bpf: fix net.core.bpf_jit_enable race

global bpf_jit_enable variable is tested multiple times in JITs,
blinding and verifier core. The malicious root can try to toggle
it while loading the programs. This race condition was accounted
for and there should be no issues, but it's safer to avoid
this race condition.

Signed-off-by: Alexei Starovoitov <[email protected]>
Acked-by: Daniel Borkmann <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>

Author: 4ast

arch/arm/net/bpf_jit_32.c

@@ -1824,7 +1824,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
 	/* If BPF JIT was not enabled then we must fall back to
 	 * the interpreter.
 	 */
-	if (!bpf_jit_enable)
+	if (!prog->jit_requested)
 		return orig_prog;
 
 	/* If constant blinding was enabled and we failed during blinding

arch/arm64/net/bpf_jit_comp.c

@@ -844,7 +844,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
 	int image_size;
 	u8 *image_ptr;
 
-	if (!bpf_jit_enable)
+	if (!prog->jit_requested)
 		return orig_prog;
 
 	tmp = bpf_jit_blind_constants(prog);

arch/mips/net/ebpf_jit.c

@@ -1869,7 +1869,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
 	unsigned int image_size;
 	u8 *image_ptr;
 
-	if (!bpf_jit_enable || !cpu_has_mips64r2)
+	if (!prog->jit_requested || !cpu_has_mips64r2)
 		return prog;
 
 	tmp = bpf_jit_blind_constants(prog);

arch/powerpc/net/bpf_jit_comp64.c

@@ -993,7 +993,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *fp)
 	struct bpf_prog *tmp_fp;
 	bool bpf_blinded = false;
 
-	if (!bpf_jit_enable)
+	if (!fp->jit_requested)
 		return org_fp;
 
 	tmp_fp = bpf_jit_blind_constants(org_fp);

arch/s390/net/bpf_jit_comp.c

@@ -1300,7 +1300,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *fp)
 	struct bpf_jit jit;
 	int pass;
 
-	if (!bpf_jit_enable)
+	if (!fp->jit_requested)
 		return orig_fp;
 
 	tmp = bpf_jit_blind_constants(fp);

arch/sparc/net/bpf_jit_comp_64.c

@@ -1517,7 +1517,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
 	u8 *image_ptr;
 	int pass;
 
-	if (!bpf_jit_enable)
+	if (!prog->jit_requested)
 		return orig_prog;
 
 	tmp = bpf_jit_blind_constants(prog);

arch/x86/net/bpf_jit_comp.c

@@ -1121,7 +1121,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
 	int pass;
 	int i;
 
-	if (!bpf_jit_enable)
+	if (!prog->jit_requested)
 		return orig_prog;
 
 	tmp = bpf_jit_blind_constants(prog);

include/linux/filter.h

@@ -458,6 +458,7 @@ struct bpf_binary_header {
 struct bpf_prog {
 	u16			pages;		/* Number of allocated pages */
 	u16			jited:1,	/* Is our filter JIT'ed? */
+				jit_requested:1,/* archs need to JIT the prog */
 				locked:1,	/* Program image locked? */
 				gpl_compatible:1, /* Is filter GPL compatible? */
 				cb_access:1,	/* Is control block accessed? */
@@ -804,15 +805,15 @@ static inline bool bpf_prog_ebpf_jited(const struct bpf_prog *fp)
 	return fp->jited && bpf_jit_is_ebpf();
 }
 
-static inline bool bpf_jit_blinding_enabled(void)
+static inline bool bpf_jit_blinding_enabled(struct bpf_prog *prog)
 {
 	/* These are the prerequisites, should someone ever have the
 	 * idea to call blinding outside of them, we make sure to
 	 * bail out.
 	 */
 	if (!bpf_jit_is_ebpf())
 		return false;
-	if (!bpf_jit_enable)
+	if (!prog->jit_requested)
 		return false;
 	if (!bpf_jit_harden)
 		return false;

kernel/bpf/core.c

@@ -94,6 +94,7 @@ struct bpf_prog *bpf_prog_alloc(unsigned int size, gfp_t gfp_extra_flags)
 	fp->pages = size / PAGE_SIZE;
 	fp->aux = aux;
 	fp->aux->prog = fp;
+	fp->jit_requested = ebpf_jit_enabled();
 
 	INIT_LIST_HEAD_RCU(&fp->aux->ksym_lnode);
 
@@ -721,7 +722,7 @@ struct bpf_prog *bpf_jit_blind_constants(struct bpf_prog *prog)
 	struct bpf_insn *insn;
 	int i, rewritten;
 
-	if (!bpf_jit_blinding_enabled())
+	if (!bpf_jit_blinding_enabled(prog))
 		return prog;
 
 	clone = bpf_prog_clone_create(prog, GFP_USER);

kernel/bpf/verifier.c

@@ -5080,7 +5080,7 @@ static int fixup_bpf_calls(struct bpf_verifier_env *env)
 		/* BPF_EMIT_CALL() assumptions in some of the map_gen_lookup
 		 * handlers are currently limited to 64 bit only.
 		 */
-		if (ebpf_jit_enabled() && BITS_PER_LONG == 64 &&
+		if (prog->jit_requested && BITS_PER_LONG == 64 &&
 		    insn->imm == BPF_FUNC_map_lookup_elem) {
 			map_ptr = env->insn_aux_data[i + delta].map_ptr;
 			if (map_ptr == BPF_MAP_PTR_POISON ||