From: Daniel Borkmann daniel@iogearbox.net
commit 58990d1ff3f7896ee341030e9a7c2e4002570683 upstream.
As commit 28e33f9d78ee ("bpf: disallow arithmetic operations on context pointer") already describes, f1174f77b50c ("bpf/verifier: rework value tracking") removed the specific white-listed cases we had previously where we would allow for pointer arithmetic in order to further generalize it, and allow e.g. context access via modified registers. While the dereferencing of modified context pointers had been forbidden through 28e33f9d78ee, syzkaller did recently manage to trigger several KASAN splats for slab out of bounds access and use after frees by simply passing a modified context pointer to a helper function which would then do the bad access since verifier allowed it in adjust_ptr_min_max_vals().
Rejecting arithmetic on ctx pointer in adjust_ptr_min_max_vals() generally could break existing programs as there's a valid use case in tracing in combination with passing the ctx to helpers as bpf_probe_read(), where the register then becomes unknown at verification time due to adding a non-constant offset to it. An access sequence may look like the following:
offset = args->filename; /* field __data_loc filename */ bpf_probe_read(&dst, len, (char *)args + offset); // args is ctx
There are two options: i) we could special case the ctx and as soon as we add a constant or bounded offset to it (hence ctx type wouldn't change) we could turn the ctx into an unknown scalar, or ii) we generalize the sanity test for ctx member access into a small helper and assert it on the ctx register that was passed as a function argument. Fwiw, latter is more obvious and less complex at the same time, and one case that may potentially be legitimate in future for ctx member access at least would be for ctx to carry a const offset. Therefore, fix follows approach from ii) and adds test cases to BPF kselftests.
Fixes: f1174f77b50c ("bpf/verifier: rework value tracking") Reported-by: syzbot+3d0b2441dbb71751615e@syzkaller.appspotmail.com Reported-by: syzbot+c8504affd4fdd0c1b626@syzkaller.appspotmail.com Reported-by: syzbot+e5190cb881d8660fb1a3@syzkaller.appspotmail.com Reported-by: syzbot+efae31b384d5badbd620@syzkaller.appspotmail.com Signed-off-by: Daniel Borkmann daniel@iogearbox.net Acked-by: Alexei Starovoitov ast@kernel.org Acked-by: Yonghong Song yhs@fb.com Acked-by: Edward Cree ecree@solarflare.com Signed-off-by: Alexei Starovoitov ast@kernel.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- kernel/bpf/verifier.c | 45 ++++++++++++++------- tools/testing/selftests/bpf/test_verifier.c | 58 +++++++++++++++++++++++++++- 2 files changed, 87 insertions(+), 16 deletions(-)
--- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -1251,6 +1251,30 @@ static int check_ptr_alignment(struct bp return check_generic_ptr_alignment(reg, pointer_desc, off, size, strict); }
+static int check_ctx_reg(struct bpf_verifier_env *env, + const struct bpf_reg_state *reg, int regno) +{ + /* Access to ctx or passing it to a helper is only allowed in + * its original, unmodified form. + */ + + if (reg->off) { + verbose("dereference of modified ctx ptr R%d off=%d disallowed\n", + regno, reg->off); + return -EACCES; + } + + if (!tnum_is_const(reg->var_off) || reg->var_off.value) { + char tn_buf[48]; + + tnum_strn(tn_buf, sizeof(tn_buf), reg->var_off); + verbose("variable ctx access var_off=%s disallowed\n", tn_buf); + return -EACCES; + } + + return 0; +} + /* truncate register to smaller size (in bytes) * must be called with size < BPF_REG_SIZE */ @@ -1320,22 +1344,10 @@ static int check_mem_access(struct bpf_v verbose("R%d leaks addr into ctx\n", value_regno); return -EACCES; } - /* ctx accesses must be at a fixed offset, so that we can - * determine what type of data were returned. - */ - if (reg->off) { - verbose("dereference of modified ctx ptr R%d off=%d+%d, ctx+const is allowed, ctx+const+const is not\n", - regno, reg->off, off - reg->off); - return -EACCES; - } - if (!tnum_is_const(reg->var_off) || reg->var_off.value) { - char tn_buf[48]; + err = check_ctx_reg(env, reg, regno); + if (err < 0) + return err;
- tnum_strn(tn_buf, sizeof(tn_buf), reg->var_off); - verbose("variable ctx access var_off=%s off=%d size=%d", - tn_buf, off, size); - return -EACCES; - } err = check_ctx_access(env, insn_idx, off, size, t, ®_type); if (!err && t == BPF_READ && value_regno >= 0) { /* ctx access returns either a scalar, or a @@ -1573,6 +1585,9 @@ static int check_func_arg(struct bpf_ver expected_type = PTR_TO_CTX; if (type != expected_type) goto err_type; + err = check_ctx_reg(env, reg, regno); + if (err < 0) + return err; } else if (arg_type == ARG_PTR_TO_MEM || arg_type == ARG_PTR_TO_UNINIT_MEM) { expected_type = PTR_TO_STACK; --- a/tools/testing/selftests/bpf/test_verifier.c +++ b/tools/testing/selftests/bpf/test_verifier.c @@ -7281,7 +7281,7 @@ static struct bpf_test tests[] = { offsetof(struct __sk_buff, mark)), BPF_EXIT_INSN(), }, - .errstr = "dereference of modified ctx ptr R1 off=68+8, ctx+const is allowed, ctx+const+const is not", + .errstr = "dereference of modified ctx ptr", .result = REJECT, .prog_type = BPF_PROG_TYPE_SCHED_CLS, }, @@ -7944,6 +7944,62 @@ static struct bpf_test tests[] = { .errstr = "BPF_XADD stores into R2 packet", .prog_type = BPF_PROG_TYPE_XDP, }, + { + "pass unmodified ctx pointer to helper", + .insns = { + BPF_MOV64_IMM(BPF_REG_2, 0), + BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, + BPF_FUNC_csum_update), + BPF_MOV64_IMM(BPF_REG_0, 0), + BPF_EXIT_INSN(), + }, + .prog_type = BPF_PROG_TYPE_SCHED_CLS, + .result = ACCEPT, + }, + { + "pass modified ctx pointer to helper, 1", + .insns = { + BPF_ALU64_IMM(BPF_ADD, BPF_REG_1, -612), + BPF_MOV64_IMM(BPF_REG_2, 0), + BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, + BPF_FUNC_csum_update), + BPF_MOV64_IMM(BPF_REG_0, 0), + BPF_EXIT_INSN(), + }, + .prog_type = BPF_PROG_TYPE_SCHED_CLS, + .result = REJECT, + .errstr = "dereference of modified ctx ptr", + }, + { + "pass modified ctx pointer to helper, 2", + .insns = { + BPF_ALU64_IMM(BPF_ADD, BPF_REG_1, -612), + BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, + BPF_FUNC_get_socket_cookie), + BPF_MOV64_IMM(BPF_REG_0, 0), + BPF_EXIT_INSN(), + }, + .result_unpriv = REJECT, + .result = REJECT, + .errstr_unpriv = "dereference of modified ctx ptr", + .errstr = "dereference of modified ctx ptr", + }, + { + "pass modified ctx pointer to helper, 3", + .insns = { + BPF_LDX_MEM(BPF_W, BPF_REG_3, BPF_REG_1, 0), + BPF_ALU64_IMM(BPF_AND, BPF_REG_3, 4), + BPF_ALU64_REG(BPF_ADD, BPF_REG_1, BPF_REG_3), + BPF_MOV64_IMM(BPF_REG_2, 0), + BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, + BPF_FUNC_csum_update), + BPF_MOV64_IMM(BPF_REG_0, 0), + BPF_EXIT_INSN(), + }, + .prog_type = BPF_PROG_TYPE_SCHED_CLS, + .result = REJECT, + .errstr = "variable ctx access var_off=(0x0; 0x4)", + }, };
static int probe_filter_length(const struct bpf_insn *fp)