Re: [PATCH 4.4 00/37] 4.4.110-stable review

[Guenter Roeck]

On Thu, Jan 11, 2018 at 11:47:23PM +0100, Thomas Gleixner wrote:
> On Thu, 11 Jan 2018, Steven Sistare wrote:
> > On 1/11/2018 5:30 PM, Thomas Gleixner wrote:
> > > On Thu, 11 Jan 2018, Thomas Gleixner wrote:
> > >> On Thu, 11 Jan 2018, Linus Torvalds wrote:
> > >>
> > >>> On Thu, Jan 11, 2018 at 12:37 PM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> > >>>>
> > >>>> 67a9108ed431 ("x86/efi: Build our own page table structures")
> > >>>>
> > >>>> got rid of EFI depending on real_mode_header->trampoline_pgd
> > >>>
> > >>> So I think it only got rid of by default - the codepath is still
> > >>> there, the allocation is still there, it's just that it's not actually
> > >>> used unless somebody does that "efi=old_mmap" thing.
> > >>
> > >> Yes, the trampoline_pgd is still around, but I can't figure out how it
> > >> would be used after boot. Confused, digging more.
> > > 
> > > So coming back to the same commit. From the changelog:
> > > 
> > >     This is caused by mapping EFI regions with RWX permissions.
> > >     There isn't much we can do to restrict the permissions for these
> > >     regions due to the way the firmware toolchains mix code and
> > >     data, but we can at least isolate these mappings so that they do
> > >     not appear in the regular kernel page tables.
> > >     
> > >     In commit d2f7cbe7b26a ("x86/efi: Runtime services virtual
> > >     mapping") we started using 'trampoline_pgd' to map the EFI
> > >     regions because there was an existing identity mapping there
> > >     which we use during the SetVirtualAddressMap() call and for
> > >     broken firmware that accesses those addresses.
> > > 
> > > So this very commit gets rid of the (ab)use of trampoline_pgd and allocates
> > > efi_pgd, which we made use the proper size.
> > > 
> > > trampoline_pgd is since then only used to get into long mode in
> > > realmode/rm/trampoline_64.S and for reboot in machine_real_restart().
> > > 
> > > The runtime services stuff does not use it in kernel versions >= 4.6
> > > 
> > > Thanks,
> > > 
> > > 	tglx
> > 
> > Yes, and addressing Linus' concern about EFI_OLD_MEMMAP, those paths are 
> > independent of it.  When EFI_OLD_MMAP is enabled, the efi pgd is not 
> > used, and the bug will not bite.
> 
> We have a fix queued in tip/x86/pti which addresses a missing NX clear, but
> that's a different story.
> 
Since you are talking about NX, I see this in last night's -next:

kernel tried to execute NX-protected page - exploit attempt? (uid: 0)
BUG: unable to handle kernel paging request at fffffe0000007000
IP: 0xfffffe0000006e9d
PGD ffd6067 P4D ffd6067 PUD ffd5067 PMD ff73067 PTE 800000000fc09063
Oops: 0011 [#1] PREEMPT SMP PTI
Modules linked in:
CPU: 0 PID: 1 Comm: init Tainted: G        W
4.15.0-rc7-next-20180111-yocto-standard #1
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS
rel-1.10.2-0-g5f4c7b1-prebuilt.qemu-project.org 04/01/2014
RIP: 0010:0xfffffe0000006e9d
RSP: 0018:ffffaee28000ffd0 EFLAGS: 00000006
RAX: 000000000000000c RBX: 0000000000400040 RCX: 00007f2c4186ad6a
RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffffffffb6a00000
RBP: 0000000000000008 R08: 000000000000037f R09: 0000000000000064
R10: 00000000078bfbfd R11: 0000000000000246 R12: 00007f2c41856a60
R13: 0000000000000000 R14: 0000000000402368 R15: 0000000000001000
FS:  0000000000000000(0000) GS:ffff95fecfc00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: fffffe0000007000 CR3: 000000000d88a000 CR4: 00000000003406f0
Call Trace:
Code: 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90
90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 <90> 90 90 90 90 90 90 90
90 90 90 90 90 90 90 90 90 90 90 90 90 
RIP: 0xfffffe0000006e9d RSP: ffffaee28000ffd0
CR2: fffffe0000007000
---[ end trace a82b8742114c1785 ]---

Is this the issue you are talking about, or is the fix triggering 
the crash ?

Guenter