Comment 0 for bug 1757193

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 #6
[ 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: ffff88003f6fa000
[ 0.339388] RIP: 0010:[<ffffffff014142f0>] [<ffffffff014142f0>] 0xffffffff014142f0
[ 0.339388] RSP: 0000:ffff88003f6fbe98 EFLAGS: 00050246
[ 0.339388] RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
[ 0.339388] RDX: 0000000000000000 RSI: ffff88003deb9eb4 RDI: ffffffff818b8590
[ 0.339388] RBP: ffff88003f6fbe98 R08: 0000000000000000 R09: ffff88003fa14ae0
[ 0.339388] R10: ffffffff81264c68 R11: ffffea0000fdd000 R12: ffffffff818b8590
[ 0.339388] R13: 00000000000000ad R14: 0000000000000000 R15: 0000000000000000
[ 0.339388] FS: 0000000000000000(0000) GS:ffff88003fa00000(0000) knlGS:0000000000000000
[ 0.339388] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 0.339388] CR2: ffffffff014142f0 CR3: 000000000180c000 CR4: 0000000000360770
[ 0.339388] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[ 0.339388] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[ 0.339388] Stack:
[ 0.339388] ffff88003f6fbf08 ffffffff81000402 ffff88003f6fbf00 ffffffff81065f88
[ 0.339388] ffff88003f6fbef0 ffff88003ffd96a1 ffffffff817e9d28 000000ad00060006
[ 0.339388] ffffffff817b013d ffffffff8196cef0 ffffffff8196d018 0000000000000006
[ 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! exitcode=0x00000009
[ 0.357539]
[ 0.358073] Kernel Offset: 0x0 from 0xffffffff81000000 (relocation range: 0xffffffff80000000-0xffffffff9fffffff)

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

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

    BugLink: http://bugs.launchpad.net/bugs/1750786

    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 assumptions
    on order of the segments that this comparison implies.

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