On Sun, Feb 02, 2020 at 06:54:48PM +0100, Ard Biesheuvel wrote: > On Sun, 2 Feb 2020 at 18:13, Arvind Sankar <nivedita@xxxxxxxxxxxx> wrote: > > > > At handover entry in efi32_stub_entry, the firmware's GDT is still > > installed. We save the GDTR for later use in __efi64_thunk but we are > > assuming that descriptor 2 (__KERNEL_CS) is a valid 32-bit code segment > > descriptor and that descriptor 3 (__KERNEL_DS/__BOOT_DS) is a valid data > > segment descriptor. > > > > This happens to be true for OVMF (it actually uses descriptor 1 for data > > segments, but descriptor 3 is also setup as data), but we shouldn't > > depend on this being the case. > > > > Fix this by saving the code and data selectors in addition to the GDTR > > in efi32_stub_entry, and restoring them in __efi64_thunk before calling > > the firmware. The UEFI specification guarantees that selectors will be > > flat, so using the DS selector for all the segment registers should be > > enough. > > > > We also need to install our own GDT before initializing segment > > registers in startup_32, so move the GDT load up to the beginning of the > > function. > > > > Signed-off-by: Arvind Sankar <nivedita@xxxxxxxxxxxx> > > It might be useful to mention /somewhere/ in the commit log that this > applies to mixed mode > Good point. I'll wait for comments from the x86 guys and include that in the next re-spin.
