[PATCH V2] KVM: sev: Fix potential overflow send|recieve_update_data

[PATCH V2] KVM: sev: Fix potential overflow send|recieve_update_data

[Peter Gonda]

KVM_SEV_SEND_UPDATE_DATA and KVM_SEV_RECEIVE_UPDATE_DATA have an integer
overflow issue. Params.guest_len and offset are both 32bite wide, with a
large params.guest_len the check to confirm a page boundary is not
crossed can falsely pass:

    /* Check if we are crossing the page boundary *
    offset = params.guest_uaddr & (PAGE_SIZE - 1);
    if ((params.guest_len + offset > PAGE_SIZE))

Add an additional check to this conditional to confirm that
params.guest_len itself is not greater than PAGE_SIZE.

The current code is can only overflow with a params.guest_len of greater
than 0xfffff000. And the FW spec says these commands fail with lengths
greater than 16KB. So this issue should not be a security concern

Fixes: 15fb7de1a7f5 ("KVM: SVM: Add KVM_SEV_RECEIVE_UPDATE_DATA command")
Fixes: d3d1af85e2c7 ("KVM: SVM: Add KVM_SEND_UPDATE_DATA command")
Reported-by: Andy Nguyen <theflow@google.com>
Suggested-by: Thomas Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Peter Gonda <pgonda@google.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Sean Christopherson <seanjc@google.com>
Cc: kvm@vger.kernel.org
Cc: stable@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
---

V2
 * Updated conditional based on feedback from Tom.

---
 arch/x86/kvm/svm/sev.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 273cba809328..3d74facaead8 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -1294,7 +1294,7 @@ static int sev_send_update_data(struct kvm *kvm, struct kvm_sev_cmd *argp)
 
 	/* Check if we are crossing the page boundary */
 	offset = params.guest_uaddr & (PAGE_SIZE - 1);
-	if ((params.guest_len + offset > PAGE_SIZE))
+	if (params.guest_len > PAGE_SIZE || (params.guest_len + offset) > PAGE_SIZE)
 		return -EINVAL;
 
 	/* Pin guest memory */
@@ -1474,7 +1474,7 @@ static int sev_receive_update_data(struct kvm *kvm, struct kvm_sev_cmd *argp)
 
 	/* Check if we are crossing the page boundary */
 	offset = params.guest_uaddr & (PAGE_SIZE - 1);
-	if ((params.guest_len + offset > PAGE_SIZE))
+	if (params.guest_len > PAGE_SIZE || (params.guest_len + offset) > PAGE_SIZE)
 		return -EINVAL;
 
 	hdr = psp_copy_user_blob(params.hdr_uaddr, params.hdr_len);
-- 
2.39.1.519.gcb327c4b5f-goog

Re: [PATCH V2] KVM: sev: Fix potential overflow send|recieve_update_data

[Tom Lendacky]

On 2/7/23 11:13, Peter Gonda wrote:
> KVM_SEV_SEND_UPDATE_DATA and KVM_SEV_RECEIVE_UPDATE_DATA have an integer
> overflow issue. Params.guest_len and offset are both 32bite wide, with a
> large params.guest_len the check to confirm a page boundary is not
> crossed can falsely pass:
> 
>      /* Check if we are crossing the page boundary *
>      offset = params.guest_uaddr & (PAGE_SIZE - 1);
>      if ((params.guest_len + offset > PAGE_SIZE))
> 
> Add an additional check to this conditional to confirm that
> params.guest_len itself is not greater than PAGE_SIZE.
> 
> The current code is can only overflow with a params.guest_len of greater
> than 0xfffff000. And the FW spec says these commands fail with lengths
> greater than 16KB. So this issue should not be a security concern
> 
> Fixes: 15fb7de1a7f5 ("KVM: SVM: Add KVM_SEV_RECEIVE_UPDATE_DATA command")
> Fixes: d3d1af85e2c7 ("KVM: SVM: Add KVM_SEND_UPDATE_DATA command")
> Reported-by: Andy Nguyen <theflow@google.com>
> Suggested-by: Thomas Lendacky <thomas.lendacky@amd.com>
> Signed-off-by: Peter Gonda <pgonda@google.com>
> Cc: David Rientjes <rientjes@google.com>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Sean Christopherson <seanjc@google.com>
> Cc: kvm@vger.kernel.org
> Cc: stable@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org

Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>

> ---
> 
> V2
>   * Updated conditional based on feedback from Tom.
> 
> ---
>   arch/x86/kvm/svm/sev.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index 273cba809328..3d74facaead8 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -1294,7 +1294,7 @@ static int sev_send_update_data(struct kvm *kvm, struct kvm_sev_cmd *argp)
>   
>   	/* Check if we are crossing the page boundary */
>   	offset = params.guest_uaddr & (PAGE_SIZE - 1);
> -	if ((params.guest_len + offset > PAGE_SIZE))
> +	if (params.guest_len > PAGE_SIZE || (params.guest_len + offset) > PAGE_SIZE)
>   		return -EINVAL;
>   
>   	/* Pin guest memory */
> @@ -1474,7 +1474,7 @@ static int sev_receive_update_data(struct kvm *kvm, struct kvm_sev_cmd *argp)
>   
>   	/* Check if we are crossing the page boundary */
>   	offset = params.guest_uaddr & (PAGE_SIZE - 1);
> -	if ((params.guest_len + offset > PAGE_SIZE))
> +	if (params.guest_len > PAGE_SIZE || (params.guest_len + offset) > PAGE_SIZE)
>   		return -EINVAL;
>   
>   	hdr = psp_copy_user_blob(params.hdr_uaddr, params.hdr_len);

Re: [PATCH V2] KVM: sev: Fix potential overflow send|recieve_update_data

[Sean Christopherson]

For now at least, I want to keep with "KVM: SVM:" instead of using "KVM: SEV:".
Many commits that touch SEV aren't strictly isolated to SEV, which means the "SEV"
tag is unreliable.  There's also the question of taggin SEV vs. SEV-ES vs. SEV-SNP.
It's usually easy enough to squeeze SEV (or SEV-ES or SNP) into the shortlog, e.g.

  KVM: SVM: Fix potential overflow in SEV's send|receive_update_data()

On Tue, Feb 07, 2023, Peter Gonda wrote:
> KVM_SEV_SEND_UPDATE_DATA and KVM_SEV_RECEIVE_UPDATE_DATA have an integer
> overflow issue. Params.guest_len and offset are both 32bite wide, with a

"32 bits"

> large params.guest_len the check to confirm a page boundary is not
> crossed can falsely pass:
> 
>     /* Check if we are crossing the page boundary *
>     offset = params.guest_uaddr & (PAGE_SIZE - 1);
>     if ((params.guest_len + offset > PAGE_SIZE))
> 
> Add an additional check to this conditional to confirm that

Eh, "to this conditional" is unnecessarily precise.

> params.guest_len itself is not greater than PAGE_SIZE.
> 
> The current code is can only overflow with a params.guest_len of greater

"is can", though I vote to omit the "current code" part entirely, it should be
obvious that this is talking about the pre-patched code.

> than 0xfffff000. And the FW spec says these commands fail with lengths
> greater than 16KB. So this issue should not be a security concern

Slightly reworded, how about this for the "not a security concern" disclaimer?

  Note, this isn't a security concern as overflow can happen if and only if
  params.guest_len is greater than 0xfffff000, and the FW spec says these
  commands fail with lengths greater than 16KB, i.e. the PSP will detect
  KVM's goof.

No need to send a v3, I'll fix up the changelog when applying.  Holler if you
disagree with anything though.

Thanks!

Re: [PATCH V2] KVM: sev: Fix potential overflow send|recieve_update_data

[Sean Christopherson]

On Tue, 07 Feb 2023 09:13:54 -0800, Peter Gonda wrote:
> KVM_SEV_SEND_UPDATE_DATA and KVM_SEV_RECEIVE_UPDATE_DATA have an integer
> overflow issue. Params.guest_len and offset are both 32bite wide, with a
> large params.guest_len the check to confirm a page boundary is not
> crossed can falsely pass:
> 
>     /* Check if we are crossing the page boundary *
>     offset = params.guest_uaddr & (PAGE_SIZE - 1);
>     if ((params.guest_len + offset > PAGE_SIZE))
> 
> [...]

Applied to kvm-x86 svm, thanks!

[1/1] KVM: sev: Fix potential overflow send|recieve_update_data
      https://github.com/kvm-x86/linux/commit/f94f053aa3a5

--
https://github.com/kvm-x86/linux/tree/next
https://github.com/kvm-x86/linux/tree/fixes