Commit d4e89d21 authored by Daniel Sneddon's avatar Daniel Sneddon Committed by Dave Hansen
Browse files

x86/bpf: Call branch history clearing sequence on exit 



Classic BPF programs have been identified as potential vectors for
intra-mode Branch Target Injection (BTI) attacks. Classic BPF programs can
be run by unprivileged users. They allow unprivileged code to execute
inside the kernel. Attackers can use unprivileged cBPF to craft branch
history in kernel mode that can influence the target of indirect branches.

Introduce a branch history buffer (BHB) clearing sequence during the JIT
compilation of classic BPF programs. The clearing sequence is the same as
is used in previous mitigations to protect syscalls. Since eBPF programs
already have their own mitigations in place, only insert the call on
classic programs that aren't run by privileged users.

Signed-off-by: default avatarDaniel Sneddon <daniel.sneddon@linux.intel.com>
Signed-off-by: default avatarPawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: default avatarDave Hansen <dave.hansen@linux.intel.com>
Acked-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
Reviewed-by: default avatarAlexandre Chartre <alexandre.chartre@oracle.com>
parent 92a09c47

arch/x86/net/bpf_jit_comp.c

+31 −0
Original line number Diff line number Diff line
@@ -1502,6 +1502,30 @@ static void emit_priv_frame_ptr(u8 **pprog, void __percpu *priv_frame_ptr) 
#define PRIV_STACK_GUARD_SZ    8

#define PRIV_STACK_GUARD_VAL   0xEB9F12345678eb9fULL



static int emit_spectre_bhb_barrier(u8 **pprog, u8 *ip,

				    struct bpf_prog *bpf_prog)

{

	u8 *prog = *pprog;

	u8 *func;



	if (cpu_feature_enabled(X86_FEATURE_CLEAR_BHB_LOOP)) {

		/* The clearing sequence clobbers eax and ecx. */

		EMIT1(0x50); /* push rax */

		EMIT1(0x51); /* push rcx */

		ip += 2;



		func = (u8 *)clear_bhb_loop;

		ip += x86_call_depth_emit_accounting(&prog, func, ip);



		if (emit_call(&prog, func, ip))

			return -EINVAL;

		EMIT1(0x59); /* pop rcx */

		EMIT1(0x58); /* pop rax */

	}

	*pprog = prog;

	return 0;

}



static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image, u8 *rw_image,

		  int oldproglen, struct jit_context *ctx, bool jmp_padding)

{
@@ -2544,6 +2568,13 @@ st: if (is_imm8(insn->off)) 
			seen_exit = true;

			/* Update cleanup_addr */

			ctx->cleanup_addr = proglen;

			if (bpf_prog_was_classic(bpf_prog) &&

			    !capable(CAP_SYS_ADMIN)) {

				u8 *ip = image + addrs[i - 1];



				if (emit_spectre_bhb_barrier(&prog, ip, bpf_prog))

					return -EINVAL;

			}

			if (bpf_prog->aux->exception_boundary) {

				pop_callee_regs(&prog, all_callee_regs_used);

				pop_r12(&prog);