Re: [PATCH 05/13] efi/x86: don't map the entire kernel text RW for mixed mode

[Jiri Slaby]

Ccing Gary.

On 08. 04. 20, 12:47, Ard Biesheuvel wrote:
> On Wed, 8 Apr 2020 at 12:42, Jiri Slaby <jslaby@xxxxxxx> wrote:
>>
>> On 13. 01. 20, 18:22, Ard Biesheuvel wrote:
>>> The mixed mode thunking routine requires a part of it to be
>>> mapped 1:1, and for this reason, we currently map the entire
>>> kernel .text read/write in the EFI page tables, which is bad.
>>>
>>> In fact, the kernel_map_pages_in_pgd() invocation that installs
>>> this mapping is entirely redundant, since all of DRAM is already
>>> 1:1 mapped read/write in the EFI page tables when we reach this
>>> point, which means that .rodata is mapped read-write as well.
>>>
>>> So let's remap both .text and .rodata read-only in the EFI
>>> page tables.
>>
>> This patch causes unhandled page faults in mixed mode:
>>
>>> BUG: unable to handle page fault for address: 000000001557ee88
>>> #PF: supervisor write access in kernel mode
>>> #PF: error_code(0x0003) - permissions violation
>>> PGD fd52063 P4D fd52063 PUD fd53063 PMD 154000e1
>>> Oops: 0003 [#1] SMP PTI
>>> CPU: 1 PID: 191 Comm: systemd-escape Not tainted
>> 5.6.2-20.gb22bc26-default #1 openSUSE Tumbleweed (unreleased)
>>> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 0.0.0
>> 02/06/2015
>>> RIP: 0008:0x3d2eed95
>>> Code: 8b 45 d4 8b 4d 10 8b 40 04 89 01 89 3b 50 6a 00 8b 55 0c 6a 00
>> 8b 45 08 0f b6 4d e4 6a 01 31 f6 e8 ee c5 fc ff 83 c4 10 eb 07 <89> 03
>> be 05 00 00 80 a1 74 63 31 3d 83 c0 48 e8 44 d2 ff ff eb 05
>>> RSP: 0018:000000000fd66fa0 EFLAGS: 00010002
>>> RAX: 0000000000000001 RBX: 000000001557ee88 RCX: 000000003d1f1120
>>> RDX: 0000000000000001 RSI: 0000000000000000 RDI: 0000000000000001
>>> RBP: 000000000fd66fd8 R08: 000000001557ee88 R09: 0000000000000000
>>> R10: 0000000000000055 R11: 0000000000000000 R12: 0000000015bcf000
>>> R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
>>> FS:  00007f36ee9dc940(0000) GS:ffff9b903d700000(0000)
>> knlGS:0000000000000000
>>> CS:  0008 DS: 0018 ES: 0018 CR0: 0000000080050033
>>> CR2: 000000001557ee88 CR3: 000000000fd5e000 CR4: 00000000000006e0
>>> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>>> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>>> Call Trace:
>>> Modules linked in: efivarfs
>>> CR2: 000000001557ee88
>>
>> EFI apparently tries to write to now read-only memory.
>>
>> See:
>> https://bugzilla.suse.com/show_bug.cgi?id=1168645
>>
>> Reverting it on the top of 5.6 fixes the issue.
>>
>> I am using
>> /usr/share/qemu/ovmf-ia32-code.bin
>> /usr/share/qemu/ovmf-ia32-vars.bin
>> from qemu-ovmf-ia32-202002-1.1.noarch rpm.
>>
> 
> Do you have a git tree for Suse's OVMF fork? I did a lot of testing
> with upstream OVMF, and never ran into this issue.

Not really a git tree, but the sources are here:
https://build.opensuse.org/package/show/openSUSE:Factory/ovmf

-- 
js
suse labs