| Message ID | 20190509121820.16294-2-stefanha@redhat.com |
|---|---|
| State | New |
| Headers | show |
| Series | security.rst: add Security Guide to developer docs | expand |
On Thu, May 09, 2019 at 01:18:19PM +0100, Stefan Hajnoczi wrote: > At KVM Forum 2018 I gave a presentation on security in QEMU: > https://www.youtube.com/watch?v=YAdRf_hwxU8 (video) > https://vmsplice.net/~stefan/stefanha-kvm-forum-2018.pdf (slides) > > This patch adds a guide to secure coding practices. This document > covers things that developers should know about security in QEMU. It is > just a starting point that we can expand on later. I hope it will be > useful as a resource for new contributors and will save code reviewers > from explaining the same concepts many times. > > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com> > Acked-by: Stefano Garzarella <sgarzare@redhat.com> > Reviewed-by: Alex Bennée <alex.bennee@linaro.org> > Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com> > --- > docs/devel/index.rst | 1 + > docs/devel/secure-coding-practices.rst | 106 +++++++++++++++++++++++++ > 2 files changed, 107 insertions(+) > create mode 100644 docs/devel/secure-coding-practices.rst Reviewed-by: Daniel P. Berrangé <berrange@redhat.com> Regards, Daniel
Stefan Hajnoczi <stefanha@redhat.com> 于2019年5月9日周四 下午8:20写道： > At KVM Forum 2018 I gave a presentation on security in QEMU: > https://www.youtube.com/watch?v=YAdRf_hwxU8 (video) > https://vmsplice.net/~stefan/stefanha-kvm-forum-2018.pdf (slides) > > This patch adds a guide to secure coding practices. This document > covers things that developers should know about security in QEMU. It is > just a starting point that we can expand on later. I hope it will be > useful as a resource for new contributors and will save code reviewers > from explaining the same concepts many times. > > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com> > Acked-by: Stefano Garzarella <sgarzare@redhat.com> > Reviewed-by: Alex Bennée <alex.bennee@linaro.org> > Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com> > Reviewed-by: Li Qiang <liq3ea@gmail.com> > --- > docs/devel/index.rst | 1 + > docs/devel/secure-coding-practices.rst | 106 +++++++++++++++++++++++++ > 2 files changed, 107 insertions(+) > create mode 100644 docs/devel/secure-coding-practices.rst > > diff --git a/docs/devel/index.rst b/docs/devel/index.rst > index ebbab636ce..2a4ddf40ad 100644 > --- a/docs/devel/index.rst > +++ b/docs/devel/index.rst > @@ -20,3 +20,4 @@ Contents: > stable-process > testing > decodetree > + secure-coding-practices > diff --git a/docs/devel/secure-coding-practices.rst > b/docs/devel/secure-coding-practices.rst > new file mode 100644 > index 0000000000..cbfc8af67e > --- /dev/null > +++ b/docs/devel/secure-coding-practices.rst > @@ -0,0 +1,106 @@ > +======================= > +Secure Coding Practices > +======================= > +This document covers topics that both developers and security researchers > must > +be aware of so that they can develop safe code and audit existing code > +properly. > + > +Reporting Security Bugs > +----------------------- > +For details on how to report security bugs or ask questions about > potential > +security bugs, see the `Security Process wiki page > +<https://wiki.qemu.org/SecurityProcess>`_. > + > +General Secure C Coding Practices > +--------------------------------- > +Most CVEs (security bugs) reported against QEMU are not specific to > +virtualization or emulation. They are simply C programming bugs. > Therefore > +it's critical to be aware of common classes of security bugs. > + > +There is a wide selection of resources available covering secure C > coding. For > +example, the `CERT C Coding Standard > +<https://wiki.sei.cmu.edu/confluence/display/c/SEI+CERT+C+Coding+Standard > >`_ > +covers the most important classes of security bugs. > + > +Instead of describing them in detail here, only the names of the most > important > +classes of security bugs are mentioned: > + > +* Buffer overflows > +* Use-after-free and double-free > +* Integer overflows > +* Format string vulnerabilities > + > +Some of these classes of bugs can be detected by analyzers. Static > analysis is > +performed regularly by Coverity and the most obvious of these bugs are > even > +reported by compilers. Dynamic analysis is possible with valgrind, tsan, > and > +asan. > + > +Input Validation > +---------------- > +Inputs from the guest or external sources (e.g. network, files) cannot be > +trusted and may be invalid. Inputs must be checked before using them in > a way > +that could crash the program, expose host memory to the guest, or > otherwise be > +exploitable by an attacker. > + > +The most sensitive attack surface is device emulation. All hardware > register > +accesses and data read from guest memory must be validated. A typical > example > +is a device that contains multiple units that are selectable by the guest > via > +an index register:: > + > + typedef struct { > + ProcessingUnit unit[2]; > + ... > + } MyDeviceState; > + > + static void mydev_writel(void *opaque, uint32_t addr, uint32_t val) > + { > + MyDeviceState *mydev = opaque; > + ProcessingUnit *unit; > + > + switch (addr) { > + case MYDEV_SELECT_UNIT: > + unit = &mydev->unit[val]; <-- this input wasn't validated! > + ... > + } > + } > + > +If ``val`` is not in range [0, 1] then an out-of-bounds memory access > will take > +place when ``unit`` is dereferenced. The code must check that ``val`` is > 0 or > +1 and handle the case where it is invalid. > + > +Unexpected Device Accesses > +-------------------------- > +The guest may access device registers in unusual orders or at unexpected > +moments. Device emulation code must not assume that the guest follows the > +typical "theory of operation" presented in driver writer manuals. The > guest > +may make nonsense accesses to device registers such as starting operations > +before the device has been fully initialized. > + > +A related issue is that device emulation code must be prepared for > unexpected > +device register accesses while asynchronous operations are in progress. A > +well-behaved guest might wait for a completion interrupt before accessing > +certain device registers. Device emulation code must handle the case > where the > +guest overwrites registers or submits further requests before an ongoing > +request completes. Unexpected accesses must not cause memory corruption > or > +leaks in QEMU. > + > +Invalid device register accesses can be reported with > +``qemu_log_mask(LOG_GUEST_ERROR, ...)