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a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1791815092; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding; bh=uYRcGFVZkmu0VNiOiRLHaQQxEggzFQTZ+2VnK3h9yck=; b=A5AWzSF3iA6BN5RTAs4npYrw3u56G3Nbnr8erSmvR9s8g29ozQCDTqpYEPH9zDp8SDVm42 v5uSyWfDnnWHNFb5PoQeWELIso3KZrR1uhO0xfBECabxNr2A11AvGjAtya3yPzKGPicmni CNWhjnIwtXalD1u/hqnT7bIOCZ4C9Tk= X-MC-Unique: 4citZWetM_et1XzqI1qtcA-1 X-Mimecast-MFC-AGG-ID: 4citZWetM_et1XzqI1qtcA_1791815090 From: =?UTF-8?q?Daniel=20P=2E=20Berrang=C3=A9?= To: qemu-devel@nongnu.org Cc: =?UTF-8?q?Daniel=20P=2E=20Berrang=C3=A9?= Subject: [PATCH] seccomp: drop redundant match for clone() flags == 0 Date: Mon, 12 Oct 2026 15:24:47 +0100 Message-ID: <20261012142447.446500-1-berrange@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.6 on 10.30.177.95 Received-SPF: pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) client-ip=209.51.188.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists1p.gnu.org; Received-SPF: pass client-ip=170.10.129.124; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: 10 X-Spam_score: 1.0 X-Spam_bar: + X-Spam_report: (1.0 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.24, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_SBL_CSS=3.335, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=no autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: qemu development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+importer=patchew.org@nongnu.org Sender: qemu-devel-bounces+importer=patchew.org@nongnu.org X-ZohoMail-DKIM: pass (identity @redhat.com) X-ZM-MESSAGEID: 1791815142145158500 We have seccomp matches for clone() which mandate the presence of several flags, in order to permit pthreads_create: REQUIRE_CLONE_FLAG(CLONE_VM); REQUIRE_CLONE_FLAG(CLONE_FS); ... Given those mandatory matches, it is impossible for flags =3D=3D 0 to pass the filter. This is fortunate, since the explicit match on flags =3D=3D 0 is in fact broken, as it has a silent size extension from 32-bits to 64-bits. This results in seccomp generating psuedo code of: if ($arch =3D=3D 3221225534) # filter for syscall "clone" (56) [priority: 65531] if ($syscall =3D=3D 56) if ($a0.hi32 & 0xffffffff =3D=3D 0) if ($a0.lo32 & 0xffffff00 =3D=3D 0) action ERRNO(1); # default action action ALLOW; # invalid architecture action action KILL; Since the clone() flag is passed as a 32-bit value in the syscall, the high 32-bits are liable to be undefined/uninitialized garbage The desired match for flags =3D=3D 0 was if ($arch =3D=3D 3221225534) # filter for syscall "clone" (56) [priority: 65532] if ($syscall =3D=3D 56) if ($a0.hi32 & 0x00000000 =3D=3D 0) if ($a0.lo32 & 0xffffff00 =3D=3D 0) action ERRNO(1); # default action action ALLOW; # invalid architecture action action KILL; That is redundant, however, as can be seen if we look at the (fixed) program with the CLONE_THREAD rule also present: if ($arch =3D=3D 3221225534) # filter for syscall "clone" (56) [priority: 65532] if ($syscall =3D=3D 56) if ($a0.hi32 & 0x00000000 =3D=3D 0) if ($a0.lo32 & 0xffffff00 =3D=3D 0) action ERRNO(1); if ($a0.lo32 & 0x00010000 =3D=3D 0) action ERRNO(1); # default action action ALLOW; # invalid architecture action action KILL; That "($a0.lo32 & 0x00010000 =3D=3D 0)" match will catch the case of flags =3D=3D 0 and do the right thing. Reported-by: Ricardo Branco (https://gitlab.com/ricardobranco777) Related: https://gitlab.com/qemu-project/qemu/-/work_items/4671 Signed-off-by: Daniel P. Berrang=C3=A9 Reviewed-by: Marc-Andr=C3=A9 Lureau --- system/qemu-seccomp.c | 9 --------- 1 file changed, 9 deletions(-) diff --git a/system/qemu-seccomp.c b/system/qemu-seccomp.c index 3889d48b4f2..8f8f8d3cf00 100644 --- a/system/qemu-seccomp.c +++ b/system/qemu-seccomp.c @@ -76,13 +76,6 @@ const struct scmp_arg_cmp sched_setscheduler_arg[] =3D { { SCMP_SYS(clone), QEMU_SECCOMP_SET_SPAWN, \ ARRAY_SIZE(clone_arg ## flag), clone_arg ## flag, SCMP_ACT_ERRNO(EPE= RM) } =20 -/* If no CLONE_* flags are set, except CSIGNAL, deny */ -const struct scmp_arg_cmp clone_arg_none[] =3D { - { .arg =3D CLONE_FLAGS_ARG, - .op =3D SCMP_CMP_MASKED_EQ, - .datum_a =3D ~(CSIGNAL), .datum_b =3D 0 } -}; - /* * pthread_create should always set all of these. */ @@ -219,8 +212,6 @@ static const struct QemuSeccompSyscall denylist[] =3D { 0, NULL, SCMP_ACT_ERRNO(EPERM) }, { SCMP_SYS(execve), QEMU_SECCOMP_SET_SPAWN, 0, NULL, SCMP_ACT_ERRNO(EPERM) }, - { SCMP_SYS(clone), QEMU_SECCOMP_SET_SPAWN, - ARRAY_SIZE(clone_arg_none), clone_arg_none, SCMP_ACT_ERRNO(EPERM) }, RULE_CLONE_FLAG(CLONE_VM), RULE_CLONE_FLAG(CLONE_FS), RULE_CLONE_FLAG(CLONE_FILES), --=20 2.56.0