Re: [PATCH v2 1/2] x86/stackprotector/32: Make the canary into a regular percpu variable

Brian Gerst
2022-11-14T21:43:24Z

On Thu, Nov 10, 2022 at 2:36 PM Ferry Toth <fntoth@gmail.com> wrote:
>
> Hi,
>
> Op 09-11-2022 om 23:33 schreef Brian Gerst:
> > On Wed, Nov 9, 2022 at 9:50 AM Andy Shevchenko
> > <andriy.shevchenko@intel.com> wrote:
> >> On Fri, Sep 30, 2022 at 11:18:51PM +0200, Ferry Toth wrote:
> >>> Op 30-09-2022 om 22:30 schreef Ferry Toth:
> >>>> Op 29-09-2022 om 16:20 schreef Andy Shevchenko:
> >>>>> On Thu, Sep 29, 2022 at 04:56:07PM +0300, Andy Shevchenko wrote:
> >>>>>> On Sat, Feb 13, 2021 at 11:19:44AM -0800, Andy Lutomirski wrote:
> >>>>>>> On 32-bit kernels, the stackprotector canary is quite nasty -- it is
> >>>>>>> stored at %gs:(20), which is nasty because 32-bit kernels use %fs for
> >>>>>>> percpu storage. It's even nastier because it means that whether %gs
> >>>>>>> contains userspace state or kernel state while running kernel code
> >>>>>>> depends on whether stackprotector is enabled (this is
> >>>>>>> CONFIG_X86_32_LAZY_GS), and this setting radically changes the way
> >>>>>>> that segment selectors work. Supporting both variants is a
> >>>>>>> maintenance and testing mess.
> >>>>>>>
> >>>>>>> Merely rearranging so that percpu and the stack canary
> >>>>>>> share the same segment would be messy as the 32-bit percpu address
> >>>>>>> layout isn't currently compatible with putting a variable at a fixed
> >>>>>>> offset.
> >>>>>>>
> >>>>>>> Fortunately, GCC 8.1 added options that allow the stack canary to be
> >>>>>>> accessed as %fs:__stack_chk_guard, effectively turning it
> >>>>>>> into an ordinary
> >>>>>>> percpu variable. This lets us get rid of all of the code to
> >>>>>>> manage the
> >>>>>>> stack canary GDT descriptor and the CONFIG_X86_32_LAZY_GS mess.
> >>>>>>>
> >>>>>>> (That name is special. We could use any symbol we want for the
> >>>>>>> %fs-relative mode, but for CONFIG_SMP=n, gcc refuses to
> >>>>>>> let us use any
> >>>>>>> name other than __stack_chk_guard.)
> >>>>>>>
> >>>>>>> This patch forcibly disables stackprotector on older compilers that
> >>>>>>> don't support the new options and makes the stack canary into a
> >>>>>>> percpu variable. The "lazy GS" approach is now used for all 32-bit
> >>>>>>> configurations.
> >>>>>>>
> >>>>>>> This patch also makes load_gs_index() work on 32-bit kernels. On
> >>>>>>> 64-bit kernels, it loads the GS selector and updates the user
> >>>>>>> GSBASE accordingly. (This is unchanged.) On 32-bit kernels,
> >>>>>>> it loads the GS selector and updates GSBASE, which is now
> >>>>>>> always the user base. This means that the overall effect is
> >>>>>>> the same on 32-bit and 64-bit, which avoids some ifdeffery.
> >>>>>> This patch broke 32-bit boot on Intel Merrifield
> >>>>>>
> >>>>>> git bisect start
> >>>>>> # good: [9f4ad9e425a1d3b6a34617b8ea226d56a119a717] Linux 5.12
> >>>>>> git bisect good 9f4ad9e425a1d3b6a34617b8ea226d56a119a717
> >>>>>> # bad: [62fb9874f5da54fdb243003b386128037319b219] Linux 5.13
> >>>>>> git bisect bad 62fb9874f5da54fdb243003b386128037319b219
