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>

1 files changed, 0 insertions, 87 deletions

diff --git a/arch/arm64/kernel/kaslr.c b/arch/arm64/kernel/kaslr.c
index 3edee81d8ea7..325455d16dbc 100644
--- a/arch/arm64/kernel/kaslr.c
+++ b/arch/arm64/kernel/kaslr.c
@@ -23,95 +23,8 @@
 u64 __ro_after_init module_alloc_base;
 u16 __initdata memstart_offset_seed;
 
-static __init u64 get_kaslr_seed(void *fdt)
-{
-	int node, len;
-	fdt64_t *prop;
-	u64 ret;
-
-	node = fdt_path_offset(fdt, "/chosen");
-	if (node < 0)
-		return 0;
-
-	prop = fdt_getprop_w(fdt, node, "kaslr-seed", &len);
-	if (!prop || len != sizeof(u64))
-		return 0;
-
-	ret = fdt64_to_cpu(*prop);
-	*prop = 0;
-	return ret;
-}
-
 struct arm64_ftr_override kaslr_feature_override __initdata;
 
-/*
- * This routine will be executed with the kernel mapped at its default virtual
- * address, and if it returns successfully, the kernel will be remapped, and
- * start_kernel() will be executed from a randomized virtual offset. The
- * relocation will result in all absolute references (e.g., static variables
- * containing function pointers) to be reinitialized, and zero-initialized
- * .bss variables will be reset to 0.
- */
-u64 __init kaslr_early_init(void)
-{
-	void *fdt;
-	u64 seed, offset, mask;
-	unsigned long raw;
-
-	/*
-	 * Try to map the FDT early. If this fails, we simply bail,
-	 * and proceed with KASLR disabled. We will make another
-	 * attempt at mapping the FDT in setup_machine()
-	 */
-	fdt = get_early_fdt_ptr();
-	if (!fdt) {
-		return 0;
-	}
-
-	/*
-	 * Retrieve (and wipe) the seed from the FDT
-	 */
-	seed = get_kaslr_seed(fdt);
-
-	/*
-	 * Check if 'nokaslr' appears on the command line, and
-	 * return 0 if that is the case.
-	 */
-	if (kaslr_feature_override.val & kaslr_feature_override.mask & 0xf) {
-		return 0;
-	}
-
-	/*
-	 * Mix in any entropy obtainable architecturally if enabled
-	 * and supported.
-	 */
-
-	if (arch_get_random_seed_long_early(&raw))
-		seed ^= raw;
-
-	if (!seed) {
-		return 0;
-	}
-
-	/*
-	 * OK, so we are proceeding with KASLR enabled. Calculate a suitable
-	 * kernel image offset from the seed. Let's place the kernel in the
-	 * middle half of the VMALLOC area (VA_BITS_MIN - 2), and stay clear of
-	 * the lower and upper quarters to avoid colliding with other
-	 * allocations.
-	 * Even if we could randomize at page granularity for 16k and 64k pages,
-	 * let's always round to 2 MB so we don't interfere with the ability to
-	 * map using contiguous PTEs
-	 */
-	mask = ((1UL << (VA_BITS_MIN - 2)) - 1) & ~(SZ_2M - 1);
-	offset = BIT(VA_BITS_MIN - 3) + (seed & mask);
-
-	/* use the top 16 bits to randomize the linear region */
-	memstart_offset_seed = seed >> 48;
-
-	return offset;
-}
-
 static int __init kaslr_init(void)
 {
 	u64 module_range;