[3.19.y-ckt stable] Patch "x86/setup: Extend low identity map to cover whole kernel range" has been added to staging queue

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    x86/setup: Extend low identity map to cover whole kernel range

to the linux-3.19.y-queue branch of the 3.19.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-3.19.y-queue

This patch is scheduled to be released in version 3.19.8-ckt11.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.19.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

------

>From 6693e7baf7092908d60419e8e2e2402b1675e874 Mon Sep 17 00:00:00 2001
From: Paolo Bonzini <pbonzini at redhat.com>
Date: Wed, 14 Oct 2015 13:30:45 +0200
Subject: x86/setup: Extend low identity map to cover whole kernel range

commit f5f3497cad8c8416a74b9aaceb127908755d020a upstream.

On 32-bit systems, the initial_page_table is reused by
efi_call_phys_prolog as an identity map to call
SetVirtualAddressMap.  efi_call_phys_prolog takes care of
converting the current CPU's GDT to a physical address too.

For PAE kernels the identity mapping is achieved by aliasing the
first PDPE for the kernel memory mapping into the first PDPE
of initial_page_table.  This makes the EFI stub's trick "just work".

However, for non-PAE kernels there is no guarantee that the identity
mapping in the initial_page_table extends as far as the GDT; in this
case, accesses to the GDT will cause a page fault (which quickly becomes
a triple fault).  Fix this by copying the kernel mappings from
swapper_pg_dir to initial_page_table twice, both at PAGE_OFFSET and at
identity mapping.

For some reason, this is only reproducible with QEMU's dynamic translation
mode, and not for example with KVM.  However, even under KVM one can clearly
see that the page table is bogus:

    $ qemu-system-i386 -pflash OVMF.fd -M q35 vmlinuz0 -s -S -daemonize
    $ gdb
    (gdb) target remote localhost:1234
    (gdb) hb *0x02858f6f
    Hardware assisted breakpoint 1 at 0x2858f6f
    (gdb) c
    Continuing.

    Breakpoint 1, 0x02858f6f in ?? ()
    (gdb) monitor info registers
    ...
    GDT=     0724e000 000000ff
    IDT=     fffbb000 000007ff
    CR0=0005003b CR2=ff896000 CR3=032b7000 CR4=00000690
    ...

The page directory is sane:

    (gdb) x/4wx 0x32b7000
    0x32b7000:	0x03398063	0x03399063	0x0339a063	0x0339b063
    (gdb) x/4wx 0x3398000
    0x3398000:	0x00000163	0x00001163	0x00002163	0x00003163
    (gdb) x/4wx 0x3399000
    0x3399000:	0x00400003	0x00401003	0x00402003	0x00403003

but our particular page directory entry is empty:

    (gdb) x/1wx 0x32b7000 + (0x724e000 >> 22) * 4
    0x32b7070:	0x00000000

[ It appears that you can skate past this issue if you don't receive
  any interrupts while the bogus GDT pointer is loaded, or if you avoid
  reloading the segment registers in general.

  Andy Lutomirski provides some additional insight:

   "AFAICT it's entirely permissible for the GDTR and/or LDT
    descriptor to point to unmapped memory.  Any attempt to use them
    (segment loads, interrupts, IRET, etc) will try to access that memory
    as if the access came from CPL 0 and, if the access fails, will
    generate a valid page fault with CR2 pointing into the GDT or
    LDT."

  Up until commit 23a0d4e8fa6d ("efi: Disable interrupts around EFI
  calls, not in the epilog/prolog calls") interrupts were disabled
  around the prolog and epilog calls, and the functional GDT was
  re-installed before interrupts were re-enabled.

  Which explains why no one has hit this issue until now. ]

Signed-off-by: Paolo Bonzini <pbonzini at redhat.com>
Reported-by: Laszlo Ersek <lersek at redhat.com>
Cc: Borislav Petkov <bp at alien8.de>
Cc: "H. Peter Anvin" <hpa at zytor.com>
Cc: Thomas Gleixner <tglx at linutronix.de>
Cc: Ingo Molnar <mingo at kernel.org>
Cc: Andy Lutomirski <luto at amacapital.net>
Signed-off-by: Matt Fleming <matt.fleming at intel.com>
[ Updated changelog. ]

Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 arch/x86/kernel/setup.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
index ab4734e..5f660ca 100644
--- a/arch/x86/kernel/setup.c
+++ b/arch/x86/kernel/setup.c
@@ -1188,6 +1188,14 @@ void __init setup_arch(char **cmdline_p)
 	clone_pgd_range(initial_page_table + KERNEL_PGD_BOUNDARY,
 			swapper_pg_dir     + KERNEL_PGD_BOUNDARY,
 			KERNEL_PGD_PTRS);
+
+	/*
+	 * sync back low identity map too.  It is used for example
+	 * in the 32-bit EFI stub.
+	 */
+	clone_pgd_range(initial_page_table,
+			swapper_pg_dir     + KERNEL_PGD_BOUNDARY,
+			KERNEL_PGD_PTRS);
 #endif

 	tboot_probe();
--
1.9.1