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 db26b477dd45..1fbafea3ed69 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);