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

[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!
> >
> > 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.