arm64: asm-bug: Add .align 2 to the end of __BUG_ENTRY

In the Linux kernel, the following vulnerability has been resolved:

arm64: asm-bug: Add .align 2 to the end of __BUG_ENTRY

When CONFIG_DEBUG_BUGVERBOSE=n, we fail to add necessary padding bytes
to bug_table entries, and as a result the last entry in a bug table will
be ignored, potentially leading to an unexpected panic(). All prior
entries in the table will be handled correctly.

The arm64 ABI requires that struct fields of up to 8 bytes are
naturally-aligned, with padding added within a struct such that struct
are suitably aligned within arrays.

When CONFIG_DEBUG_BUGVERPOSE=y, the layout of a bug_entry is:

struct bug_entry {
signed int bug_addr_disp; // 4 bytes
signed int file_disp; // 4 bytes
unsigned short line; // 2 bytes
unsigned short flags; // 2 bytes
}

... with 12 bytes total, requiring 4-byte alignment.

When CONFIG_DEBUG_BUGVERBOSE=n, the layout of a bug_entry is:

struct bug_entry {
signed int bug_addr_disp; // 4 bytes
unsigned short flags; // 2 bytes
< implicit padding > // 2 bytes
}

... with 8 bytes total, with 6 bytes of data and 2 bytes of trailing
padding, requiring 4-byte alginment.

When we create a bug_entry in assembly, we align the start of the entry
to 4 bytes, which implicitly handles padding for any prior entries.
However, we do not align the end of the entry, and so when
CONFIG_DEBUG_BUGVERBOSE=n, the final entry lacks the trailing padding
bytes.

For the main kernel image this is not a problem as find_bug() doesn't
depend on the trailing padding bytes when searching for entries:

for (bug = __start___bug_table; bug < __stop___bug_table; ++bug)
if (bugaddr == bug_addr(bug))
return bug;

However for modules, module_bug_finalize() depends on the trailing
bytes when calculating the number of entries:

mod->num_bugs = sechdrs[i].sh_size / sizeof(struct bug_entry);

... and as the last bug_entry lacks the necessary padding bytes, this entry
will not be counted, e.g. in the case of a single entry:

sechdrs[i].sh_size == 6
sizeof(struct bug_entry) == 8;

sechdrs[i].sh_size / sizeof(struct bug_entry) == 0;

Consequently module_find_bug() will miss the last bug_entry when it does:

for (i = 0; i < mod->num_bugs; ++i, ++bug)
if (bugaddr == bug_addr(bug))
goto out;

... which can lead to a kenrel panic due to an unhandled bug.

This can be demonstrated with the following module:

static int __init buginit(void)
{
WARN(1, "hello\n");
return 0;
}

static void __exit bugexit(void)
{
}

module_init(buginit);
module_exit(bugexit);
MODULE_LICENSE("GPL");

... which will trigger a kernel panic when loaded:

------------[ cut here ]------------
hello
Unexpected kernel BRK exception at EL1
Internal error: BRK handler: 00000000f2000800 [#1] PREEMPT SMP
Modules linked in: hello(O+)
CPU: 0 PID: 50 Comm: insmod Tainted: G O 6.9.1 #8
Hardware name: linux,dummy-virt (DT)
pstate: 60400005 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
pc : buginit+0x18/0x1000 [hello]
lr : buginit+0x18/0x1000 [hello]
sp : ffff800080533ae0
x29: ffff800080533ae0 x28: 0000000000000000 x27: 0000000000000000
x26: ffffaba8c4e70510 x25: ffff800080533c30 x24: ffffaba8c4a28a58
x23: 0000000000000000 x22: 0000000000000000 x21: ffff3947c0eab3c0
x20: ffffaba8c4e3f000 x19: ffffaba846464000 x18: 0000000000000006
x17: 0000000000000000 x16: ffffaba8c2492834 x15: 0720072007200720
x14: 0720072007200720 x13: ffffaba8c49b27c8 x12: 0000000000000312
x11: 0000000000000106 x10: ffffaba8c4a0a7c8 x9 : ffffaba8c49b27c8
x8 : 00000000ffffefff x7 : ffffaba8c4a0a7c8 x6 : 80000000fffff000
x5 : 0000000000000107 x4 : 0000000000000000 x3 : 0000000000000000
x2 : 0000000000000000 x1 : 0000000000000000 x0 : ffff3947c0eab3c0
Call trace:
buginit+0x18/0x1000 [hello]
do_one_initcall+0x80/0x1c8
do_init_module+0x60/0x218
load_module+0x1ba4/0x1d70
__do_sys_init_module+0x198/0x1d0
__arm64_sys_init_module+0x1c/0x28
invoke_syscall+0x48/0x114
el0_svc
---truncated---