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  Boot crash with Trusty 3.13

Status in linux package in Ubuntu:
Status in linux source package in Trusty:
  Fix Committed

Bug description:
  == SRU Justification ==
  Custom compilation of the Trusty 3.13 kernel codebase results in a 
(reproducible) QEMU boot crash (see below).

  == Fix ==
  Replace UBUNTU SAUCE patch with proper upstream commit:
  548acf19234d ("x86/mm: Expand the exception table logic to allow new handling 

  == Regression Potential ==
  Medium. The patch is quite large but the backport was a simple context 
adjustment. Ran the x86 selftests and perf NMI tests for several hours to 
verify stability.

  == Test Case ==
  Compile the Trusty 3.13 kernel code using the default config (make defconfig) 
and run the resulting kernel in QMEU. Crashes every time.

  Original bug description:

  While doing kernel testing using the Trusty 3.13 code base, I get the
  following boot crash with QEMU:

  [    0.338393] BUG: unable to handle kernel paging request at ffffffff014142f0
  [    0.338987] IP: [<ffffffff014142f0>] 0xffffffff014142f0
  [    0.339388] PGD 180f067 PUD 0
  [    0.339388] Oops: 0010 [#1] SMP
  [    0.339388] Modules linked in:
  [    0.339388] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 3.13.11-ckt39-trusty 
  [    0.339388] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 
1.10.2-1ubuntu1 04/01/2014
  [    0.339388] task: ffff88003f708000 ti: ffff88003f6fa000 task.ti: 
  [    0.339388] RIP: 0010:[<ffffffff014142f0>]  [<ffffffff014142f0>] 
  [    0.339388] RSP: 0000:ffff88003f6fbe98  EFLAGS: 00050246
  [    0.339388] RAX: 0000000000000000 RBX: 0000000000000000 RCX: 
  [    0.339388] RDX: 0000000000000000 RSI: ffff88003deb9eb4 RDI: 
  [    0.339388] RBP: ffff88003f6fbe98 R08: 0000000000000000 R09: 
  [    0.339388] R10: ffffffff81264c68 R11: ffffea0000fdd000 R12: 
  [    0.339388] R13: 00000000000000ad R14: 0000000000000000 R15: 
  [    0.339388] FS:  0000000000000000(0000) GS:ffff88003fa00000(0000) 
  [    0.339388] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  [    0.339388] CR2: ffffffff014142f0 CR3: 000000000180c000 CR4: 
  [    0.339388] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 
  [    0.339388] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 
  [    0.339388] Stack:
  [    0.339388]  ffff88003f6fbf08 ffffffff81000402 ffff88003f6fbf00 
  [    0.339388]  ffff88003f6fbef0 ffff88003ffd96a1 ffffffff817e9d28 
  [    0.339388]  ffffffff817b013d ffffffff8196cef0 ffffffff8196d018 
  [    0.339388] Call Trace:
  [    0.339388]  [<ffffffff81000402>] do_one_initcall+0xf2/0x140
  [    0.339388]  [<ffffffff81065f88>] ? parse_args+0x1e8/0x320
  [    0.339388]  [<ffffffff8189df8f>] kernel_init_freeable+0x14c/0x1d1
  [    0.339388]  [<ffffffff8189d842>] ? do_early_param+0x88/0x88
  [    0.339388]  [<ffffffff813fac20>] ? rest_init+0x80/0x80
  [    0.339388]  [<ffffffff813fac29>] kernel_init+0x9/0x120
  [    0.339388]  [<ffffffff8140fcae>] ret_from_fork+0x6e/0xa0
  [    0.339388]  [<ffffffff813fac20>] ? rest_init+0x80/0x80
  [    0.339388] Code:  Bad RIP value.
  [    0.339388] RIP  [<ffffffff014142f0>] 0xffffffff014142f0
  [    0.339388]  RSP <ffff88003f6fbe98>
  [    0.339388] CR2: ffffffff014142f0
  [    0.339388] ---[ end trace a71242bdac7e8632 ]---
  [    0.339388] note: swapper/0[1] exited with preempt_count 1
  [    0.357079] swapper/0 (1) used greatest stack depth: 5424 bytes left
  [    0.357539] Kernel panic - not syncing: Attempted to kill init! 
  [    0.357539]
  [    0.358073] Kernel Offset: 0x0 from 0xffffffff81000000 (relocation range: 

  Git bisect identified the following commit as the culprit:
  commit 56764fdc3a847371531b8044155c70412fc5be76
  Author: Andy Whitcroft <>
  Date:   Thu Feb 22 11:24:00 2018 +0100

      UBUNTU: SAUCE: x86, extable: fix uaccess fixup detection


      The existing code intends to identify a subset of fixups which need
      special handling, uaccess related faults need to record the failure.
      This is done by adjusting the fixup code pointer by a (random) constant
      0x7ffffff0.  This is detected in fixup_exception by comparing the two
      pointers.  The intent of this code is to detect the the delta between
      the original code and its fixup code being greater than the constant.
      However, the code as written triggers undefined comparison behaviour.
      In this kernel this prevents the condition triggering, leading to panics
      when jumping to the corrupted fixup address.

      Convert the code to better implement the intent.  Convert both of the
      offsets to final addresses and compare the delta between those.  Also add
      a massive comment to explain all of this including the implicit 
      on order of the segments that this comparison implies.

      Fixes: 706276543b69 ("x86, extable: Switch to relative exception table 
      Signed-off-by: Andy Whitcroft <>
      Acked-by: Colin Ian King <>
      Acked-by: Khalid Elmously <>
      Signed-off-by: Kleber Sacilotto de Souza <>

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