[Bug 1866892] Re: guest OS catches a page fault bug when running dotnet

[Peter Maydell <peter.maydell@linaro.org>]

That function is for "do a guest memory load as if from the kernel" (ie
with the permissions and page table settings that guest kernel-mode
accesses would use). We'd need to look at what the required semantics
for the instruction are for user-mode iretq: are the loads supposed to
be done with only the user-mode access and page tables?

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Title:
  guest OS catches a page  fault bug when running dotnet

Status in QEMU:
  New

Bug description:
  The linux guest OS catches a page fault bug when running the dotnet
  application.

  host = metal = x86_64
  host OS = ubuntu 19.10
  qemu emulation, without KVM, with "tiny code generator" tcg; no plugins; built from head/master
  guest emulation = x86_64
  guest OS = ubuntu 19.10
  guest app = dotnet, running any program

  qemu sha=7bc4d1980f95387c4cc921d7a066217ff4e42b70 (head/master Mar 10,
  2020)

  qemu invocation is:

  qemu/build/x86_64-softmmu/qemu-system-x86_64 \
    -m size=4096 \
    -smp cpus=1 \
    -machine type=pc-i440fx-5.0,accel=tcg \
    -cpu Skylake-Server-v1 \
    -nographic \
    -bios OVMF-pure-efi.fd \
    -drive if=none,id=hd0,file=ubuntu-19.10-server-cloudimg-amd64.img \
    -device virtio-blk,drive=hd0 \
    -drive if=none,id=cloud,file=linux_cloud_config.img \
    -device virtio-blk,drive=cloud \
    -netdev user,id=user0,hostfwd=tcp::2223-:22 \
    -device virtio-net,netdev=user0

  
  Here's the guest kernel console output:

  
  [ 2834.005449] BUG: unable to handle page fault for address: 00007fffffffc2c0
  [ 2834.009895] #PF: supervisor read access in user mode
  [ 2834.013872] #PF: error_code(0x0001) - permissions violation
  [ 2834.018025] IDT: 0xfffffe0000000000 (limit=0xfff) GDT: 0xfffffe0000001000 (limit=0x7f)
  [ 2834.022242] LDTR: NULL
  [ 2834.026306] TR: 0x40 -- base=0xfffffe0000003000 limit=0x206f
  [ 2834.030395] PGD 80000000360d0067 P4D 80000000360d0067 PUD 36105067 PMD 36193067 PTE 8000000076d8e867
  [ 2834.038672] Oops: 0001 [#4] SMP PTI
  [ 2834.042707] CPU: 0 PID: 13537 Comm: dotnet Tainted: G      D           5.3.0-29-generic #31-Ubuntu
  [ 2834.050591] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 0.0.0 02/06/2015
  [ 2834.054785] RIP: 0033:0x1555547eaeda
  [ 2834.059017] Code: d0 00 00 00 4c 8b a7 d8 00 00 00 4c 8b af e0 00 00 00 4c 8b b7 e8 00 00 00 4c 8b bf f0 00 00 00 48 8b bf b0 00 00 00 9d 74 02 <48> cf 48 8d 64 24 30 5d c3 90 cc c3 66 90 55 4c 8b a7 d8 00 00 00
  [ 2834.072103] RSP: 002b:00007fffffffc2c0 EFLAGS: 00000202
  [ 2834.076507] RAX: 0000000000000000 RBX: 00001554b401af38 RCX: 0000000000000001
  [ 2834.080832] RDX: 0000000000000000 RSI: 0000000000000000 RDI: 00007fffffffcfb0
  [ 2834.085010] RBP: 00007fffffffd730 R08: 0000000000000000 R09: 00007fffffffd1b0
  [ 2834.089184] R10: 0000155555331dd5 R11: 00001555553ad8d0 R12: 0000000000000002
  [ 2834.093350] R13: 0000000000000001 R14: 0000000000000001 R15: 00001554b401d388
  [ 2834.097309] FS:  0000155554fa5740 GS:  0000000000000000
  [ 2834.101131] Modules linked in: isofs nls_iso8859_1 dm_multipath scsi_dh_rdac scsi_dh_emc scsi_dh_alua ppdev input_leds serio_raw parport_pc parport sch_fq_codel ip_tables x_tables autofs4 btrfs zstd_compress raid10 raid456 async_raid6_recov async_memcpy async_pq async_xor async_tx xor raid6_pq libcrc32c raid1 raid0 multipath linear crct10dif_pclmul crc32_pclmul ghash_clmulni_intel aesni_intel aes_x86_64 crypto_simd cryptd glue_helper virtio_net psmouse net_failover failover virtio_blk floppy
  [ 2834.122539] CR2: 00007fffffffc2c0
  [ 2834.126867] ---[ end trace dfae51f1d9432708 ]---
  [ 2834.131239] RIP: 0033:0x14d793262eda
  [ 2834.135715] Code: Bad RIP value.
  [ 2834.140243] RSP: 002b:00007ffddb4e2980 EFLAGS: 00000202
  [ 2834.144615] RAX: 0000000000000000 RBX: 000014d6f402acb8 RCX: 0000000000000002
  [ 2834.148943] RDX: 0000000001cd6950 RSI: 0000000000000000 RDI: 00007ffddb4e3670
  [ 2834.153335] RBP: 00007ffddb4e3df0 R08: 0000000000000001 R09: 00007ffddb4e3870
  [ 2834.157774] R10: 000014d793da9dd5 R11: 000014d793e258d0 R12: 0000000000000002
  [ 2834.162132] R13: 0000000000000001 R14: 0000000000000001 R15: 000014d6f402d040
  [ 2834.166239] FS:  0000155554fa5740(0000) GS:ffff97213ba00000(0000) knlGS:0000000000000000
  [ 2834.170529] CS:  0033 DS: 0000 ES: 0000 CR0: 0000000080050033
  [ 2834.174751] CR2: 000014d793262eb0 CR3: 0000000036130000 CR4: 00000000007406f0
  [ 2834.178892] PKRU: 55555554

  I run the application from a shell with `ulimit -s unlimited`
  (unlimited stack to size).

  The application creates a number of threads, and those threads make a
  lot of calls to sigaltstack() and mprotect(); see the relevant source
  for dotnet here
  https://github.com/dotnet/runtime/blob/15ec69e47b4dc56098e6058a11ccb6ae4d5d4fa1/src/coreclr/src/pal/src/thread/thread.cpp#L2467

  using strace -f on the app shows that no alt stacks come anywhere near
  the failing address; all alt stacks are in the heap, as expected.
  None of the mmap/mprotect/munmap syscalls were given arguments in the
  high memory 0x7fffffff0000 and up.

  gdb (with default signal stop/print/pass semantics) does not report
  any signals prior to the kernel bug being tripped, so I doubt the
  alternate signal stack is actually used.

  When I run the same dotnet binary on the host (eg, on "bare metal"),
  the host kernel seems happy and dotnet runs as expected.

  I have not tried different qemu or guest or host O/S.

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