ARM: 9410/1: vfp: Use asm volatile in fmrx/fmxr macros
In the Linux kernel, the following vulnerability has been resolved:

ARM: 9410/1: vfp: Use asm volatile in fmrx/fmxr macros

Floating point instructions in userspace can crash some arm kernels
built with clang/LLD 17.0.6:

    BUG: unsupported FP instruction in kernel mode
    FPEXC == 0xc0000780
    Internal error: Oops - undefined instruction: 0 [#1] ARM
    CPU: 0 PID: 196 Comm: vfp-reproducer Not tainted 6.10.0 #1
    Hardware name: BCM2835
    PC is at vfp_support_entry+0xc8/0x2cc
    LR is at do_undefinstr+0xa8/0x250
    pc : [<c0101d50>]    lr : [<c010a80c>]    psr: a0000013
    sp : dc8d1f68  ip : 60000013  fp : bedea19c
    r10: ec532b17  r9 : 00000010  r8 : 0044766c
    r7 : c0000780  r6 : ec532b17  r5 : c1c13800  r4 : dc8d1fb0
    r3 : c10072c4  r2 : c0101c88  r1 : ec532b17  r0 : 0044766c
    Flags: NzCv  IRQs on  FIQs on  Mode SVC_32  ISA ARM  Segment none
    Control: 00c5387d  Table: 0251c008  DAC: 00000051
    Register r0 information: non-paged memory
    Register r1 information: vmalloc memory
    Register r2 information: non-slab/vmalloc memory
    Register r3 information: non-slab/vmalloc memory
    Register r4 information: 2-page vmalloc region
    Register r5 information: slab kmalloc-cg-2k
    Register r6 information: vmalloc memory
    Register r7 information: non-slab/vmalloc memory
    Register r8 information: non-paged memory
    Register r9 information: zero-size pointer
    Register r10 information: vmalloc memory
    Register r11 information: non-paged memory
    Register r12 information: non-paged memory
    Process vfp-reproducer (pid: 196, stack limit = 0x61aaaf8b)
    Stack: (0xdc8d1f68 to 0xdc8d2000)
    1f60:                   0000081f b6f69300 0000000f c10073f4 c10072c4 dc8d1fb0
    1f80: ec532b17 0c532b17 0044766c b6f9ccd8 00000000 c010a80c 00447670 60000010
    1fa0: ffffffff c1c13800 00c5387d c0100f10 b6f68af8 00448fc0 00000000 bedea188
    1fc0: bedea314 00000001 00448ebc b6f9d000 00447608 b6f9ccd8 00000000 bedea19c
    1fe0: bede9198 bedea188 b6e1061c 0044766c 60000010 ffffffff 00000000 00000000
    Call trace:
    [<c0101d50>] (vfp_support_entry) from [<c010a80c>] (do_undefinstr+0xa8/0x250)
    [<c010a80c>] (do_undefinstr) from [<c0100f10>] (__und_usr+0x70/0x80)
    Exception stack(0xdc8d1fb0 to 0xdc8d1ff8)
    1fa0:                                     b6f68af8 00448fc0 00000000 bedea188
    1fc0: bedea314 00000001 00448ebc b6f9d000 00447608 b6f9ccd8 00000000 bedea19c
    1fe0: bede9198 bedea188 b6e1061c 0044766c 60000010 ffffffff
    Code: 0a000061 e3877202 e594003c e3a09010 (eef16a10)
    ---[ end trace 0000000000000000 ]---
    Kernel panic - not syncing: Fatal exception in interrupt
    ---[ end Kernel panic - not syncing: Fatal exception in interrupt ]---

This is a minimal userspace reproducer on a Raspberry Pi Zero W:

    #include <stdio.h>
    #include <math.h>

    int main(void)
    {
            double v = 1.0;
            printf("%fn", NAN + *(volatile double *)&v);
            return 0;
    }

Another way to consistently trigger the oops is:

    calvin@raspberry-pi-zero-w ~$ python -c "import json"

The bug reproduces only when the kernel is built with DYNAMIC_DEBUG=n,
because the pr_debug() calls act as barriers even when not activated.

This is the output from the same kernel source built with the same
compiler and DYNAMIC_DEBUG=y, where the userspace reproducer works as
expected:

    VFP: bounce: trigger ec532b17 fpexc c0000780
    VFP: emulate: INST=0xee377b06 SCR=0x00000000
    VFP: bounce: trigger eef1fa10 fpexc c0000780
    VFP: emulate: INST=0xeeb40b40 SCR=0x00000000
    VFP: raising exceptions 30000000

    calvin@raspberry-pi-zero-w ~$ ./vfp-reproducer
    nan

Crudely grepping for vmsr/vmrs instructions in the otherwise nearly
idential text for vfp_support_entry() makes the problem obvious:

    vmlinux.llvm.good [0xc0101cb8] <+48>:  vmrs   r7, fpexc
    vmlinux.llvm.good [0xc0101cd8] <+80>:  vmsr   fpexc, r0
    vmlinux.llvm.good [0xc0101d20
---truncated---