[PATCH 3/3] arm64: enforce x1|x2|x3 == 0 upon kernel entry as per boot protocol

Ard Biesheuvel
2015-03-17T10:11:14Z

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.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
---
arch/arm64/kernel/head.S | 8 ++++++++
arch/arm64/kernel/setup.c | 13 +++++++++++++
2 files changed, 21 insertions(+)

diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
index 1651c0fd50e6..fe5354eae069 100644
--- a/arch/arm64/kernel/head.S
+++ b/arch/arm64/kernel/head.S
@@ -231,6 +231,10 @@ section_table:
#endif

ENTRY(stext)
+ adr x8, boot_regs // record the contents of
+ stp x0, x1, [x8] // x0 .. x3 at kernel entry
+ stp x2, x3, [x8, #16]
+
mov x21, x0 // x21=FDT
bl el2_setup // Drop to EL1, w20=cpu_boot_mode
adrp x24, KERNEL_START - TEXT_OFFSET // x24=PHYS_OFFSET
@@ -251,6 +255,10 @@ ENTRY(stext)
b __cpu_setup // initialise processor
ENDPROC(stext)

+ .align 3
+ENTRY(boot_regs)
+ .skip 4 * 8 // x0 .. x3
+
/*
* 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 6c5fb5aff325..2b81d0a907ce 100644
--- a/arch/arm64/kernel/setup.c
+++ b/arch/arm64/kernel/setup.c
@@ -532,3 +532,16 @@ const struct seq_operations cpuinfo_op = {
.stop = c_stop,
.show = c_show
};
+
+static int verify_boot_protocol(void)
+{
+ extern u64 boot_regs[];
+
+ if (boot_regs[1] || boot_regs[2] || boot_regs[3]) {
+ pr_err("WARNING: boot protocol violation detected (x1 == %llx, x2 == %llx, x3 == %llx)\n",
+ boot_regs[1], boot_regs[2], boot_regs[3]);
+ pr_err("WARNING: your bootloader may fail to load newer kernels\n");
+ }
+ return 0;
+}
+late_initcall(verify_boot_protocol);
--
1.8.3.2