From ba9d2e0cc870d1a16195c801aa18b33cd70125b9 Mon Sep 17 00:00:00 2001 From: Daniel Borkmann Date: Wed, 3 Apr 2019 18:39:09 +0000 Subject: [PATCH] bpf: restrict stack pointer arithmetic for unprivileged commit e4298d25830a866cc0f427d4bccb858e76715859 upstream. Restrict stack pointer arithmetic for unprivileged users in that arithmetic itself must not go out of bounds as opposed to the actual access later on. Therefore after each adjust_ptr_min_max_vals() with a stack pointer as a destination we simulate a check_stack_access() of 1 byte on the destination and once that fails the program is rejected for unprivileged program loads. This is analog to map value pointer arithmetic and needed for masking later on. Signed-off-by: Daniel Borkmann Acked-by: Alexei Starovoitov Signed-off-by: Alexei Starovoitov [backported to 4.14 sblbir] Signed-off-by: Balbir Singh Signed-off-by: Greg Kroah-Hartman --- kernel/bpf/verifier.c | 62 ++++++++++++++++++++++++++++--------------- 1 file changed, 41 insertions(+), 21 deletions(-) diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index 096a15a884c9..bc073789b693 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -940,6 +940,31 @@ static int check_stack_read(struct bpf_verifier_state *state, int off, int size, } } +static int check_stack_access(struct bpf_verifier_env *env, + const struct bpf_reg_state *reg, + int off, int size) +{ + /* Stack accesses must be at a fixed offset, so that we + * can determine what type of data were returned. See + * check_stack_read(). + */ + if (!tnum_is_const(reg->var_off)) { + char tn_buf[48]; + + tnum_strn(tn_buf, sizeof(tn_buf), reg->var_off); + verbose("variable stack access var_off=%s off=%d size=%d", + tn_buf, off, size); + return -EACCES; + } + + if (off >= 0 || off < -MAX_BPF_STACK) { + verbose("invalid stack off=%d size=%d\n", off, size); + return -EACCES; + } + + return 0; +} + /* check read/write into map element returned by bpf_map_lookup_elem() */ static int __check_map_access(struct bpf_verifier_env *env, u32 regno, int off, int size) @@ -1322,23 +1347,10 @@ static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, u32 regn } } else if (reg->type == PTR_TO_STACK) { - /* stack accesses must be at a fixed offset, so that we can - * determine what type of data were returned. - * See check_stack_read(). - */ - if (!tnum_is_const(reg->var_off)) { - char tn_buf[48]; - - tnum_strn(tn_buf, sizeof(tn_buf), reg->var_off); - verbose("variable stack access var_off=%s off=%d size=%d", - tn_buf, off, size); - return -EACCES; - } off += reg->var_off.value; - if (off >= 0 || off < -MAX_BPF_STACK) { - verbose("invalid stack off=%d size=%d\n", off, size); - return -EACCES; - } + err = check_stack_access(env, reg, off, size); + if (err) + return err; if (env->prog->aux->stack_depth < -off) env->prog->aux->stack_depth = -off; @@ -2195,11 +2207,19 @@ static int adjust_ptr_min_max_vals(struct bpf_verifier_env *env, /* For unprivileged we require that resulting offset must be in bounds * in order to be able to sanitize access later on. */ - if (!env->allow_ptr_leaks && dst_reg->type == PTR_TO_MAP_VALUE && - check_map_access(env, dst, dst_reg->off, 1, false)) { - verbose(env, "R%d pointer arithmetic of map value goes out of range, prohibited for !root\n", - dst); - return -EACCES; + if (!env->allow_ptr_leaks) { + if (dst_reg->type == PTR_TO_MAP_VALUE && + check_map_access(env, dst, dst_reg->off, 1)) { + verbose("R%d pointer arithmetic of map value goes out of range, " + "prohibited for !root\n", dst); + return -EACCES; + } else if (dst_reg->type == PTR_TO_STACK && + check_stack_access(env, dst_reg, dst_reg->off + + dst_reg->var_off.value, 1)) { + verbose("R%d stack pointer arithmetic goes out of range, " + "prohibited for !root\n", dst); + return -EACCES; + } } return 0; -- 2.20.1