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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
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