[PATCH v2 1/2] arm64: ftrace: don't validate branch via PLT in ftrace_make_nop()
Will Deacon
will.deacon at arm.com
Mon Jun 5 10:15:52 PDT 2017
On Tue, May 30, 2017 at 01:52:19PM +0000, Ard Biesheuvel wrote:
> When turning branch instructions into NOPs, we attempt to validate the
> action by comparing the old value at the call site with the opcode of
> a direct relative branch instruction pointing at the old target.
>
> However, these call sites are statically initialized to call _mcount(),
> and may be redirected via a PLT entry if the module is loaded far away
> from the kernel text, leading to false negatives and spurious errors.
>
> So skip the validation if CONFIG_ARM64_MODULE_PLTS is configured.
>
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
> ---
> arch/arm64/kernel/ftrace.c | 9 ++++++---
> 1 file changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/arch/arm64/kernel/ftrace.c b/arch/arm64/kernel/ftrace.c
> index 40ad08ac569a..a8db6857cad6 100644
> --- a/arch/arm64/kernel/ftrace.c
> +++ b/arch/arm64/kernel/ftrace.c
> @@ -84,12 +84,15 @@ int ftrace_make_nop(struct module *mod, struct dyn_ftrace *rec,
> unsigned long addr)
> {
> unsigned long pc = rec->ip;
> - u32 old, new;
> + u32 old = 0, new;
>
> - old = aarch64_insn_gen_branch_imm(pc, addr, AARCH64_INSN_BRANCH_LINK);
> + if (!IS_ENABLED(CONFIG_ARM64_MODULE_PLTS))
> + old = aarch64_insn_gen_branch_imm(pc, addr,
> + AARCH64_INSN_BRANCH_LINK);
> new = aarch64_insn_gen_nop();
>
> - return ftrace_modify_code(pc, old, new, true);
> + return ftrace_modify_code(pc, old, new,
> + !IS_ENABLED(CONFIG_ARM64_MODULE_PLTS));
Even in the case of PLTs, the instruction is still a branch instruction,
so could we validate the basic opcode fields rather than disable the check
altogether?
Will
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