Re: [PATCH v2] x86/mm: warn on W+x mappings

[Ingo Molnar]

* Matt Fleming <matt@xxxxxxxxxxxxxxxxxxx> wrote:

> On Mon, 12 Oct, at 01:36:05PM, Borislav Petkov wrote:
> > On Tue, Oct 06, 2015 at 11:37:57AM -0400, Stephen Smalley wrote:
> > > > What does this range correspond to on your kernel?
> > 
> > Got a W+X splat here too, on the UEFI box with rc5+tip/master:
> > 
> > [    6.792949] rtc_cmos 00:02: setting system clock to 2015-10-12 11:17:03 UTC (1444648623)
> > [    6.807863] Freeing unused kernel memory: 1312K (ffffffff81f5f000 - ffffffff820a7000)
> > [    6.815831] usb 3-1: new high-speed USB device number 2 using ehci-pci
> > [    6.823261] Write protecting the kernel read-only data: 14336k
> > [    6.832196] Freeing unused kernel memory: 1796K (ffff88000383f000 - ffff880003a00000)
> > [    6.842210] Freeing unused kernel memory: 284K (ffff880003db9000 - ffff880003e00000)
> > [    6.850524] ------------[ cut here ]------------
> > [    6.855682] WARNING: CPU: 5 PID: 1 at arch/x86/mm/dump_pagetables.c:225 note_page+0x61e/0x7e0()
> > [    6.864944] x86/mm: Found insecure W+X mapping at address ffff88000005e000/0xffff88000005e000
> > [    6.874022] Modules linked in:
> > [    6.877643] CPU: 5 PID: 1 Comm: swapper/0 Not tainted 4.3.0-rc5+ #1
> > [    6.884462] Hardware name: Dell Inc. Precision T3600/0PTTT9, BIOS A13 05/11/2014
> > [    6.892416]  ffffffff81caf1f7 ffff88043bdffd60 ffffffff813aab2c ffff88043bdffda8
> > [    6.900460]  ffff88043bdffd98 ffffffff81066776 ffff880004e55308 0000000000000004
> > [    6.907816] usb 4-1: new high-speed USB device number 2 using ehci-pci
> > [    6.915499]  8000000000000163 ffff88043bdffe98 0000000000000000 ffff88043bdffdf8
> > [    6.923520] Call Trace:
> > [    6.926512]  [<ffffffff813aab2c>] dump_stack+0x4e/0x82
> > [    6.931551] usb 3-1: New USB device found, idVendor=8087, idProduct=0024
> > [    6.931552] usb 3-1: New USB device strings: Mfr=0, Product=0, SerialNumber=0
> > [    6.933120] hub 3-1:1.0: USB hub found
> > [    6.933369] hub 3-1:1.0: 6 ports detected
> > [    6.955784]  [<ffffffff81066776>] warn_slowpath_common+0x86/0xc0
> > [    6.962341]  [<ffffffff810667fc>] warn_slowpath_fmt+0x4c/0x50
> > [    6.968631]  [<ffffffff8105bb7e>] note_page+0x61e/0x7e0
> > [    6.974404]  [<ffffffff8105c09f>] ptdump_walk_pgd_level_core+0x35f/0x3f0
> > [    6.981651]  [<ffffffff8105c1d7>] ptdump_walk_pgd_level_checkwx+0x17/0x20
> > [    6.988996]  [<ffffffff81051b0e>] mark_rodata_ro+0xee/0x100
> > [    6.995124]  [<ffffffff81828610>] ? rest_init+0x140/0x140
> > [    7.001064]  [<ffffffff8182862d>] kernel_init+0x1d/0xe0
> > [    7.006841]  [<ffffffff81836f6f>] ret_from_fork+0x3f/0x70
> > [    7.012774]  [<ffffffff81828610>] ? rest_init+0x140/0x140
> > [    7.018706] ---[ end trace 920055014e07ef1e ]---
> > [    7.024302] x86/mm: Checked W+X mappings: FAILED, 69568 W+X pages found.
> > 
> > And yes, there are a bunch of those mappings here too:
> > 
> > $ grep -c 'RW.*x' /sys/kernel/debug/kernel_page_tables
> > 75
> > 
> > Some of them are the UEFI runtime regions. I guess we can try to map
> > them as RO maybe, they need to be X. Matt, any reasons against that?
> 
> I'm glad you asked (but you won't be)!
> 
> Basically, it's guaranteed that there exist some machines that contain
> data in EfiRuntimeCode regions (and so require write permission) and
> code in EfiRuntimeData regions (and therefore require eXecute),
> because the whole point of the new EFI_PROPERTIES_TABLE feature in
> UEFI v2.5 was to make it explicit when the firmware does not include
> such regions.

So why not unmap them after bootup? Is there any reason to call into EFI code 
while the system is up and running?

Thanks,

	Ingo
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