[bpf-next,08/11] bpf: Allow nospec-protected var-offset stack access

Commit Message

Luis Gerhorst March 13, 2025, 5:41 p.m. UTC

Insert a nospec before the access to prevent it from ever using an index
that is subject to speculative scalar-confusion.

Signed-off-by: Luis Gerhorst <[e-mail]>
Acked-by: Henriette Herzog <[e-mail]>
Cc: Maximilian Ott <[e-mail]>
Cc: Milan Stephan <[e-mail]>
---
 kernel/bpf/verifier.c | 24 ++++++++++++------------
 1 file changed, 12 insertions(+), 12 deletions(-)

Patch

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 8093a5bac4d1..683a76aceffa 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -7858,6 +7858,11 @@  static int check_atomic(struct bpf_verifier_env *env, struct bpf_insn *insn)
 	}
 }
 
+static struct bpf_insn_aux_data *cur_aux(const struct bpf_verifier_env *env)
+{
+	return &env->insn_aux_data[env->insn_idx];
+}
+
 /* When register 'regno' is used to read the stack (either directly or through
  * a helper function) make sure that it's within stack boundary and, depending
  * on the access type and privileges, that all elements of the stack are
@@ -7897,18 +7902,18 @@  static int check_stack_range_initialized(
 	if (tnum_is_const(reg->var_off)) {
 		min_off = max_off = reg->var_off.value + off;
 	} else {
-		/* Variable offset is prohibited for unprivileged mode for
+		/* Variable offset requires a nospec for unprivileged mode for
 		 * simplicity since it requires corresponding support in
 		 * Spectre masking for stack ALU.
 		 * See also retrieve_ptr_limit().
 		 */
 		if (!env->bypass_spec_v1) {
-			char tn_buf[48];
-
-			tnum_strn(tn_buf, sizeof(tn_buf), reg->var_off);
-			verbose(env, "R%d variable offset stack access prohibited for !root, var_off=%s\n",
-				regno, tn_buf);
-			return -EACCES;
+			/* Allow the access, but prevent it from using a
+			 * speculative offset using a nospec before the
+			 * dereference op.
+			 */
+			cur_aux(env)->nospec = true;
+			WARN_ON_ONCE(cur_aux(env)->alu_state);
 		}
 		/* Only initialized buffer on stack is allowed to be accessed
 		 * with variable offset. With uninitialized buffer it's hard to
@@ -11096,11 +11101,6 @@  static int check_get_func_ip(struct bpf_verifier_env *env)
 	return -ENOTSUPP;
 }
 
-static struct bpf_insn_aux_data *cur_aux(const struct bpf_verifier_env *env)
-{
-	return &env->insn_aux_data[env->insn_idx];
-}
-
 static bool loop_flag_is_zero(struct bpf_verifier_env *env)
 {
 	struct bpf_reg_state *regs = cur_regs(env);