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Subject[PATCH 4.9 66/66] bpf: reject stores into ctx via st and xadd
Date
4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Daniel Borkmann <daniel@iogearbox.net>

[ 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>
---
kernel/bpf/verifier.c | 19 +++++++++++++++++++
1 file changed, 19 insertions(+)

--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -702,6 +702,13 @@ static bool is_pointer_value(struct bpf_
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_verifie
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_
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,

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