Re: [PATCH] Arm: mm: ftrace: Only set text back to ro after kernel has been marked ro

From: Russell King - ARM Linux
Date: Tue Dec 05 2017 - 08:36:25 EST

On Tue, Dec 05, 2017 at 01:30:11PM +0000, Phil Elwell wrote:
> On 05/12/2017 13:23, Matthias Reichl wrote:
> > On Tue, Dec 05, 2017 at 01:14:17PM +0000, Russell King - ARM Linux wrote:
> >> On Tue, Dec 05, 2017 at 12:47:09PM +0100, Matthias Reichl wrote:
> >>> On Wed, Aug 23, 2017 at 03:03:51PM -0400, Steven Rostedt wrote:
> >>>> On Wed, 23 Aug 2017 11:48:13 -0700
> >>>> Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> >>>>
> >>>>>> diff --git a/arch/arm/mm/init.c b/arch/arm/mm/init.c
> >>>>>> index ad80548..fd75f38 100644
> >>>>>> --- a/arch/arm/mm/init.c
> >>>>>> +++ b/arch/arm/mm/init.c
> >>>>>> @@ -745,19 +745,29 @@ static int __mark_rodata_ro(void *unused)
> >>>>>> return 0;
> >>>>>> }
> >>>>>>
> >>>>>> +static int kernel_set_to_readonly;
> >>>>>
> >>>>> Adding a comment here might be a good idea, something like:
> >>>>>
> >>>>> /* Has system boot-up reached mark_rodata_ro() yet? */
> >>>>
> >>>> I don't mind adding a comment, but the above is rather self explanatory
> >>>> (one can easily see that it is set in mark_rodata_ro() with a simple
> >>>> search).
> >>>>
> >>>> If a comment is to be added, something a bit more descriptive of the
> >>>> functionality of the variable would be appropriate:
> >>>>
> >>>> /*
> >>>> * Ignore modifying kernel text permissions until the kernel core calls
> >>>> * make_rodata_ro() at system start up.
> >>>> */
> >>>>
> >>>> I can resend with the comment, or whoever takes this could add it
> >>>> themselves.
> >>>
> >>> Gentle ping: this patch doesn't seem to have landed in upstream
> >>> trees yet. Is any more work required?
> >>>
> >>> It would be nice to have this fix added. Just tested next-20171205
> >>> on RPi B+, it oopses when the function tracer is enabled during boot.
> >>> next-20171205 plus this patch boots up fine.
> >>
> >> When does it oops?
> >
> > Rather early in the boot process:
> >
> > [ 0.000000] Booting Linux on physical CPU 0x0
> > [ 0.000000] Linux version 4.15.0-rc2-next-20171205 (hias@camel2) (gcc version 4.9.3 (crosstool-NG crosstool-ng-1.22.0-88-g8460611)) #3 Tue Dec 5 12:35:08 CET 2017
> > [ 0.000000] CPU: ARMv6-compatible processor [410fb767] revision 7 (ARMv7), cr=00c5387d
> > [ 0.000000] CPU: PIPT / VIPT nonaliasing data cache, VIPT nonaliasing instruction cache
> > [ 0.000000] OF: fdt: Machine model: Raspberry Pi Model B Plus Rev 1.2
> > [ 0.000000] earlycon: pl11 at MMIO32 0x20201000 (options '')
> > [ 0.000000] bootconsole [pl11] enabled
> > [ 0.000000] Memory policy: Data cache writeback
> > [ 0.000000] cma: Reserved 32 MiB at 0x13c00000
> > [ 0.000000] CPU: All CPU(s) started in SVC mode.
