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

Andy Shevchenko
2022-11-09T14:50:25Z

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.
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?

> > > > 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.

--
With Best Regards,
Andy Shevchenko