arm64: enforce x1|x2|x3 == 0 upon kernel entry as per boot protocol

According to the arm64 boot protocol, registers x1 to x3 should be
zero upon kernel entry, and non-zero values are reserved for future
use. This future use is going to be problematic if we never enforce
the current rules, so start enforcing them now, by emitting a warning
if non-zero values are detected.

Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>

2 files changed, 29 insertions, 1 deletions

diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
index f5ac337f9598..1fdf42041f42 100644
--- a/arch/arm64/kernel/head.S
+++ b/arch/arm64/kernel/head.S
@@ -233,7 +233,7 @@ section_table:
 #endif
 
 ENTRY(stext)
-        mov     x21, x0                         // x21=FDT
+        bl      preserve_boot_args
         bl      el2_setup                       // Drop to EL1, w20=cpu_boot_mode
         adrp    x24, __PHYS_OFFSET
         bl      set_cpu_boot_mode_flag
@@ -253,6 +253,23 @@ ENTRY(stext)
 ENDPROC(stext)
 
 /*
+ * Preserve the arguments passed by the bootloader in x0 .. x3
+ */
+preserve_boot_args:
+        mov     x21, x0                         // x21=FDT
+
+        adr_l   x0, boot_args                   // record the contents of
+        stp     x21, x1, [x0]                   // x0 .. x3 at kernel entry
+        stp     x2, x3, [x0, #16]
+
+        dmb     sy                              // needed before dc ivac with
+                                                // MMU off
+
+        add     x1, x0, #0x20                   // 4 x 8 bytes
+        b       __inval_cache_range             // tail call
+ENDPROC(preserve_boot_args)
+
+/*
  * Determine validity of the x21 FDT pointer.
  * The dtb must be 8-byte aligned and live in the first 512M of memory.
  */
diff --git a/arch/arm64/kernel/setup.c b/arch/arm64/kernel/setup.c
index 1783b38cf4c0..51ef97274b52 100644
--- a/arch/arm64/kernel/setup.c
+++ b/arch/arm64/kernel/setup.c
@@ -115,6 +115,11 @@ void __init early_print(const char *str, ...)
         printk("%s", buf);
 }
 
+/*
+ * The recorded values of x0 .. x3 upon kernel entry.
+ */
+u64 __cacheline_aligned boot_args[4];
+
 void __init smp_setup_processor_id(void)
 {
         u64 mpidr = read_cpuid_mpidr() & MPIDR_HWID_BITMASK;
@@ -412,6 +417,12 @@ void __init setup_arch(char **cmdline_p)
         conswitchp = &dummy_con;
 #endif
 #endif
+        if (boot_args[1] || boot_args[2] || boot_args[3]) {
+                pr_err("WARNING: x1-x3 nonzero in violation of boot protocol:\n"
+                        "\tx1: %016llx\n\tx2: %016llx\n\tx3: %016llx\n"
+                        "This indicates a broken bootloader or old kernel\n",
+                        boot_args[1], boot_args[2], boot_args[3]);
+        }
 }
 
 static int __init arm64_device_init(void)