Re: [PATCH] x86/mm: Drop TS_COMPAT on 64-bit exec() syscall

[Dmitry Safonov]

On Fri, 2018-05-18 at 19:05 -0700, Andy Lutomirski wrote:
> > On May 18, 2018, at 4:10 PM, Dmitry Safonov <0x7f454c46@xxxxxxxxx>
> > cpu family    : 6
> > model        : 142
> > model name    : Intel(R) Core(TM) i7-7600U CPU @ 2.80GHz
> > But I usually test kernels in VM. So, I use virt-manager as it's
> > easier to manage
> > multiple VMs. The thing is that I've chosen "Copy host CPU
> > configuration"
> > and for some reason, I don't quite follow virt-manager makes model
> 
> "Opteron_G4".
> > I'm on Fedora 27, virt-manager 1.4.3, qemu 2.9.1(qemu-2.9.1-
> > 2.fc26).
> > So, cpuinfo in VM says:
> > cpu family    : 21
> > model        : 1
> > model name    : AMD Opteron 62xx class CPU
> 
> What does guest cpuinfo say for vendor_id?
> 
> There are multiple potential screwups here.
> 
> 1. (What I *thought* was going on) AMD CPUs have screwy IRET behavior
> thatâs different from Intelâs, and the test case was definitely
> wrong. But
> KVM has no way to influence it.  Are you sure youâre using KVM and
> not QEMU
> TCG? Anyway, the IRET thing is minor compared to your other problems,
> so
> letâs try to fix them first.
> 
> 2. Compat fast syscalls are wildly different on AMD and Intel.
> Because of
> this issue, QEMU with KVM is supposed to always report the real
> vendor_id
> no matter -cpu asks for.  If we get the wrong vendor_id, then weâre
> at the
> mercy of KVMâs emulation and performance will suck.  On older
> kernels, this
> would cause hideous kernel crashes.  On new kernels, I would expect
> it to
> merely crash 32-bit user programs or be slow.

Heh, I didn't know those details, so it looks like it's (2),
vendor_id	: AuthenticAMD
in guest.

> 
> > What's worse than registers changes is that some selftests actually
> > lead
> 
> to
> > Oops's. The same reason for criu-ia32 fails.
> > I've tested so far v4.15 and v4.16 releases besides master
> > (2c71d338bef2),
> > so it looks to be not a recent regression.
> > Full Oopses:
> > [  189.100174] BUG: unable to handle kernel paging request at
> 
> 00000000417bafe8
> > [  189.100174] PGD 69ed4067 P4D 69ed4067 PUD 707fc067 PMD 6c535067
> > PTE
> 
> 6991f067
> > [  189.100174] Oops: 0001 [#3] SMP NOPTI
> 
> Whoa there!  0001 means a failed *kernel* access.
> 
> > [  189.100174] Modules linked in:
> > [  189.100174] CPU: 0 PID: 2443 Comm: sysret_ss_attrs Tainted: G
> 
> Was this sysret_ss_attrs_32 or sysret_ss_attrs_64?

sysret_ss_attrs_32 survives

> 
> > D           4.17.0-rc5+ #11
> > [  189.103187] Hardware name: QEMU Standard PC (i440FX + PIIX,
> > 1996),
> > BIOS 1.10.2-1.fc26 04/01/2014
> > [  189.103187] RIP: 0033:0x40085a
> 
> The oops was caused from CPL 3 at what looks like a totally sensible
> user
> address.  Can you disassemble the offending binary and tell me what
> the
> code at 0x40085a is?

Here is the function:
0000000000400842 <call32_from_64>:
  400842:       53                      push   %rbx
  400843:       55                      push   %rbp
  400844:       41 54                   push   %r12
  400846:       41 55                   push   %r13
  400848:       41 56                   push   %r14
  40084a:       41 57                   push   %r15
  40084c:       9c                      pushfq 
  40084d:       48 89 27                mov    %rsp,(%rdi)
  400850:       48 89 fc                mov    %rdi,%rsp
  400853:       6a 23                   pushq  $0x23
  400855:       68 5c 08 40 00          pushq  $0x40085c
  40085a:       48 cb                   lretq  
  40085c:       ff d6                   callq  *%rsi
  40085e:       ea                      (bad)  
  40085f:       65 08 40 00             or     %al,%gs:0x0(%rax)
  400863:       33 00                   xor    (%rax),%eax
  400865:       48 8b 24 24             mov    (%rsp),%rsp
  400869:       9d                      popfq  
  40086a:       41 5f                   pop    %r15
  40086c:       41 5e                   pop    %r14
  40086e:       41 5d                   pop    %r13
  400870:       41 5c                   pop    %r12
  400872:       5d                      pop    %rbp
  400873:       5b                      pop    %rbx
  400874:       c3                      retq   
  400875:       66 2e 0f 1f 84 00 00    nopw   %cs:0x0(%rax,%rax,1)
  40087c:       00 00 00 
  40087f:       90                      nop

Looks like mov between registers caused it? The hell.

> 
> > [  189.103187] RSP: 002b:00000000417bafe8 EFLAGS: 00000206
> > [  189.103187] RAX: 0000000000000000 RBX: 00000000000003e8 RCX:
> 
> 0000000000000000
> > [  189.103187] RDX: 0000000000000000 RSI: 0000000000400830 RDI:
> 
> 00000000417baff8
> > [  189.103187] RBP: 00000000417baff8 R08: 0000000000000000 R09:
> 
> 0000000000000077
> > [  189.103187] R10: 0000000000000006 R11: 0000000000000000 R12:
> 
> 00000000417ba000
> > [  189.103187] R13: 00007ffc05207840 R14: 0000000000000000 R15:
> 
> 0000000000000000
> > [  189.103187] FS:  00007f98566ecb40(0000)
> > GS:ffff9740ffc00000(0000)
> > knlGS:0000000000000000
> > [  189.103187] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> 
> CS here is the value of CS that the *kernel* has, so 0x10 is normal.
> 
> > [  189.103187] CR2: 00000000417bafe8 CR3: 0000000069dc4000 CR4:
> 
> 00000000007406f0
> 
> CR2 is in user space.
> 
> So the big question is: what happened here?  Why did the CPU (or
> emulated
> CPU) attempt a privileged access to a user address while running user
> code?

No idea, but looks like it's not a kernel fault.

-- 
Thanks,
             Dmitry