bitmap.h>
@@ -2307,6 +2308,7 @@ static struct s390_io_adapter *get_io_adapter(struct kvm *kvm, unsigned int id)
  {
  	if (id >= MAX_S390_IO_ADAPTERS)
  		return NULL;
+	id = array_index_nospec(id, MAX_S390_IO_ADAPTERS);
  	return kvm->arch.adapters[id];

return kvm->arch.adapters[array_index_nospec(id, MAX_S390_IO_ADAPTERS)];

should exactly fit into a single line if I am not wrong.

Yeah, just. As Paolo pointed out, that's not common usage. Though of the four other hits I see, only one of them is the same as this instance, in that "id" is passed as a variable and then we immediately return with an array entry (even if NULL) rather than doing something else in that function. So maybe all-in-one-line here is a little cleaner.


  }
@@ -2320,8 +2322,13 @@ static int register_io_adapter(struct kvm_device *dev,
  			   (void __user *)attr->addr, sizeof(adapter_info)))
  		return -EFAULT;
- if ((adapter_info.id >= MAX_S390_IO_ADAPTERS) ||
-	    (dev->kvm->arch.adapters[adapter_info.id] != NULL))
+	if (adapter_info.id >= MAX_S390_IO_ADAPTERS)
+		return -EINVAL;
+
+	adapter_info.id = array_index_nospec(adapter_info.id,
+					     MAX_S390_IO_ADAPTERS);

I dislike that we are modifying adapter_info here. Can you use a local
variable instead?

I guess, but adapter_info is a local variable too. So sanitization this way seems fine to me. But if you dislike it more than I don't care, I'll add another local variable. :)


+
+	if (dev->kvm->arch.adapters[adapter_info.id] != NULL)
  		return -EINVAL;
adapter = kzalloc(sizeof(*adapter), GFP_KERNEL);






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