[PATCH bpf-next v2 10/11] bpf: Allow nospec-protected var-offset stack access

[Luis Gerhorst <luis.gerhorst@fau.de>]

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

The access itself can either happen directly in the BPF program (reads
only, check_stack_read_var_off()) or in a helper (read/write,
check_helper_mem_access()).

This relies on the fact that the speculative scalar confusion that leads
to the variable-stack access going OOBs must stem from a prior
speculative store or branch bypass. Adding a nospec before the
variable-stack access will force all previously bypassed stores/branches
to complete and cause the stack access to only ever go to the stack slot
that is accessed architecturally.

Alternatively, the variable-offset stack access might be a write that
can itself be subject to speculative store bypass (this can happen in
theory even if this code adds a nospec /before/ the variable-offset
write). Only indirect writes by helpers might be affected here (e.g.,
those taking ARG_PTR_TO_MAP_VALUE). (Because check_stack_write_var_off()
does not use check_stack_range_initialized(), in-program variable-offset
writes are not affected.) If the in-helper write can be subject to
Spectre v4 and the helper writes/overwrites pointers on the BPF stack,
they are already a problem for fixed-offset stack accesses and should be
subject to Spectre v4 sanitization.

Signed-off-by: Luis Gerhorst <luis.gerhorst@fau.de>
Acked-by: Henriette Herzog <henriette.herzog@rub.de>
Cc: Maximilian Ott <ott@cs.fau.de>
Cc: Milan Stephan <milan.stephan@fau.de>
---
 kernel/bpf/verifier.c | 24 ++++++++++++------------
 1 file changed, 12 insertions(+), 12 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 92490964eb3b..2cd925b915e0 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -7894,6 +7894,11 @@ 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
@@ -7933,18 +7938,18 @@ 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
@@ -11172,11 +11177,6 @@ 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);
-- 
2.49.0