Re: [PATCH 3.12 25/91] x86/setup: Extend low identity map to cover whole kernel range

[Paolo Bonzini]

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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