kvm, cpuid: silence a buffer overflow warning

RFC PATCH In review Dan Carpenter 3 participants 5 messages Feb 20, 2014

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Dan Carpenter

author

2014-02-20 12:34 UTC #0 lore raw

RFC PATCH 1/1 kvm, cpuid: silence a buffer overflow warning

This seems like a harmless off by one overflow if "i" is the last element in the vcpu->arch.cpuid_entries[] array.

Signed-off-by Dan Carpenter <[email protected]>

---
Not tested. I always wonder if it's worth fixing these or if it's worth reporting them? Either of those seem like a lot of work for something harmless.

arch/x86/kvm/cpuid.c +1 -1

index c6976257eff5..7d02c0fc768c 100644
@@ -660,7 +660,7 @@ static int move_to_next_stateful_cpuid_entry(struct kvm_vcpu *vcpu, int i)
 
 	e->flags &= ~KVM_CPUID_FLAG_STATE_READ_NEXT;
 	/* when no next entry is found, the current entry[i] is reselected */
-	for (j = i + 1; ; j = (j + 1) % nent) {
+	for (j = (i + 1) % nent; ; j = (j + 1) % nent) {
 		struct kvm_cpuid_entry2 *ej = &vcpu->arch.cpuid_entries[j];
 		if (ej->function = e->function) {
 			ej->flags |= KVM_CPUID_FLAG_STATE_READ_NEXT;

Paolo Bonzini

maintainer

2014-02-20 13:07 UTC #1 lore raw

Il 20/02/2014 13:34, Dan Carpenter ha scritto:

Quoted 8 lines

This seems like a harmless off by one overflow if "i" is the last element in the vcpu->arch.cpuid_entries[] array.
Signed-off-by Dan Carpenter <[email protected]>
---
Not tested. I always wonder if it's worth fixing these or if it's worth reporting them? Either of those seem like a lot of work for something harmless.

Could it oops if cpuid_nent is INT_MAX? If so, it's not entirely harmless. In this case I'd rather take the occasion to cleanup the code like this (compile-tested):

arch/x86/kvm/cpuid.c +10 -9

index 9fed5bedaad6..2fd6e7169936 100644
@@ -656,18 +656,19 @@ out:
 static int move_to_next_stateful_cpuid_entry(struct kvm_vcpu *vcpu, int i)
 {
 	struct kvm_cpuid_entry2 *e = &vcpu->arch.cpuid_entries[i];
-	int j, nent = vcpu->arch.cpuid_nent;
+	struct kvm_cpuid_entry2 *ej;
+	int j = i, nent = vcpu->arch.cpuid_nent;
 
 	e->flags &= ~KVM_CPUID_FLAG_STATE_READ_NEXT;
+
 	/* when no next entry is found, the current entry[i] is reselected */
-	for (j = i + 1; ; j = (j + 1) % nent) {
-		struct kvm_cpuid_entry2 *ej = &vcpu->arch.cpuid_entries[j];
-		if (ej->function = e->function) {
-			ej->flags |= KVM_CPUID_FLAG_STATE_READ_NEXT;
-			return j;
-		}
-	}
-	return 0; /* silence gcc, even though control never reaches here */
+	do {
+		j = (j + 1) % nent;
+		ej = &vcpu->arch.cpuid_entries[j];
+	} while (ej->function != e->function);
+
+	ej->flags |= KVM_CPUID_FLAG_STATE_READ_NEXT;
+	return j;
 }
 
 /* find an entry with matching function, matching index (if needed), and that

What do you think?

Paolo

walter harms

2014-02-20 13:18 UTC #2 lore raw

Am 20.02.2014 14:07, schrieb Paolo Bonzini:

Quoted 48 lines

Il 20/02/2014 13:34, Dan Carpenter ha scritto:
Quoted 8 lines
This seems like a harmless off by one overflow if "i" is the last element in the vcpu->arch.cpuid_entries[] array.
Signed-off-by Dan Carpenter <[email protected]>
---
Not tested. I always wonder if it's worth fixing these or if it's worth reporting them? Either of those seem like a lot of work for something harmless.
Could it oops if cpuid_nent is INT_MAX? If so, it's not entirely harmless. In this case I'd rather take the occasion to cleanup the code like this (compile-tested):
arch/x86/kvm/cpuid.c +10 -9
index 9fed5bedaad6..2fd6e7169936 100644
@@ -656,18 +656,19 @@ out:
 static int move_to_next_stateful_cpuid_entry(struct kvm_vcpu *vcpu, int i)
 {
 	struct kvm_cpuid_entry2 *e = &vcpu->arch.cpuid_entries[i];
-	int j, nent = vcpu->arch.cpuid_nent;
+	struct kvm_cpuid_entry2 *ej;
+	int j = i, nent = vcpu->arch.cpuid_nent;
 
