* Linus Torvalds <torva...@linux-foundation.org> wrote:
> On Wed, Aug 5, 2020 at 4:03 AM Jason A. Donenfeld <ja...@zx2c4.com> wrote:
> >
> > The commit 8bb9bf242d1f ("x86/mm/64: Do not sync vmalloc/ioremap
> > mappings") causes the OOPS below, in Linus' tree and in linux-next,
> > unearthed by my CI on <https://www.wireguard.com/build-status/>.
> > Bisecting reveals 8bb9bf242d1f, and reverting this makes the OOPS go
> > away.
>
> The oops happens early in the function, and the "Code:" line actually
> gets almost the whole function prologue in it (missing first two bytes
> are probably "push %rbp"):
>
> 0: 41 56 push %r14
> 2: 41 55 push %r13
> 4: 41 54 push %r12
> 6: 55 push %rbp
> 7: 48 89 f5 mov %rsi,%rbp
> a: 53 push %rbx
> b: 48 89 fb mov %rdi,%rbx
> e: 48 83 ec 08 sub $0x8,%rsp
> 12: 48 8b 06 mov (%rsi),%rax
> 15: 4c 8b 67 40 mov 0x40(%rdi),%r12
> 19: 49 89 c6 mov %rax,%r14
> 1c: 45 30 f6 xor %r14b,%r14b
> 1f: a8 04 test $0x4,%al
> 21: b8 00 00 00 00 mov $0x0,%eax
> 26: 4c 0f 44 f0 cmove %rax,%r14
> 2a:* 49 8b 46 08 mov 0x8(%r14),%rax <-- trapping instruction
>
>
> > BUG: unable to handle page fault for address: ffffe8ffffd00608
> > #PF: supervisor read access in kernel mode
> > #PF: error_code(0x0000) - not-present page
> > PGD 0 P4D 0
>
> Yeah, missing page table because it wasn't copied.
>
> Presumably because that kthread is using the active_mm of some random
> user space process that didn't get sync'ed.
>
> And the sync_global_pgds() may have ended up being sufficient
> synchronization with whoever allocated thigns, even if it wasn't about
> the TLB contents themselves.
>
> So apparently the "the page-table pages are all pre-allocated now" is
> simply not true. Joerg?
>
> Unless somebody can figure this out fairly quickly, I think it should
> just be reverted.
Agreed. Joerg?
Thanks,
Ingo