> > [ 0.000000] random: fast init done
> > [ 0.000000] Built 1 zonelists, mobility grouping on. Total pages: 89408
> > [ 0.000000] Kernel command line: bcm2708_fb.fbwidth=1280 bcm2708_fb.fbheight=1024 bcm2708_fb.fbswap=1 dma.dmachans=0x7f35 bcm2708.boardrev=0x10 bcm2708.serial=0x59ce1e57 bcm2708.uart_clock=48000000 bcm2708.disk_led_gpio=47 bcm2708.disk_led_active_low=0 smsc95xx.macaddr=B8:27:EB:CE:1E:57 vc_mem.mem_base=0x1ec00000 vc_mem.mem_size=0x20000000 root=/dev/mmcblk0p2 rw rootwait elevator=noop earlycon=pl011,mmio32,0x20201000 console=/dev/ttyAMA0,115200 ftrace=function
> > [ 0.000000] Dentry cache hash table entries: 65536 (order: 6, 262144 bytes)
> > [ 0.000000] Inode-cache hash table entries: 32768 (order: 5, 131072 bytes)
> > [ 0.000000] Memory: 311776K/360448K available (7168K kernel code, 561K rwdata, 2212K rodata, 1024K init, 683K bss, 15904K reserved, 32768K cma-reserved)
> > [ 0.000000] Virtual kernel memory layout:
> > [ 0.000000] vector : 0xffff0000 - 0xffff1000 ( 4 kB)
> > [ 0.000000] fixmap : 0xffc00000 - 0xfff00000 (3072 kB)
> > [ 0.000000] vmalloc : 0xd6800000 - 0xff800000 ( 656 MB)
> > [ 0.000000] lowmem : 0xc0000000 - 0xd6000000 ( 352 MB)
> > [ 0.000000] modules : 0xbf000000 - 0xc0000000 ( 16 MB)
> > [ 0.000000] .text : 0x(ptrval) - 0x(ptrval) (8160 kB)
> > [ 0.000000] .init : 0x(ptrval) - 0x(ptrval) (1024 kB)
> > [ 0.000000] .data : 0x(ptrval) - 0x(ptrval) ( 562 kB)
> > [ 0.000000] .bss : 0x(ptrval) - 0x(ptrval) ( 684 kB)
> > [ 0.000000] SLUB: HWalign=64, Order=0-3, MinObjects=0, CPUs=1, Nodes=1
> > [ 0.000000] ftrace: allocating 25789 entries in 76 pages
> > [ 0.000000] Starting tracer 'function'
> > [ 0.000000] NR_IRQS: 16, nr_irqs: 16, preallocated irqs: 16
> > [ 0.000052] sched_clock: 32 bits at 1000kHz, resolution 1000ns, wraps every 2147483647500ns
> > [ 0.008889] clocksource: timer: mask: 0xffffffff max_cycles: 0xffffffff, max_idle_ns: 1911260446275 ns
> > [ 0.018979] bcm2835: system timer (irq = 27)
> > [ 0.028070] Console: colour dummy device 80x30
> > [ 0.033154] Calibrating delay loop... 697.95 BogoMIPS (lpj=3489792)
> > [ 0.090179] pid_max: default: 32768 minimum: 301
> > [ 0.097851] Mount-cache hash table entries: 1024 (order: 0, 4096 bytes)
> > [ 0.104852] Mountpoint-cache hash table entries: 1024 (order: 0, 4096 bytes)
> > [ 0.117353] CPU: Testing write buffer coherency: ok
> > [ 0.127472] Setting up static identity map for 0x100000 - 0x100054
> > [ 0.145654] devtmpfs: initialized
> > [ 0.235561] VFP support v0.3: implementor 41 architecture 1 part 20 variant b rev 5
> > [ 0.245751] Unable to handle kernel paging request at virtual address c09eb214
> > [ 0.253373] pgd = 8aaa5336
> > [ 0.256250] [c09eb214] *pgd=0080840e(bad)
> > [ 0.260567] Internal error: Oops: 80d [#1] ARM
> > [ 0.265073] Modules linked in:
> > [ 0.268188] CPU: 0 PID: 1 Comm: swapper Not tainted 4.15.0-rc2-next-20171205 #3
> > [ 0.275592] Hardware name: BCM2835
> > [ 0.279046] task: 11ad8790 task.stack: 886fda4c
> > [ 0.283670] PC is at ksysfs_init+0x64/0xb0
> > [ 0.287840] LR is at internal_create_group+0x294/0x2c4
> > [ 0.293049] pc : [<c0b0a33c>] lr : [<c0292898>] psr: 20000053
> > [ 0.299400] sp : d3a3ded0 ip : c028e908 fp : d3a3dee4
> > [ 0.304694] r10: 00000000 r9 : c0c8c500 r8 : c0c8c500
> > [ 0.309991] r7 : 00000000 r6 : c0c04048 r5 : c0c8d19c r4 : 00000000
