Date: Wed, 25 Jul 2018 17:00:15 +0000 From: Xen.org security team <security@....org> To: xen-announce@...ts.xen.org, xen-devel@...ts.xen.org, xen-users@...ts.xen.org, oss-security@...ts.openwall.com CC: Xen.org security team <security-team-members@....org> Subject: Xen Security Advisory 274 - Linux: Uninitialized state in PV syscall return path -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA256 Xen Security Advisory XSA-274 Linux: Uninitialized state in PV syscall return path ISSUE DESCRIPTION ================= Linux has a `failsafe` callback, invoked by Xen under certain conditions. Normally in this failsafe callback, error_entry is paired with error_exit; and error_entry uses %ebx to communicate to error_exit whether to use the user or kernel return path. Unfortunately, on 64-bit PV Xen on x86, error_exit is called without error_entry being called first, leaving %ebx with an invalid value. IMPACT ====== A rogue user-space program could crash a guest kernel. Privilege escalation cannot be ruled out. VULNERABLE SYSTEMS ================== Only 64-bit x86 PV Linux systems are vulnerable. All versions of Linux are vulnerable. MITIGATION ========== Switching to HVM or PVH guests will mitigate this issue. CREDITS ======= This issue was discovered by M. Vefa Bicakci, and recognized as a security issue by Andy Lutorminski. RESOLUTION ========== Applying the appropriate attached patch resolves this issue. NB this patch has not been accepted into Linux upstream yet. An updated advisory will be sent if the fix upstreamed looks significantly different. xsa274-linux-4.17.patch Linux 4.17 $ sha256sum xsa274* 0c30cb13d1d573f446c8cb8d4824ffad8ef9149a7589a19ef9bcc83c07bddcf5 xsa274-linux-4.17.patch $ NOTE ON THE LACK OF EMBARGO =========================== The patch for this issue was published on linux-kernel without being first reported to the XenProject Security Team. -----BEGIN PGP SIGNATURE----- iQFABAEBCAAqFiEEI+MiLBRfRHX6gGCng/4UyVfoK9kFAltYp7EMHHBncEB4ZW4u b3JnAAoJEIP+FMlX6CvZipwIAINGjP6d5vABI2CEdbromimlXiwGvTUBWOoIsvu1 bfLyeab334UBIpmouz+UhgKXFdujIFNpWqGpCc68xoNSsJiY+95GykbkxfghxzkL GQXzGloJVrHSzRGT+wUlTg9qCpbj1YVr1YtnACa34eXJTGhUBnOl0L3gBRbrjILb esECY3/EAKcnB8z1d2AzCRamYVGvfMO8xcolYrP1DzlNYQPnfrKvZu/7vkiyhbrO M9nM6+9MdS63JPGp5dX8xRO3TzyRDpgpSpkoMY8Lqhrr5/oLC9dhtdm/yK2kNtJ/ JluBn6q+EfZKoW/UcwTsehiTOOTKb/WYhC3e1jsRpm/+drU= =7MDt -----END PGP SIGNATURE----- Download attachment "xsa274-linux-4.17.patch" of type "application/octet-stream" (4131 bytes)
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