Re: [PATCH] x86: setup: extend low identity map to cover whole kernel range

[Matt Fleming]

(Pulling in luto for low-level x86 fu)

On Wed, 14 Oct, at 01:30:45PM, Paolo Bonzini wrote:
> 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.
 
Oops, good catch guys. This is clearly a bug, but...

> 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
 
... I'm a little surprised you managed to trigger this at all, because
the GDT we load in efi_call_phys_prolog() is part of the per-cpu data
section and therefore part of the kernel image.

The kernel image *is* mapped in the identity range, even for non-PAE
kernels.

So yes, you're right this is a bug but I'm not sure how you're
actually triggering it.

> Reported-by: Laszlo Ersek <lersek@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> ---
>  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 80f874bf999e..24154bd12307 100644
> --- a/arch/x86/kernel/setup.c
> +++ b/arch/x86/kernel/setup.c
> @@ -1198,6 +1198,14 @@ 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();
> -- 
> 2.5.0
> 
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-- 
Matt Fleming, Intel Open Source Technology Center
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