arm64: kernel: Create initial ID map from C code

The asm code that creates the initial ID map is rather intricate and
hard to follow. This is problematic because it makes adding support for
things like LPA2 or WXN more difficult than necessary. Also, it is
parameterized like the rest of the MM code to run with a configurable
number of levels, which is rather pointless, given that all AArch64 CPUs
implement support for 48-bit virtual addressing, and that many systems
exist with DRAM located outside of the 39-bit addressable range, which
is the only smaller VA size that is widely used, and we need additional
tricks to make things work in that combination.

So let's bite the bullet, and rip out all the asm macros, and fiddly
code, and replace it with a C implementation based on the newly added
routines for creating the early kernel VA mappings. And while at it,
create the initial ID map based on 48-bit virtual addressing as well,
regardless of the number of configured levels for the kernel proper.

Note that this code may execute with the MMU and caches disabled, and is
therefore not permitted to make unaligned accesses. This shouldn't
generally happen in any case for the algorithm as implemented, but to be
sure, let's pass -mstrict-align to the compiler just in case.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20240214122845.2033971-66-ardb+git@google.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arm64: head: Move early kernel mapping routines into C code

The asm version of the kernel mapping code works fine for creating a
coarse grained identity map, but for mapping the kernel down to its
exact boundaries with the right attributes, it is not suitable. This is
why we create a preliminary RWX kernel mapping first, and then rebuild
it from scratch later on.

So let's reimplement this in C, in a way that will make it unnecessary
to create the kernel page tables yet another time in paging_init().

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20240214122845.2033971-63-ardb+git@google.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arm64: head: move dynamic shadow call stack patching into early C runtime

Once we update the early kernel mapping code to only map the kernel once
with the right permissions, we can no longer perform code patching via
this mapping.

So move this code to an earlier stage of the boot, right after applying
the relocations.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20240214122845.2033971-54-ardb+git@google.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arm64: idreg-override: Move to early mini C runtime

We will want to parse the ID register overrides even earlier, so that we
can take them into account before creating the kernel mapping. So
migrate the code and make it work in the context of the early C runtime.
We will move the invocation to an earlier stage in a subsequent patch.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20240214122845.2033971-49-ardb+git@google.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arm64: head: move relocation handling to C code

Now that we have a mini C runtime before the kernel mapping is up, we
can move the non-trivial relocation processing code out of head.S and
reimplement it in C.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20240214122845.2033971-48-ardb+git@google.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arm64: kernel: Don't rely on objcopy to make code under pi/ __init

We will add some code under pi/ that contains global variables that
should not end up in __initdata, as they will not be writable via the
initial ID map. So only rely on objcopy for making the libfdt code
__init, and use explicit annotations for the rest.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20240214122845.2033971-47-ardb+git@google.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arm64: kernel: Manage absolute relocations in code built under pi/

The mini C runtime runs before relocations are processed, and so it
cannot rely on statically initialized pointer variables.

Add a check to ensure that such code does not get introduced by
accident, by going over the relocations in each object, identifying the
ones that operate on data sections that are part of the executable
image, and raising an error if any relocations of type R_AARCH64_ABS64
exist. Note that such relocations are permitted in other places (e.g.,
debug sections) and will never occur in compiler generated code sections
when using the small code model, so only check sections that have
SHF_ALLOC set and SHF_EXECINSTR cleared.

To accommodate cases where statically initialized symbol references are
unavoidable, introduce a special case for ELF input data sections that
have ".rodata.prel64" in their names, and in these cases, instead of
rejecting any encountered ABS64 relocations, convert them into PREL64
relocations, which don't require any runtime fixups. Note that the code
in question must still be modified to deal with this, as it needs to
convert the 64-bit signed offsets into absolute addresses before use.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20240214122845.2033971-46-ardb+git@google.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arm64: kernel: Disable latent_entropy GCC plugin in early C runtime

In subsequent patches, mark portions of the early C code will be marked
as __init. Unfortunarely, __init implies __latent_entropy, and this
would result in the early C code being instrumented in an unsafe manner.

Disable the latent entropy plugin for the early C code.

Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20231129111555.3594833-44-ardb@google.com
Signed-off-by: Will Deacon <will@kernel.org>

arm64: unwind: add asynchronous unwind tables to kernel and modules

Enable asynchronous unwind table generation for both the core kernel as
well as modules, and emit the resulting .eh_frame sections as init code
so we can use the unwind directives for code patching at boot or module
load time.

This will be used by dynamic shadow call stack support, which will rely
on code patching rather than compiler codegen to emit the shadow call
stack push and pop instructions.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Tested-by: Sami Tolvanen <samitolvanen@google.com>
Link: https://lore.kernel.org/r/20221027155908.1940624-2-ardb@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>

arm64: head: avoid relocating the kernel twice for KASLR

Currently, when KASLR is in effect, we set up the kernel virtual address
space twice: the first time, the KASLR seed is looked up in the device
tree, and the kernel virtual mapping is torn down and recreated again,
after which the relocations are applied a second time. The latter step
means that statically initialized global pointer variables will be reset
to their initial values, and to ensure that BSS variables are not set to
values based on the initial translation, they are cleared again as well.

All of this is needed because we need the command line (taken from the
DT) to tell us whether or not to randomize the virtual address space
before entering the kernel proper. However, this code has expanded
little by little and now creates global state unrelated to the virtual
randomization of the kernel before the mapping is torn down and set up
again, and the BSS cleared for a second time. This has created some
issues in the past, and it would be better to avoid this little dance if
possible.

So instead, let's use the temporary mapping of the device tree, and
execute the bare minimum of code to decide whether or not KASLR should
be enabled, and what the seed is. Only then, create the virtual kernel
mapping, clear BSS, etc and proceed as normal. This avoids the issues
around inconsistent global state due to BSS being cleared twice, and is
generally more maintainable, as it permits us to defer all the remaining
DT parsing and KASLR initialization to a later time.

This means the relocation fixup code runs only a single time as well,
allowing us to simplify the RELR handling code too, which is not
idempotent and was therefore required to keep track of the offset that
was applied the first time around.

Note that this means we have to clone a pair of FDT library objects, so
that we can control how they are built - we need the stack protector
and other instrumentation disabled so that the code can tolerate being
called this early. Note that only the kernel page tables and the
temporary stack are mapped read-write at this point, which ensures that
the early code does not modify any global state inadvertently.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20220624150651.1358849-21-ardb@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>