Linux 6.0.8 generates L1TF-vulnerable PTE if Xen's PAT is modified

Linux 6.0.8 generates L1TF-vulnerable PTE if Xen's PAT is modified

[Demi Marie Obenour]

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If Xen is patched to use the same PAT Linux does, it appears to break
L1TF mitigations in PV Linux 6.0.8.  Linux 5.15.81 works fine.  The
symptom is that Linux fails to boot, with Xen complaining about an
L1TF-vulnerable PTE with shadow paging disabled.

Details are at https://github.com/QubesOS/qubes-issues/issues/7935.
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab

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Re: Linux 6.0.8 generates L1TF-vulnerable PTE if Xen's PAT is modified

[Marek Marczykowski-Górecki]

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On Fri, Dec 09, 2022 at 01:40:53PM -0500, Demi Marie Obenour wrote:
> If Xen is patched to use the same PAT Linux does, it appears to break
> L1TF mitigations in PV Linux 6.0.8.  Linux 5.15.81 works fine.  The
> symptom is that Linux fails to boot, with Xen complaining about an
> L1TF-vulnerable PTE with shadow paging disabled.
> 
> Details are at https://github.com/QubesOS/qubes-issues/issues/7935.

Call trace:

(early) [    0.417527] RIP: e030:xen_hypercall_mmu_update+0x8/0x20
(early) [    0.417534] Code: cc cc 51 41 53 b8 00 00 00 00 0f 05 41 5b 59 c3 cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc 51 41 53 b8 01 00 00 00 <0f> 05 41 5b 59 c3 cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc
(early) [    0.417537] RSP: e02b:ffffffff82c03d68 EFLAGS: 00000046
(early) [    0.417540] RAX: 0000000000000001 RBX: 000000039f788000 RCX: ffffffff81e2502a
(early) [    0.417543] RDX: 0000000000000000 RSI: 0000000080000001 RDI: ffffffff82c03d80
(early) [    0.417546] RBP: 8010000013600066 R08: ffff888018f88000 R09: 0000000000000000
(early) [    0.417548] R10: 0000000000007ff0 R11: 0000000000000246 R12: 0000000000000000
(early) [    0.417550] R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
(early) [    0.417557] FS:  0000000000000000(0000) GS:ffff888013e00000(0000) knlGS:0000000000000000
(early) [    0.417560] CS:  10000e030 DS: 0000 ES: 0000 CR0: 0000000080050033
(early) [    0.417562] CR2: ffffc900007cf000 CR3: 0000000002c10000 CR4: 0000000000040660
(early) [    0.417567] Call Trace:
(early) [    0.417570]  <TASK>
(early) [    0.417573]  ? __xen_set_pte+0xdc/0x210
(early) [    0.417578]  ? kfence_protect_page+0x68/0xd0
(early) [    0.417582]  ? kfence_init_pool+0x12d/0x280
(early) [    0.417586]  ? kfence_init_pool_early+0x4c/0x281
(early) [    0.417591]  ? kfence_init+0x3f/0xc4
(early) [    0.417594]  ? start_kernel+0x40d/0x6ef
(early) [    0.417599]  ? xen_start_kernel+0x5c4/0x5e9
(early) [    0.417603]  ? startup_xen+0x1f/0x1f
(early) [    0.417607]  </TASK>

Automated bisect in progress.

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab

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Re: Linux 6.0.8 generates L1TF-vulnerable PTE if Xen's PAT is modified

[Juergen Gross]

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On 11.12.22 05:34, Marek Marczykowski-Górecki wrote:
> On Fri, Dec 09, 2022 at 01:40:53PM -0500, Demi Marie Obenour wrote:
>> If Xen is patched to use the same PAT Linux does, it appears to break
>> L1TF mitigations in PV Linux 6.0.8.  Linux 5.15.81 works fine.  The
>> symptom is that Linux fails to boot, with Xen complaining about an
>> L1TF-vulnerable PTE with shadow paging disabled.
>>
>> Details are at https://github.com/QubesOS/qubes-issues/issues/7935.
> 
> Call trace:
> 
> (early) [    0.417527] RIP: e030:xen_hypercall_mmu_update+0x8/0x20
> (early) [    0.417534] Code: cc cc 51 41 53 b8 00 00 00 00 0f 05 41 5b 59 c3 cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc 51 41 53 b8 01 00 00 00 <0f> 05 41 5b 59 c3 cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc
> (early) [    0.417537] RSP: e02b:ffffffff82c03d68 EFLAGS: 00000046
> (early) [    0.417540] RAX: 0000000000000001 RBX: 000000039f788000 RCX: ffffffff81e2502a
> (early) [    0.417543] RDX: 0000000000000000 RSI: 0000000080000001 RDI: ffffffff82c03d80
> (early) [    0.417546] RBP: 8010000013600066 R08: ffff888018f88000 R09: 0000000000000000
> (early) [    0.417548] R10: 0000000000007ff0 R11: 0000000000000246 R12: 0000000000000000
> (early) [    0.417550] R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
> (early) [    0.417557] FS:  0000000000000000(0000) GS:ffff888013e00000(0000) knlGS:0000000000000000
> (early) [    0.417560] CS:  10000e030 DS: 0000 ES: 0000 CR0: 0000000080050033
> (early) [    0.417562] CR2: ffffc900007cf000 CR3: 0000000002c10000 CR4: 0000000000040660
> (early) [    0.417567] Call Trace:
> (early) [    0.417570]  <TASK>
> (early) [    0.417573]  ? __xen_set_pte+0xdc/0x210
> (early) [    0.417578]  ? kfence_protect_page+0x68/0xd0
> (early) [    0.417582]  ? kfence_init_pool+0x12d/0x280
> (early) [    0.417586]  ? kfence_init_pool_early+0x4c/0x281
> (early) [    0.417591]  ? kfence_init+0x3f/0xc4
> (early) [    0.417594]  ? start_kernel+0x40d/0x6ef
> (early) [    0.417599]  ? xen_start_kernel+0x5c4/0x5e9
> (early) [    0.417603]  ? startup_xen+0x1f/0x1f
> (early) [    0.417607]  </TASK>
> 
> Automated bisect in progress.
> 

I think this is a bug in the kfence code. Just setting a pte to invalid
without clearing the pfn is going to result in L1TF vulnerable PTEs.

Why this would be related to Xen's PAT settings I have no idea, though.


Juergen

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