On Mon, Sep 13, 2021 at 11:52:36AM +0200, Joerg Roedel wrote: > From: Joerg Roedel <jroedel@xxxxxxx> > > The trampoline_pgd only maps the 0xfffffff000000000-0xffffffffffffffff > range of kernel memory (with 4-level paging). This range contains the > kernels text+data+bss mappings and the module mapping space, but not the > direct mapping and the vmalloc area. > > This is enough to get an application processors out of real-mode, but > for code that switches back to real-mode the trampoline_pgd is missing > important parts of the address space. For example, consider this code > from arch/x86/kernel/reboot.c, function machine_real_restart() for a > 64-bit kernel: > > #ifdef CONFIG_X86_32 > load_cr3(initial_page_table); > #else > write_cr3(real_mode_header->trampoline_pgd); > > /* Exiting long mode will fail if CR4.PCIDE is set. */ > if (boot_cpu_has(X86_FEATURE_PCID)) > cr4_clear_bits(X86_CR4_PCIDE); > #endif > > /* Jump to the identity-mapped low memory code */ > #ifdef CONFIG_X86_32 > asm volatile("jmpl *%0" : : > "rm" (real_mode_header->machine_real_restart_asm), > "a" (type)); > #else > asm volatile("ljmpl *%0" : : > "m" (real_mode_header->machine_real_restart_asm), > "D" (type)); > #endif > > The code switches to the trampoline_pgd, which unmaps the direct mapping > and also the kernel stack. The call to cr4_clear_bits() will find no > stack and crash the machine. The real_mode_header pointer below points > into the direct mapping, and dereferencing it also causes a crash. > > The reason this does not crash always is only that kernel mappings are > global and the CR3 switch does not flush those mappings. But if theses > mappings are not in the TLB already, the above code will crash before it > can jump to the real-mode stub. > > Extend the trampoline_pgd to contain all kernel mappings to prevent > these crashes and to make code which runs on this page-table more > robust. > > Cc: stable@xxxxxxxxxxxxxxx > Signed-off-by: Joerg Roedel <jroedel@xxxxxxx> > --- > arch/x86/realmode/init.c | 12 +++++++++++- > 1 file changed, 11 insertions(+), 1 deletion(-) > > diff --git a/arch/x86/realmode/init.c b/arch/x86/realmode/init.c > index 31b5856010cb..7a08c96cb42a 100644 > --- a/arch/x86/realmode/init.c > +++ b/arch/x86/realmode/init.c > @@ -72,6 +72,7 @@ static void __init setup_real_mode(void) > #ifdef CONFIG_X86_64 > u64 *trampoline_pgd; > u64 efer; > + int i; > #endif > > base = (unsigned char *)real_mode_header; > @@ -128,8 +129,17 @@ static void __init setup_real_mode(void) > trampoline_header->flags = 0; > > trampoline_pgd = (u64 *) __va(real_mode_header->trampoline_pgd); > + > + /* > + * Map all of kernel memory into the trampoline PGD so that it includes > + * the direct mapping and vmalloc space. This is needed to keep the > + * stack and real_mode_header mapped when switching to this page table. > + */ > + for (i = pgd_index(__PAGE_OFFSET); i < PTRS_PER_PGD; i++) > + trampoline_pgd[i] = init_top_pgt[i].pgd; Don't we need to update the trampoline_pgd in sync_global_pgds() as well? > + > + /* Map the real mode stub as virtual == physical */ > trampoline_pgd[0] = trampoline_pgd_entry.pgd; > - trampoline_pgd[511] = init_top_pgt[511].pgd; > #endif > > sme_sev_setup_real_mode(trampoline_header); > -- > 2.33.0 > -- Sincerely yours, Mike.