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

[Juergen Gross <jgross@xxxxxxxx>]

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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