EFI urgent fix

From: Matt Fleming <[email protected]>

Folks, the below fix from Paolo addresses an issue causing 32-bit non-PAE kernels to triple fault on EFI boot. The issue is that the physical address of the GDT that gets used in efi_call_phys_prolog() won't be covered by the identitty mapping in initial_page_table.

The following changes since commit 8a53554e12e98d1759205afd7b8e9e2ea0936f48:

x86/efi: Fix multiple GOP device support (2015-10-14 16:02:43 +0200)

are available in the git repository at:

git://git.kernel.org/pub/scm/linux/kernel/git/mfleming/efi.git tags/efi-urgent

for you to fetch changes up to f5f3497cad8c8416a74b9aaceb127908755d020a:

x86/setup: Extend low identity map to cover whole kernel range (2015-10-16 10:52:29 +0100)

----------------------------------------------------------------
* Ensure that the identity mapping in initial_page_table is updated to cover the entire kernel range. This fixes a triple fault on non-PAE kernels when booting on 32-bit EFI due to accessing an unmapped GDT in efi_call_phys_prolog() - Paolo Bonzini

----------------------------------------------------------------
Paolo Bonzini (1):
x86/setup: Extend low identity map to cover whole kernel range

 arch/x86/kernel/setup.c | 8 ++++++++
 1 file changed, 8 insertions(+)

From: Paolo Bonzini <[email protected]>

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. ]

---
 arch/x86/kernel/setup.c | 8 ++++++++
 1 file changed, 8 insertions(+)

index fdb7f2a2d328..a3cccbfc5f77 100644
@@ -1173,6 +1173,14 @@ 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();

2.1.0

Pulled, thanks Matt!

Ingo