 	e->flags &= ~KVM_CPUID_FLAG_STATE_READ_NEXT;
+
 	/* when no next entry is found, the current entry[i] is reselected */
-	for (j = i + 1; ; j = (j + 1) % nent) {
-		struct kvm_cpuid_entry2 *ej = &vcpu->arch.cpuid_entries[j];
-		if (ej->function = e->function) {
-			ej->flags |= KVM_CPUID_FLAG_STATE_READ_NEXT;
-			return j;
-		}
-	}
-	return 0; /* silence gcc, even though control never reaches here */
+	do {
+		j = (j + 1) % nent;
+		ej = &vcpu->arch.cpuid_entries[j];
+	} while (ej->function != e->function);
+
+	ej->flags |= KVM_CPUID_FLAG_STATE_READ_NEXT;
+	return j;
 }
 
 /* find an entry with matching function, matching index (if needed), and that

Quoted 2 lines

What do you think?

Is there any guaranty that this will not loop forever ?

an if (i=j) return 0; would be on the save side. (I guess that these was the idea behind the for).

re,
wh

Quoted 6 lines

Paolo
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Paolo Bonzini

maintainer

2014-02-20 13:19 UTC #3 lore raw

Il 20/02/2014 14:18, walter harms ha scritto:

Quoted 51 lines

Am 20.02.2014 14:07, schrieb Paolo Bonzini:
Quoted 48 lines
Il 20/02/2014 13:34, Dan Carpenter ha scritto:
Quoted 8 lines
This seems like a harmless off by one overflow if "i" is the last element in the vcpu->arch.cpuid_entries[] array.
Signed-off-by Dan Carpenter <[email protected]>
---
Not tested. I always wonder if it's worth fixing these or if it's worth reporting them? Either of those seem like a lot of work for something harmless.
Could it oops if cpuid_nent is INT_MAX? If so, it's not entirely harmless. In this case I'd rather take the occasion to cleanup the code like this (compile-tested):
arch/x86/kvm/cpuid.c +10 -9
index 9fed5bedaad6..2fd6e7169936 100644
@@ -656,18 +656,19 @@ out:
 static int move_to_next_stateful_cpuid_entry(struct kvm_vcpu *vcpu, int i)
 {
 	struct kvm_cpuid_entry2 *e = &vcpu->arch.cpuid_entries[i];
-	int j, nent = vcpu->arch.cpuid_nent;
+	struct kvm_cpuid_entry2 *ej;
+	int j = i, nent = vcpu->arch.cpuid_nent;
 
 	e->flags &= ~KVM_CPUID_FLAG_STATE_READ_NEXT;
+
 	/* when no next entry is found, the current entry[i] is reselected */
-	for (j = i + 1; ; j = (j + 1) % nent) {
-		struct kvm_cpuid_entry2 *ej = &vcpu->arch.cpuid_entries[j];
-		if (ej->function = e->function) {
-			ej->flags |= KVM_CPUID_FLAG_STATE_READ_NEXT;
-			return j;
-		}
-	}
-	return 0; /* silence gcc, even though control never reaches here */
+	do {
+		j = (j + 1) % nent;
+		ej = &vcpu->arch.cpuid_entries[j];
+	} while (ej->function != e->function);
+
+	ej->flags |= KVM_CPUID_FLAG_STATE_READ_NEXT;
+	return j;
 }
 
 /* find an entry with matching function, matching index (if needed), and that

Quoted 7 lines

Quoted 2 lines
What do you think?
Is there any guaranty that this will not loop forever ?
an if (i=j) return 0; would be on the save side. (I guess that these was the idea behind the for).

Once i=j you'll get ej->function = e->function and exit.

Paolo

Dan Carpenter

author

2014-02-20 13:23 UTC #4 lore raw

In reply to Paolo Bonzini

Quoted 11 lines · Paolo Bonzini

Il 20/02/2014 13:34, Dan Carpenter ha scritto:
Quoted 8 lines
This seems like a harmless off by one overflow if "i" is the last element in the vcpu->arch.cpuid_entries[] array.
Signed-off-by Dan Carpenter <[email protected]>
---
Not tested. I always wonder if it's worth fixing these or if it's worth reporting them? Either of those seem like a lot of work for something harmless.
Could it oops if cpuid_nent is INT_MAX?

cpuid_nent is capped at KVM_MAX_CPUID_ENTRIES (80).

regards,
dan carpenter