> >>>>>> # bad: [85f3f17b5db2dd9f8a094a0ddc665555135afd22] Merge branch
> >>>>>> 'md-fixes' of
> >>>>>> https://git.kernel.org/pub/scm/linux/kernel/git/song/md into
> >>>>>> block-5.13
> >>>>>> git bisect bad 85f3f17b5db2dd9f8a094a0ddc665555135afd22
> >>>>>> # good: [ca62e9090d229926f43f20291bb44d67897baab7] Merge tag
> >>>>>> 'regulator-v5.13' of
> >>>>>> git://git.kernel.org/pub/scm/linux/kernel/git/broonie/regulator
> >>>>>> git bisect good ca62e9090d229926f43f20291bb44d67897baab7
> >>>>>> # bad: [68a32ba14177d4a21c4a9a941cf1d7aea86d436f] Merge tag
> >>>>>> 'drm-next-2021-04-28' of git://anongit.freedesktop.org/drm/drm
> >>>>>> git bisect bad 68a32ba14177d4a21c4a9a941cf1d7aea86d436f
> >>>>>> # good: [49c70ece54b0d1c51bc31b2b0c1070777c992c26]
> >>>>>> drm/amd/display: Change input parameter for set_drr
> >>>>>> git bisect good 49c70ece54b0d1c51bc31b2b0c1070777c992c26
> >>>>>> # good: [0b276e470a4d43e1365d3eb53c608a3d208cabd4] media: coda:
> >>>>>> fix macroblocks count control usage
> >>>>>> git bisect good 0b276e470a4d43e1365d3eb53c608a3d208cabd4
> >>>>>> # bad: [c6536676c7fe3f572ba55842e59c3c71c01e7fb3] Merge tag
> >>>>>> 'x86_core_for_v5.13' of
> >>>>>> git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
> >>>>>> git bisect bad c6536676c7fe3f572ba55842e59c3c71c01e7fb3
> >>>>>> # good: [d1466bc583a81830cef2399a4b8a514398351b40] Merge branch
> >>>>>> 'work.inode-type-fixes' of
> >>>>>> git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs
> >>>>>> git bisect good d1466bc583a81830cef2399a4b8a514398351b40
> >>>>>> # good: [fafe1e39ed213221c0bce6b0b31669334368dc97] Merge tag
> >>>>>> 'afs-netfs-lib-20210426' of
> >>>>>> git://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs
> >>>>>> git bisect good fafe1e39ed213221c0bce6b0b31669334368dc97
> >>>>>> # bad: [b1f480bc0686e65d5413c035bd13af2ea4888784] Merge branch
> >>>>>> 'x86/cpu' into WIP.x86/core, to merge the NOP changes & resolve
> >>>>>> a semantic conflict
> >>>>>> git bisect bad b1f480bc0686e65d5413c035bd13af2ea4888784
> >>>>>> # bad: [0c925c61dae18ee3cb93a61cc9dd9562a066034d]
> >>>>>> x86/tools/insn_decoder_test: Convert to insn_decode()
> >>>>>> git bisect bad 0c925c61dae18ee3cb93a61cc9dd9562a066034d
> >>>>>> # bad: [514ef77607b9ff184c11b88e8f100bc27f07460d]
> >>>>>> x86/boot/compressed/sev-es: Convert to insn_decode()
> >>>>>> git bisect bad 514ef77607b9ff184c11b88e8f100bc27f07460d
> >>>>>> # bad: [9e761296c52dcdb1aaa151b65bd39accb05740d9] x86/insn:
> >>>>>> Rename insn_decode() to insn_decode_from_regs()
> >>>>>> git bisect bad 9e761296c52dcdb1aaa151b65bd39accb05740d9
> >>>>>> # bad: [d0962f2b24c99889a386f0658c71535f56358f77] x86/entry/32:
> >>>>>> Remove leftover macros after stackprotector cleanups
> >>>>>> git bisect bad d0962f2b24c99889a386f0658c71535f56358f77
> >>>>>> # bad: [3fb0fdb3bbe7aed495109b3296b06c2409734023]
> >>>>>> x86/stackprotector/32: Make the canary into a regular percpu
> >>>>>> variable
> >>>>>> git bisect bad 3fb0fdb3bbe7aed495109b3296b06c2409734023
> >>>>>> # first bad commit: [3fb0fdb3bbe7aed495109b3296b06c2409734023]
