Commit f8af0b36 authored by Ard Biesheuvel's avatar Ard Biesheuvel Committed by Will Deacon
Browse files

arm64: ftrace: don't validate branch via PLT in ftrace_make_nop() 



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: default avatarArd Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: default avatarWill Deacon <will.deacon@arm.com>
parent dbbb08f5

arch/arm64/kernel/ftrace.c

+43 −3
Original line number Diff line number Diff line
@@ -84,12 +84,52 @@ int ftrace_make_nop(struct module *mod, struct dyn_ftrace *rec, 
		    unsigned long addr)

{

	unsigned long pc = rec->ip;

	u32 old, new;

	long offset = (long)pc - (long)addr;

	bool validate = true;

	u32 old = 0, new;



	if (IS_ENABLED(CONFIG_ARM64_MODULE_PLTS) &&

	    (offset < -SZ_128M || offset >= SZ_128M)) {

		u32 replaced;



		/*

		 * 'mod' is only set at module load time, but if we end up

		 * dealing with an out-of-range condition, we can assume it

		 * is due to a module being loaded far away from the kernel.

		 */

		if (!mod) {

			preempt_disable();

			mod = __module_text_address(pc);

			preempt_enable();



			if (WARN_ON(!mod))

				return -EINVAL;

		}



		/*

		 * The instruction we are about to patch may be a branch and

		 * link instruction that was redirected via a PLT entry. In

		 * this case, the normal validation will fail, but we can at

		 * least check that we are dealing with a branch and link

		 * instruction that points into the right module.

		 */

		if (aarch64_insn_read((void *)pc, &replaced))

			return -EFAULT;



		if (!aarch64_insn_is_bl(replaced) ||

		    !within_module(pc + aarch64_get_branch_offset(replaced),

				   mod))

			return -EINVAL;



		validate = false;

	} else {

		old = aarch64_insn_gen_branch_imm(pc, addr,

						  AARCH64_INSN_BRANCH_LINK);

	}



	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, validate);

}



void arch_ftrace_update_code(int command)

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