``. The ``-d guest_errors`` > command-line > +option enables these log messages. > + > +Live Migration > +-------------- > +Device state can be saved to disk image files and shared with other users. > +Live migration code must validate inputs when loading device state so an > +attacker cannot gain control by crafting invalid device states. Device > state > +is therefore considered untrusted even though it is typically generated > by QEMU > +itself. > + > +Guest Memory Access Races > +------------------------- > +Guests with multiple vCPUs may modify guest RAM while device emulation > code is > +running. Device emulation code must copy in descriptors and other guest > RAM > +structures and only process the local copy. This prevents > +time-of-check-to-time-of-use (TOCTOU) race conditions that could cause > QEMU to > +crash when a vCPU thread modifies guest RAM while device emulation is > +processing it. > -- > 2.21.0 > > >
diff --git a/docs/devel/index.rst b/docs/devel/index.rst index ebbab636ce..2a4ddf40ad 100644 --- a/docs/devel/index.rst +++ b/docs/devel/index.rst @@ -20,3 +20,4 @@ Contents: stable-process testing decodetree + secure-coding-practices diff --git a/docs/devel/secure-coding-practices.rst b/docs/devel/secure-coding-practices.rst new file mode 100644 index 0000000000..cbfc8af67e --- /dev/null +++ b/docs/devel/secure-coding-practices.rst @@ -0,0 +1,106 @@ +======================= +Secure Coding Practices +======================= +This document covers topics that both developers and security researchers must +be aware of so that they can develop safe code and audit existing code +properly. + +Reporting Security Bugs +----------------------- +For details on how to report security bugs or ask questions about potential +security bugs, see the `Security Process wiki page +<https://wiki.qemu.org/SecurityProcess>`_. + +General Secure C Coding Practices +--------------------------------- +Most CVEs (security bugs) reported against QEMU are not specific to +virtualization or emulation. They are simply C programming bugs. Therefore +it's critical to be aware of common classes of security bugs. + +There is a wide selection of resources available covering secure C coding. For +example, the `CERT C Coding Standard +<https://wiki.sei.cmu.edu/confluence/display/c/SEI+CERT+C+Coding+Standard>`_ +covers the most important classes of security bugs. + +Instead of describing them in detail here, only the names of the most important +classes of security bugs are mentioned: + +* Buffer overflows +* Use-after-free and double-free +* Integer overflows +* Format string vulnerabilities + +Some of these classes of bugs can be detected by analyzers. Static analysis is +performed regularly by Coverity and the most obvious of these bugs are even +reported by compilers. Dynamic analysis is possible with valgrind, tsan, and +asan. + +Input Validation +---------------- +Inputs from the guest or external sources (e.g. network, files) cannot be +trusted and may be invalid. Inputs must be checked before using them in a way +that could crash the program, expose host memory to the guest, or otherwise be +exploitable by an attacker. + +The most sensitive attack surface is device emulation. All hardware register +accesses and data read from guest memory must be validated. A typical example +is a device that contains multiple units that are selectable by the guest via +an index register:: + + typedef struct { + ProcessingUnit unit[2]; + ... + } MyDeviceState; + + static void mydev_writel(void *opaque, uint32_t addr, uint32_t val) + { + MyDeviceState *mydev = opaque; + ProcessingUnit *unit; + + switch (addr) { + case MYDEV_SELECT_UNIT: + unit = &mydev->unit[val]; <-- this input wasn't validated! + ... + } + } + +If ``val`` is not in range [0, 1] then an out-of-bounds memory access will take +place when ``unit`` is dereferenced. The code must check that ``val`` is 0 or +1 and handle the case where it is invalid. + +Unexpected Device Accesses +-------------------------- +The guest may access device registers in unusual orders or at unexpected +moments. Device emulation code must not assume that the guest follows the +typical "theory of operation" presented in driver writer manuals. The guest +may make nonsense accesses to device registers such as starting operations +before the device has been fully initialized. + +A related issue is that device emulation code must be prepared for unexpected +device register accesses while asynchronous operations are in progress. A +well-behaved guest might wait for a completion interrupt before accessing +certain device registers. Device emulation code must handle the case where the +guest overwrites registers or submits further requests before an ongoing +request completes. Unexpected accesses must not cause memory corruption or +leaks in QEMU. + +Invalid device register accesses can be reported with +``qemu_log_mask(LOG_GUEST_ERROR, ...)``. The ``-d guest_errors`` command-line +option enables these log messages. + +Live Migration +-------------- +Device state can be saved to disk image files and shared with other users. +Live migration code must validate inputs when loading device state so an +attacker cannot gain control by crafting invalid device states. Device state +is therefore considered untrusted even though it is typically generated by QEMU +itself. + +Guest Memory Access Races +------------------------- +Guests with multiple vCPUs may modify guest RAM while device emulation code is +running. Device emulation code must copy in descriptors and other guest RAM +structures and only process the local copy. This prevents +time-of-check-to-time-of-use (TOCTOU) race conditions that could cause QEMU to +crash when a vCPU thread modifies guest RAM while device emulation is +processing it.