> >>>>>> x86/stackprotector/32: Make the canary into a regular percpu
> >>>>>> variable
> >>>> With the bad commit the last words in dmesg are:
> >>>>
> >>>> mem auto-init: stack:off, heap alloc:off, heap free:off
> >>>> Initializing HighMem for node 0 (00036ffe:0003f500)
> >>>> Initializing Movable for node 0 (00000000:00000000)
> >>>> Checking if this processor honours the WP bit even in supervisor
> >>>> mode...Ok.
> >>>> Memory: 948444K/1004124K available (12430K kernel code, 2167K rwdata,
> >>>> 4948K rodata, 716K init, 716K bss, 55680K reserved, 0K cma-reserved,
> >>>> 136200K highmem)
> >>>> SLUB: HWalign=64, Order=0-3, MinObjects=0, CPUs=2, Nodes=1
> >>>> trace event string verifier disabled
> >>>> Dynamic Preempt: voluntary
> >>>> rcu: Preemptible hierarchical RCU implementation.
> >>>> rcu: RCU event tracing is enabled.
> >>>> rcu: RCU restricting CPUs from NR_CPUS=8 to nr_cpu_ids=2.
> >>>> Trampoline variant of Tasks RCU enabled.
> >>>> Tracing variant of Tasks RCU enabled.
> >>>> rcu: RCU calculated value of scheduler-enlistment delay is 100 jiffies.
> >>>> rcu: Adjusting geometry for rcu_fanout_leaf=16, nr_cpu_ids=2
> >>>> NR_IRQS: 2304, nr_irqs: 512, preallocated irqs: 0
> >>>>
> >>>> without the bad commit dmesg continues:
> >>>>
> >>>> random: get_random_bytes called from start_kernel+0x492/0x65a with
> >>>> crng_init=0
> >>>> Console: colour dummy device 80x25
> >>>> printk: console [tty0] enabled
> >>>> printk: bootconsole [uart0] disabled
> >>>>
> >>>> ....
> >>>>
> >>>>>> Any suggestions how to fix are welcome!
> >>>>>>
> >>> Interesting. I added the following fragment to the kernel config:
> >>>
> >>> # CONFIG_STACKPROTECTOR is not set
> >>>
> >>> And this resolves the boot issue (tested with v5.17 i686 on Intel
> >>> Merrifield)
> >> I'm not sure that's the correct approach.
> I didn't intend as a resolution, merely as a workaround. And since
> revert was not possible, as proof issue is localized in stack protector.
> >> Any answer from the Andy Lutomirski?
> >>
> >> And in general to x86 maintainers, do we support all features on x86 32-bit? If
> >> no, can it be said explicitly, please?
> > What compiler version are you using?
>
> I built with Yocto Honister which builds it's own cross-compiler gcc
> 11.2. For completeness:
>
> root@yuna:~# uname -a
> Linux yuna 5.17.0-edison-acpi-standard #1 SMP PREEMPT Sun Mar 20
> 20:14:17 UTC 2022 i686 i686 i386 GNU/Linux
>
> root@yuna:~# cat /proc/version
> Linux version 5.17.0-edison-acpi-standard (oe-user@oe-host)
> (i686-poky-linux-gcc (GCC) 11.2.0, GNU ld (GNU Binutils) 2.37.20210721)
> #1 SMP PREEMPT Sun Mar 20 20:14:17 UTC 2022
>
> root@yuna:~# cat /etc/os-release
> ID=poky-edison
> NAME="Poky (Yocto Project Reference Distro)"
> VERSION="3.4.4 (honister)"
> VERSION_ID=3.4.4
> PRETTY_NAME="Poky (Yocto Project Reference Distro) 3.4.4 (honister)"
>
> > --
> > Brian Gerst

What exactly happens when it fails (hang/reboot/oops)?

Does removing the call to boot_init_stack_canary() in init/main.c fix
the problem?

--
Brian Gerst