Re: [PATCH v2] ARM: mm: flip priority of CONFIG_DEBUG_RODATA

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Kees, Russell,

On Wed, Dec 2, 2015 at 9:27 PM, Kees Cook <keescook@chromium.org> wrote:
> The use of CONFIG_DEBUG_RODATA is generally seen as an essential part of
> kernel self-protection:
> http://www.openwall.com/lists/kernel-hardening/2015/11/30/13
> Additionally, its name has grown to mean things beyond just rodata. To
> get ARM closer to this, we ought to rearrange the names of the configs
> that control how the kernel protects its memory. What was called
> CONFIG_ARM_KERNMEM_PERMS is really doing the work that other architectures
> call CONFIG_DEBUG_RODATA.

[...]

This broke s2ram with shmobile_defconfig on r8a7791/koelsch:

    Freezing user space processes ... (elapsed 0.002 seconds) done.
    Freezing remaining freezable tasks ... (elapsed 0.003 seconds) done.
    PM: suspend of devices complete after 112.157 msecs
    PM: late suspend of devices complete after 1.605 msecs
    PM: noirq suspend of devices complete after 13.098 msecs
    Disabling non-boot CPUs ...
    s---[ end Kernel panic - not syncing: Attempted to kill the idle task!
    CPU0: stopping
    CPU: 0 PID: 2412 Comm: s2ram Tainted: G      D
4.4.0-rc6-00003-g1bb20571dcf0edfc #470
    Hardware name: Generic R8A7791 (Flattened Device Tree)
    Backtrace:
    [<c010a92c>] (dump_backtrace) from [<c010aad4>] (show_stack+0x18/0x1c)
     r6:00000000 r5:00000000 r4:00000000 r3:80404000
    [<c010aabc>] (show_stack) from [<c02b9ff4>] (dump_stack+0x78/0x94)
    [<c02b9f7c>] (dump_stack) from [<c010d4b4>] (handle_IPI+0xf4/0x19c)
     r4:c09313f0 r3:c09091ec
    [<c010d3c0>] (handle_IPI) from [<c0101430>] (gic_handle_irq+0x7c/0x98)
     r7:c0910b80 r6:ee1d5c30 r5:c0902754 r4:f0802000
    [<c01013b4>] (gic_handle_irq) from [<c010b654>] (__irq_svc+0x54/0x70)
    Exception stack(0xee1d5c30 to 0xee1d5c78)
    5c20:                                     c0955484 00000002
00000000 60070013
    5c40: c0942718 c093916c 00000005 0000000f 00000000 00000000
c0943088 ee1d5cd4
    5c60: ee1d5c08 ee1d5c80 c033fc20 c0158120 60070013 ffffffff
     r8:00000000 r7:ee1d5c64 r6:ffffffff r5:60070013 r4:c0158120 r3:c033fc20
    [<c0157ecc>] (console_unlock) from [<c0158724>] (vprintk_emit+0x448/0x4a4)
     r10:c09450a6 r9:00000000 r8:0000000e r7:00000005 r6:00000006 r5:c0932758
     r4:00000001
    [<c01582dc>] (vprintk_emit) from [<c01588e0>] (vprintk_default+0x28/0x30)
     r10:c09055e0 r9:00000001 r8:c09055e0 r7:00000010 r6:00000000 r5:00000000
     r4:00000001
    [<c01588b8>] (vprintk_default) from [<c018e538>] (printk+0x34/0x40)
    [<c018e508>] (printk) from [<c010cfb8>] (__cpu_die+0x34/0x78)
     r3:00000003 r2:c0906808 r1:00000001 r0:c0710af6
    [<c010cf84>] (__cpu_die) from [<c011d7d0>] (_cpu_down+0x168/0x290)
     r4:00000001 r3:00000005
    [<c011d668>] (_cpu_down) from [<c011dd90>] (disable_nonboot_cpus+0x70/0xf0)
     r10:00000051 r9:c0932734 r8:c0902528 r7:00000000 r6:c090245c r5:c0931b40
     r4:00000001
    [<c011dd20>] (disable_nonboot_cpus) from [<c0155fd8>]
(suspend_devices_and_enter+0x290/0x3f8)
     r8:c0714bb5 r7:eebac300 r6:00000003 r5:c0932734 r4:00000000 r3:00000000
    [<c0155d48>] (suspend_devices_and_enter) from [<c01561f4>]
(pm_suspend+0xb4/0x1c8)
     r9:c093273c r8:c0714bb5 r7:eebac300 r6:00000003 r5:c09576fc r4:00000000
    [<c0156140>] (pm_suspend) from [<c0155148>] (state_store+0xb0/0xc4)
     r6:00000004 r5:00000003 r4:00000003 r3:0000006d
    [<c0155098>] (state_store) from [<c02bbce8>] (kobj_attr_store+0x1c/0x28)
     r9:000cdc08 r8:ee1d5f80 r7:eebacb0c r6:eebacb00 r5:eebac300 r4:eebac300
    [<c02bbccc>] (kobj_attr_store) from [<c0222438>] (sysfs_kf_write+0x44/0x50)
    [<c02223f4>] (sysfs_kf_write) from [<c0221ae0>]
(kernfs_fop_write+0x13c/0x1a0)
     r4:00000004 r3:c02223f4
    [<c02219a4>] (kernfs_fop_write) from [<c01ca1b4>] (__vfs_write+0x34/0xdc)
     r10:00000000 r9:ee1d4000 r8:c0106fa4 r7:00000004 r6:ee1d5f80 r5:c02219a4
     r4:edf85d00
    [<c01ca180>] (__vfs_write) from [<c01ca3dc>] (vfs_write+0xb8/0x140)
     r7:ee1d5f80 r6:000cdc08 r5:edf85d00 r4:00000004
    [<c01ca324>] (vfs_write) from [<c01ca544>] (SyS_write+0x50/0x90)
     r9:ee1d4000 r8:c0106fa4 r7:000cdc08 r6:00000004 r5:edf85d00 r4:edf85d00
    [<c01ca4f4>] (SyS_write) from [<c0106de0>] (ret_fast_syscall+0x0/0x3c)

Before commit 1bb20571dcf0edfc ("ARM: 8470/1: mm: flip priority of
CONFIG_DEBUG_RODATA"):

    # CONFIG_ARM_KERNMEM_PERMS is not set

    Freezing user space processes ... (elapsed 0.001 seconds) done.
    Freezing remaining freezable tasks ... (elapsed 0.003 seconds) done.
    PM: suspend of devices complete after 112.163 msecs
    PM: late suspend of devices complete after 1.610 msecs
    PM: noirq suspend of devices complete after 13.109 msecs
    Disabling non-boot CPUs ...
    CPU1: shutdown

After the offending commit:

    CONFIG_DEBUG_RODATA=y
    CONFIG_DEBUG_ALIGN_RODATA=y

The "problem" is that DEBUG_RODATA now defaults to y on CPU_V7, so it gets
enabled for shmobile_defconfig. If I manually disable DEBUG_RODATA again,
s2ram does work.

The real problem is something else, though. I can trigger the same panic
without the offending commit by enabling:

    CONFIG_ARM_KERNMEM_PERMS=y
    CONFIG_DEBUG_RODATA=y

I never enabled those options before, so I have no idea if this is a recent
regression. I've just tried a few older versions: on v4.4-rc1 I see the same
panic, on v4.3 (and v4.3.3) I don't see the panic, and the "CPU1: shutdown"
line, but the system doesn't wake up.

Thanks for your suggestions!

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds

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