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

Ferry Toth
2022-09-30T21:19:01Z

Hi,

Op 30-09-2022 om 22:30 schreef Ferry Toth:
> Hi,
>
> Op 29-09-2022 om 16:20 schreef Andy Shevchenko:
>> On Thu, Sep 29, 2022 at 04:56:07PM +0300, Andy Shevchenko wrote:
>>> +Cc: Ferry
>>>
>>> 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)

>>> Configuration is based on in-tree i386_defconfig with some drivers
>>> enabled
>>> on top (no core stuff was altered, but if you wish to check, it's here:
>>> https://github.com/andy-shev/linux/blob/eds-acpi/arch/x86/configs/i386_defconfig).
>>>
>> For the record (and preventing some questions) the v6.0-rc7 still has
>> this issue.
>>
>> I can't test reverts, because it's huge pile of changes in that area
>> happened
>> for the last year or so.
>>
> I just tested this by reverting 3fb0fdb3 "x86/stackprotector/32: Make
> the canary into a regular percpu variable" and it's prerequisite
> d0962f2b "x86/entry/32: Remove leftover macros after stackprotector
> cleanups" on top of v5.13 and indeed this resolves the boot issue.
>
> I can also confirm the 2 reverts will not apply on top of v6.0-rc7.
>