Messages in this thread | | | Date | Wed, 15 Mar 2023 16:25:45 +0300 | From | Evgeniy Baskov <> | Subject | Re: [PATCH v5 09/27] x86/boot: Remove mapping from page fault handler |
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On 2023-03-14 23:33, Andy Lutomirski wrote: > On Tue, Mar 14, 2023, at 3:13 AM, Evgeniy Baskov wrote: >> After every implicit mapping is removed, this code is no longer >> needed. >> >> Remove memory mapping from page fault handler to ensure that there are >> no hidden invalid memory accesses. > > This patch is *by far* the scariest of the bunch in my boot. And it > violates a basic principle of kernel development: it's better to run > in degraded mode than to fail outright unless running in degraded mode > is dangerous for some reason. > > And this boot code is not actually meaningfully exposed to attack. > Anyone who can get the boot code to consume garbage likely *already* > controls the system, including anything that we might write to TPM or > any other verification mechanism. > > So I think this should log an error, set a flag to make sure we print > an even louder error after full boot, but still add the mapping and > keep trying.
Good point. This patch can be dropped and replaced by the loud warning, since it is not required for the functioning of the rest of the series it is here mainly to indicate bugs in the kernel rather than for the increased protection. But I would not expect anything in the working systems, I made my best to map all the things explicitly. And since no code but the extraction code is supposed to be run (interrupts are disabled and we are not using any UEFI services there), this should be practically save to remove the implicit mapping.
And at least this allowed me to find out about the insufficient size of the boot page tables which did not account for the ACPI and UEFI mappings. (patch 4 "x86/boot: Increase boot page table size")
If this patch is dropped now, I can send the follow up patch later adding the warning.
Thanks, Evgeniy Baskov
> > --Andy > >> >> Tested-by: Mario Limonciello <mario.limonciello@amd.com> >> Signed-off-by: Evgeniy Baskov <baskov@ispras.ru> >> --- >> arch/x86/boot/compressed/ident_map_64.c | 26 >> ++++++++++--------------- >> 1 file changed, 10 insertions(+), 16 deletions(-) >> ...
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