Re: kexec reboot failed due to commit 75d090fd167ac

["Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>]

On Mon, Sep 11, 2023 at 05:57:07PM +0300, Kirill A. Shutemov wrote:
> On Mon, Sep 11, 2023 at 10:56:36PM +0800, Dave Young wrote:
> > > early console in extract_kernel
> > > input_data: 0x000000807eb433a8
> > > input_len: 0x0000000000d26271
> > > output: 0x000000807b000000
> > > output_len: 0x0000000004800c10
> > > kernel_total_size: 0x0000000003e28000
> > > needed_size: 0x0000000004a00000
> > > trampoline_32bit: 0x000000000009d000
> > >
> > > Decompressing Linux... out of pgt_buf in arch/x86/boot/compressed/ident_map_64.c!?
> > > pages->pgt_buf_offset: 0x0000000000006000
> > > pages->pgt_buf_size: 0x0000000000006000
> > >
> > >
> > > Error: kernel_ident_mapping_init() failed
> > >
> > > It crashes on #PF due to stbl->nr_tables dereference in
> > > efi_get_conf_table() called from init_unaccepted_memory().
> > >
> > > I don't see anything special about stbl location: 0x775d6018.
> > >
> > > One other bit of information: disabling 5-level paging also helps the
> > > issue.
> > >
> > > I will debug further.
> 
> The problem is not limited to unaccepted memory, it also triggers if we
> reach efi_get_rsdp_addr() in the same setup.
> 
> I think we have several problems here.
> 
> - 6 pages for !RANDOMIZE_BASE is only enough for kernel, cmdline,
>   boot_data and setup_data if we assume that they are in different 1G
>   regions and do not cross the 1G boundaries. 4-level paging: 1 for PGD, 1
>   for PUD, 4 for PMD tables.
> 
>   Looks like we never map EFI/ACPI memory explicitly.
> 
>   It might work if kernel/cmdline/... are in single 1G and we have
>   spare pages to handle page faults.
> 
> - No spare memory to handle mapping for cc_info and cc_info->cpuid_phys;
> 
> - I didn't increase BOOT_INIT_PGT_SIZE when added 5-level paging support.
>   And if start pagetables from scratch ('else' case of 'if (p4d_offset...))
>   we run out of memory.
> 
> I believe similar logic would apply for BOOT_PGT_SIZE for RANDOMIZE_BASE=y
> case.
> 
> I don't know what the right fix here. We can increase the constants to be
> enough to cover existing cases, but it is very fragile. I am not sure I
> saw all users. Some of them could silently handled with pagefault handler
> in some setups. And it is hard to catch new users during code review.
> 
> Also I'm not sure why do we need pagefault handler there. Looks like it
> just masking problems. I think everything has to be mapped explicitly.
> 
> Any comments?

I struggle to come up with anything better than increasing the constant to
a value that "ought to be enough for anybody" ©, let's say 128K.

And we can eliminate logic on no-KASLR vs. KASLR vs. KASLR+VERBOSE_BOOTUP.

Objections?

-- 
  Kiryl Shutsemau / Kirill A. Shutemov