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

Ferry Toth
2022-11-17T19:28:39Z

Hi

Op 14-11-2022 om 23:16 schreef Ferry Toth:
> Hi
>
> Op 14-11-2022 om 22:43 schreef Brian Gerst:
>> 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)?
>
> It just hangs. Rebooting into another kernel with
> CONFIG_STACKPROTECTOR not set and displaying journal of the failed
> boot show the last words mentioned above.
>
>> Does removing the call to boot_init_stack_canary() in init/main.c fix
>> the problem?
> I will try tomorrow.

I just tried this on top of v6.0:

diff --git a/init/main.c b/init/main.c
index 1fe7942f5d4a..f30ec221f473 100644
--- a/init/main.c
+++ b/init/main.c
@@ -1046,7 +1046,6 @@ asmlinkage __visible void __init
__no_sanitize_address start_kernel(void)
* - random_init() to initialize the RNG from from early entropy
sources
*/
random_init(command_line);
-    boot_init_stack_canary();

perf_event_init();
profile_init();

and it just hangs, same as with 5.17. Again tried (without this patch)
"CONFIG_STACKPROTECTOR not set" and that boots fine.

>> --
>> Brian Gerst