[RFC PATCH 6/9] bpf: Allow nospec-protected var-offset stack access

[Luis Gerhorst]

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

Signed-off-by: Luis Gerhorst <luis.gerhorst at fau.de>
Acked-by: Henriette Herzog <henriette.herzog at rub.de>
Cc: Maximilian Ott <ott at cs.fau.de>
Cc: Milan Stephan <milan.stephan at fau.de>
---
 kernel/bpf/verifier.c | 15 ++++++++-------
 1 file changed, 8 insertions(+), 7 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index aee49f8da0c1..06c2f929d602 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -7638,6 +7638,8 @@ static int check_atomic(struct bpf_verifier_env *env, int insn_idx, struct bpf_i
 	return 0;
 }
 
+static struct bpf_insn_aux_data *cur_aux(struct bpf_verifier_env *env);
+
 /* 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
@@ -7677,18 +7679,17 @@ 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;
 		}
 		/* Only initialized buffer on stack is allowed to be accessed
 		 * with variable offset. With uninitialized buffer it's hard to
-- 
2.48.1