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Wed, 17 Jun 2020 09:36:16 +0000 (UTC) Date: Wed, 17 Jun 2020 10:36:13 +0100 From: "Dr. David Alan Gilbert" To: Max Reitz Subject: Re: [PATCH] virtiofsd: Whitelist fchmod Message-ID: <20200617093613.GA7075@work-vm> References: <20200608093111.14942-1-mreitz@redhat.com> MIME-Version: 1.0 In-Reply-To: <20200608093111.14942-1-mreitz@redhat.com> User-Agent: Mutt/1.14.0 (2020-05-02) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Received-SPF: pass client-ip=205.139.110.120; envelope-from=dgilbert@redhat.com; helo=us-smtp-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/06/16 23:30:45 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -30 X-Spam_score: -3.1 X-Spam_bar: --- X-Spam_report: (-3.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1, 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_MSPIKE_H4=-0.01, RCVD_IN_MSPIKE_WL=-0.01, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=_AUTOLEARN X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: virtio-fs@redhat.com, qemu-devel@nongnu.org, Stefan Hajnoczi , qemu-stable@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" * Max Reitz (mreitz@redhat.com) wrote: > lo_setattr() invokes fchmod() in a rarely used code path, so it should > be whitelisted or virtiofsd will crash with EBADSYS. > > Said code path can be triggered for example as follows: > > On the host, in the shared directory, create a file with the sticky bit > set and a security.capability xattr: > (1) # touch foo > (2) # chmod u+s foo > (3) # setcap '' foo > > Then in the guest let some process truncate that file after it has > dropped all of its capabilities (at least CAP_FSETID): > > int main(int argc, char *argv[]) > { > capng_setpid(getpid()); > capng_clear(CAPNG_SELECT_BOTH); > capng_updatev(CAPNG_ADD, CAPNG_PERMITTED | CAPNG_EFFECTIVE, 0); > capng_apply(CAPNG_SELECT_BOTH); > > ftruncate(open(argv[1], O_RDWR), 0); > } > > This will cause the guest kernel to drop the sticky bit (i.e. perform a > mode change) as part of the truncate (where FATTR_FH is set), and that > will cause virtiofsd to invoke fchmod() instead of fchmodat(). > > (A similar configuration exists further below with futimens() vs. > utimensat(), but the former is not a syscall but just a wrapper for the > latter, so no further whitelisting is required.) > > Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1842667 > Reported-by: Qian Cai > Cc: qemu-stable@nongnu.org > Signed-off-by: Max Reitz Queued. > --- > tools/virtiofsd/seccomp.c | 1 + > 1 file changed, 1 insertion(+) > > diff --git a/tools/virtiofsd/seccomp.c b/tools/virtiofsd/seccomp.c > index bd9e7b083c..3b1522acdd 100644 > --- a/tools/virtiofsd/seccomp.c > +++ b/tools/virtiofsd/seccomp.c > @@ -42,6 +42,7 @@ static const int syscall_whitelist[] = { > SCMP_SYS(exit_group), > SCMP_SYS(fallocate), > SCMP_SYS(fchdir), > + SCMP_SYS(fchmod), > SCMP_SYS(fchmodat), > SCMP_SYS(fchownat), > SCMP_SYS(fcntl), > -- > 2.26.2 > > -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK