This bug
On 05/01/2016 18:46, Jiri Slaby wrote:
> From: Paolo Bonzini <pbonzini@xxxxxxxxxx>
>
> 3.12-stable review patch. If anyone has any objections, please let me know.
>
> ===============
>
> 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. ]
Without testing the problematic scenario explicitly (32-bit UEFI
kernel), I think this patch and 26/91 should not be backported to
kernels that do not have 23a0d4e8fa6d.
Paolo
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