> > [ 0.316607] r3 : 00000024 r2 : c0abf19c r1 : c09eb20c r0 : d3a81d80
> > [ 0.323224] Flags: nzCv IRQs on FIQs off Mode SVC_32 ISA ARM Segment none
> > [ 0.330543] Control: 00c5387d Table: 00004008 DAC: 00000051
> > [ 0.336367] Process swapper (pid: 1, stack limit = 0xfa32e9e1)
> > [ 0.342280] Stack: (0xd3a3ded0 to 0xd3a3e000)
> > [ 0.346703] dec0: 00000001 c0b0a2d8 d3a3df5c d3a3dee8
> > [ 0.354999] dee0: c01027b0 c0b0a2e4 c0a280c0 00000000 d3a3df5c d3a3df00 c0138578 c0b00650
> > [ 0.363294] df00: d3a3df00 c0c0c328 00000000 c0a280d4 0000009f c0a280d4 00000001 00000001
> > [ 0.371590] df20: 000000a0 c0a27454 d5fffd21 d5fffd28 c014f4c0 4710e3ed 00000001 000000a0
> > [ 0.379886] df40: c0b5681c c0b7cb84 c0c8c500 c0c8c500 d3a3df94 d3a3df60 c0b00ef8 c01026f8
> > [ 0.388180] df60: 00000001 00000001 00000000 c0b00644 00000000 c07463a4 00000000 00000000
> > [ 0.396475] df80: 00000000 00000000 d3a3dfac d3a3df98 c07463bc c0b00df0 ffffffff 00000000
> > [ 0.404769] dfa0: 00000000 d3a3dfb0 c01010e8 c07463b0 00000000 00000000 00000000 00000000
> > [ 0.413061] dfc0: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
> > [ 0.421354] dfe0: 00000000 00000000 00000000 00000000 00000013 00000000 00000000 00000000
> > [ 0.429689] [<c0b0a33c>] (ksysfs_init) from [<c01027b0>] (do_one_initcall+0xc4/0x184)
> > [ 0.437652] [<c01027b0>] (do_one_initcall) from [<c0b00ef8>] (kernel_init_freeable+0x114/0x1d4)
> > [ 0.446490] [<c0b00ef8>] (kernel_init_freeable) from [<c07463bc>] (kernel_init+0x18/0x11c)
> > [ 0.454887] [<c07463bc>] (kernel_init) from [<c01010e8>] (ret_from_fork+0x14/0x2c)
> > [ 0.462559] Exception stack(0xd3a3dfb0 to 0xd3a3dff8)
> > [ 0.467683] dfa0: 00000000 00000000 00000000 00000000
> > [ 0.475976] dfc0: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
> > [ 0.484266] dfe0: 00000000 00000000 00000000 00000000 00000013 00000000
> > [ 0.490982] Code: e3a00000 e89da830 e59f1048 e5950000 (e5813008)
> > [ 0.497194] ---[ end trace 53d55c7b93eb8c51 ]---
> > [ 0.502064] Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b
> > [ 0.502064]
> > [ 0.511346] ---[ end Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b
> > [ 0.511346]
> >
> > so long,
> >
> > Hias
> >
> >>
> >> Reading through this code, I'm left wondering why we switch the rodata
> >> section to be writable here - if we're poking at kernel text, then
> >> surely we shouldn't be the read-only data read-write?
> >>
> >> Should kernel_set_to_readonly also be a rodata-after-init variable?
>
> This was my initial explanation:
>
> 1. Data which is marked __ro_after_init is initially writeable.
>
> 2. The ro_perms data covers kernel text, read-only data and __ro_after_init data.
>
> 3. set_kernel_text_rw marks everything in ro_perms as writeable.
>
> 4. set_kernel_text_ro marks everything in ro_perms as read-only, including the __ro_after_init data.
>
> 5. Using the function tracing code involves code modification, resulting in calls to
> __ftrace_modify_code and set_kernel_text_ro.
>
> 6. Therefore if function tracing is enabled before kernel_init has completed then the __ro_after_init
> data is made read-only prematurely.

My question still stands, but let me rephrase. Do we need
set_kernel_text_*() to touch the read-only data?

--
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