[PATCH RESEND v3] KVM: selftests: Initialise dynamically allocated configuration names

[PATCH RESEND v3] KVM: selftests: Initialise dynamically allocated configuration names

[Mark Brown]

When we dynamically generate a name for a configuration in get-reg-list
we use strcat() to append to a buffer allocated using malloc() but we
never initialise that buffer. Since malloc() offers no guarantees
regarding the contents of the memory it returns this can lead to us
corrupting, and likely overflowing, the buffer:

  vregs: PASS
  vregs+pmu: PASS
  sve: PASS
  sve+pmu: PASS
  vregs+pauth_address+pauth_generic: PASS
  X�vr+gspauth_addre+spauth_generi+pmu: PASS

Initialise the buffer to an empty string to avoid this.

Fixes: 2f9ace5d4557 ("KVM: arm64: selftests: get-reg-list: Introduce vcpu configs")
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
---
Changes in v3:
- Rebase this bugfix onto v6.7-rc1
- Link to v2: https://lore.kernel.org/r/20231017-kvm-get-reg-list-str-init-v2-1-ee30b1df3e50@kernel.org

Changes in v2:
- Update Fixes: tag.
- Link to v1: https://lore.kernel.org/r/20231013-kvm-get-reg-list-str-init-v1-1-034f370ff8ab@kernel.org
---
 tools/testing/selftests/kvm/get-reg-list.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/tools/testing/selftests/kvm/get-reg-list.c b/tools/testing/selftests/kvm/get-reg-list.c
index be7bf5224434..dd62a6976c0d 100644
--- a/tools/testing/selftests/kvm/get-reg-list.c
+++ b/tools/testing/selftests/kvm/get-reg-list.c
@@ -67,6 +67,7 @@ static const char *config_name(struct vcpu_reg_list *c)
 
 	c->name = malloc(len);
 
+	c->name[0] = '\0';
 	len = 0;
 	for_each_sublist(c, s) {
 		if (!strcmp(s->name, "base"))

---
base-commit: b85ea95d086471afb4ad062012a4d73cd328fa86
change-id: 20231012-kvm-get-reg-list-str-init-76c8ed4e19d6

Best regards,
-- 
Mark Brown <broonie@kernel.org>

Re: [PATCH RESEND v3] KVM: selftests: Initialise dynamically allocated configuration names

[Paolo Bonzini]

On 12/11/23 14:08, Mark Brown wrote:
> When we dynamically generate a name for a configuration in get-reg-list
> we use strcat() to append to a buffer allocated using malloc() but we
> never initialise that buffer. Since malloc() offers no guarantees
> regarding the contents of the memory it returns this can lead to us
> corrupting, and likely overflowing, the buffer:
> 
>    vregs: PASS
>    vregs+pmu: PASS
>    sve: PASS
>    sve+pmu: PASS
>    vregs+pauth_address+pauth_generic: PASS
>    X�vr+gspauth_addre+spauth_generi+pmu: PASS
> 
> Initialise the buffer to an empty string to avoid this.

> diff --git a/tools/testing/selftests/kvm/get-reg-list.c b/tools/testing/selftests/kvm/get-reg-list.c
> index be7bf5224434..dd62a6976c0d 100644
> --- a/tools/testing/selftests/kvm/get-reg-list.c
> +++ b/tools/testing/selftests/kvm/get-reg-list.c
> @@ -67,6 +67,7 @@ static const char *config_name(struct vcpu_reg_list *c)
>   
>   	c->name = malloc(len);
>   
> +	c->name[0] = '\0';
>   	len = 0;
>   	for_each_sublist(c, s) {
>   		if (!strcmp(s->name, "base"))
>  			continue;
>  		strcat(c->name + len, s->name);

This can be fixed just by s/strcat/strcpy/, but there's also an ugly 
hidden assumption that for_each_sublist runs at least one iteration of 
the loop; otherwise, the loop ends with a c->name[-1] = '\0';

>                 len += strlen(s->name) + 1;
>                 c->name[len - 1] = '+';
>         }
>         c->name[len - 1] = '\0';

Now this *is* a bit academic, but it remains the fact that all the 
invariants are screwed up and while we're fixing it we might at least 
fix it well.

So let's make the invariant that c->name[0..len-1] is initialized.  Then 
every write is done with either strcpy of c->name[len++] = '...'.

> ---
> base-commit: b85ea95d086471afb4ad062012a4d73cd328fa86
> change-id: 20231012-kvm-get-reg-list-str-init-76c8ed4e19d6
> 
> Best regards,