[PATCH v1 1/1] riscv: fix runtime constant support for nommu kernels

[Charles Mirabile]

the `__runtime_fixup_32` function does not handle the case where `val` is
zero correctly (as might occur when patching a nommu kernel and referring
to a physical address below the 4GiB boundary whose upper 32 bits are all
zero) because nothing in the existing logic prevents the code from taking
the `else` branch of both nop-checks and emitting two `nop` instructions.

This leaves random garbage in the register that is supposed to receive the
upper 32 bits of the pointer instead of zero that when combined with the
value for the lower 32 bits yields an invalid pointer and causes a kernel
panic when that pointer is eventually accessed.

The author clearly considered the fact that if the `lui` is converted into
a `nop` that the second instruction needs to be adjusted to become an `li`
instead of an `addi`, hence introducing the `addi_insn_mask` variable, but
didn't follow that logic through fully to the case where the `else` branch
executes. To fix it just adjust the logic to ensure that the second `else`
branch is not taken if the first instruction will be patched to a `nop`.

Fixes: a44fb5722199 ("riscv: Add runtime constant support")

Signed-off-by: Charles Mirabile <cmirabil at redhat.com>
---
 arch/riscv/include/asm/runtime-const.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/riscv/include/asm/runtime-const.h b/arch/riscv/include/asm/runtime-const.h
index 451fd76b8811..d766e2b9e6df 100644
--- a/arch/riscv/include/asm/runtime-const.h
+++ b/arch/riscv/include/asm/runtime-const.h
@@ -206,7 +206,7 @@ static inline void __runtime_fixup_32(__le16 *lui_parcel, __le16 *addi_parcel, u
 		addi_insn_mask &= 0x07fff;
 	}
 
-	if (lower_immediate & 0x00000fff) {
+	if (lower_immediate & 0x00000fff || lui_insn == RISCV_INSN_NOP4) {
 		/* replace upper 12 bits of addi with lower 12 bits of val */
 		addi_insn &= addi_insn_mask;
 		addi_insn |= (lower_immediate & 0x00000fff) << 20;
-- 
2.49.0