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

Ferry Toth
2022-11-10T19:36:21Z

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