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Thu, 09 Apr 2026 22:08:56 -0700 (PDT) Received: from TDC4045031631.e0cglfehwr0e5gttmepj3hi3hf.ux.internal.cloudapp.net ([20.63.37.123]) by smtp.gmail.com with ESMTPSA id 6a1803df08f44-8ac84c44236sm17740986d6.27.2026.04.09.22.08.55 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 09 Apr 2026 22:08:55 -0700 (PDT) From: Ashutosh Desai To: seanjc@google.com, pbonzini@redhat.com Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Ashutosh Desai Subject: [PATCH] KVM: SVM: Fix page overflow in sev_dbg_crypt() for ENCRYPT path Date: Fri, 10 Apr 2026 05:08:54 +0000 Message-Id: <20260410050854.2463447-1-ashutoshdesai993@gmail.com> X-Mailer: git-send-email 2.34.1 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" In sev_dbg_crypt(), the per-iteration transfer length is bounded by the source page offset (PAGE_SIZE - s_off) but not by the destination page offset (PAGE_SIZE - d_off). When d_off > s_off, the encrypt path (__sev_dbg_encrypt_user) performs a read-modify-write using a single-page intermediate buffer (dst_tpage): 1. __sev_dbg_decrypt() expands the size to round_up(len + (d_off & 15), 1= 6) before issuing the PSP command. If len + (d_off & 15) > PAGE_SIZE, the PSP writes beyond the end of the 4096-byte dst_tpage allocation. 2. The subsequent memcpy()/copy_from_user() into page_address(dst_tpage) + (d_off & 15) of 'len' bytes overflows by up to 15 bytes under the same condition. Trigger example: s_off =3D 0, d_off =3D 1, debug.len =3D PAGE_SIZE - the PSP is instructed to write round_up(4097, 16) =3D 4112 bytes to a 4096-byte buffer. Fix by also bounding len by (PAGE_SIZE - d_off), the same check that sev_send_update_data() already performs for its single-page guest region. Signed-off-by: Ashutosh Desai --- arch/x86/kvm/svm/sev.c | 1 + 1 file changed, 1 insertion(+) diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c index 3f9c1aa39..a7eb24179 100644 --- a/arch/x86/kvm/svm/sev.c +++ b/arch/x86/kvm/svm/sev.c @@ -1340,6 +1340,7 @@ static int sev_dbg_crypt(struct kvm *kvm, struct kvm_= sev_cmd *argp, bool dec) s_off =3D vaddr & ~PAGE_MASK; d_off =3D dst_vaddr & ~PAGE_MASK; len =3D min_t(size_t, (PAGE_SIZE - s_off), size); + len =3D min_t(size_t, len, PAGE_SIZE - d_off); =20 if (dec) ret =3D __sev_dbg_decrypt_user(kvm, --=20 2.34.1