bpf: reject stores into ctx via st and xadd

[ upstream commit f37a8cb84cce18762e8f86a70bd6a49a66ab964c ]

Alexei found that verifier does not reject stores into context
via BPF_ST instead of BPF_STX. And while looking at it, we
also should not allow XADD variant of BPF_STX.

The context rewriter is only assuming either BPF_LDX_MEM- or
BPF_STX_MEM-type operations, thus reject anything other than
that so that assumptions in the rewriter properly hold. Add
test cases as well for BPF selftests.

Fixes: d691f9e8d440 ("bpf: allow programs to write to certain skb fields")
Reported-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 5c56ff9..076e4a0 100644 (file)
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -702,6 +702,13 @@ static bool is_pointer_value(struct bpf_verifier_env *env, int regno)
        return __is_pointer_value(env->allow_ptr_leaks, &env->cur_state.regs[regno]);
 }
 
+static bool is_ctx_reg(struct bpf_verifier_env *env, int regno)
+{
+       const struct bpf_reg_state *reg = &env->cur_state.regs[regno];
+
+       return reg->type == PTR_TO_CTX;
+}
+
 static int check_ptr_alignment(struct bpf_verifier_env *env,
                               struct bpf_reg_state *reg, int off, int size)
 {
@@ -896,6 +903,12 @@ static int check_xadd(struct bpf_verifier_env *env, struct bpf_insn *insn)
                return -EACCES;
        }
 
+       if (is_ctx_reg(env, insn->dst_reg)) {
+               verbose("BPF_XADD stores into R%d context is not allowed\n",
+                       insn->dst_reg);
+               return -EACCES;
+       }
+
        /* check whether atomic_add can read the memory */
        err = check_mem_access(env, insn->dst_reg, insn->off,
                               BPF_SIZE(insn->code), BPF_READ, -1);
@@ -3012,6 +3025,12 @@ static int do_check(struct bpf_verifier_env *env)
                        if (err)
                                return err;
 
+                       if (is_ctx_reg(env, insn->dst_reg)) {
+                               verbose("BPF_ST stores into R%d context is not allowed\n",
+                                       insn->dst_reg);
+                               return -EACCES;
+                       }
+
                        /* check that memory (dst_reg + off) is writeable */
                        err = check_mem_access(env, insn->dst_reg, insn->off,
                                               BPF_SIZE(insn->code), BPF_WRITE,