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authorAndrey Ignatov <rdna@fb.com>2018-11-10 22:15:13 -0800
committerAlexei Starovoitov <ast@kernel.org>2018-11-10 22:29:59 -0800
commit46f53a65d2de3e1591636c22b626b09d8684fd71 (patch)
tree23e917e4c8dc02d130c4103b38fd66352c5ea875
parentf2cbf95826fb9c0ccdcb5dfec1a7de8203fd7269 (diff)
bpf: Allow narrow loads with offset > 0
Currently BPF verifier allows narrow loads for a context field only with offset zero. E.g. if there is a __u32 field then only the following loads are permitted: * off=0, size=1 (narrow); * off=0, size=2 (narrow); * off=0, size=4 (full). On the other hand LLVM can generate a load with offset different than zero that make sense from program logic point of view, but verifier doesn't accept it. E.g. tools/testing/selftests/bpf/sendmsg4_prog.c has code: #define DST_IP4 0xC0A801FEU /* 192.168.1.254 */ ... if ((ctx->user_ip4 >> 24) == (bpf_htonl(DST_IP4) >> 24) && where ctx is struct bpf_sock_addr. Some versions of LLVM can produce the following byte code for it: 8: 71 12 07 00 00 00 00 00 r2 = *(u8 *)(r1 + 7) 9: 67 02 00 00 18 00 00 00 r2 <<= 24 10: 18 03 00 00 00 00 00 fe 00 00 00 00 00 00 00 00 r3 = 4261412864 ll 12: 5d 32 07 00 00 00 00 00 if r2 != r3 goto +7 <LBB0_6> where `*(u8 *)(r1 + 7)` means narrow load for ctx->user_ip4 with size=1 and offset=3 (7 - sizeof(ctx->user_family) = 3). This load is currently rejected by verifier. Verifier code that rejects such loads is in bpf_ctx_narrow_access_ok() what means any is_valid_access implementation, that uses the function, works this way, e.g. bpf_skb_is_valid_access() for __sk_buff or sock_addr_is_valid_access() for bpf_sock_addr. The patch makes such loads supported. Offset can be in [0; size_default) but has to be multiple of load size. E.g. for __u32 field the following loads are supported now: * off=0, size=1 (narrow); * off=1, size=1 (narrow); * off=2, size=1 (narrow); * off=3, size=1 (narrow); * off=0, size=2 (narrow); * off=2, size=2 (narrow); * off=0, size=4 (full). Reported-by: Yonghong Song <yhs@fb.com> Signed-off-by: Andrey Ignatov <rdna@fb.com> Signed-off-by: Alexei Starovoitov <ast@kernel.org>
-rw-r--r--include/linux/filter.h16
-rw-r--r--kernel/bpf/verifier.c21
2 files changed, 17 insertions, 20 deletions
diff --git a/include/linux/filter.h b/include/linux/filter.h
index de629b706d1d..cc17f5f32fbb 100644
--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -668,24 +668,10 @@ static inline u32 bpf_ctx_off_adjust_machine(u32 size)
return size;
}
-static inline bool bpf_ctx_narrow_align_ok(u32 off, u32 size_access,
- u32 size_default)
-{
- size_default = bpf_ctx_off_adjust_machine(size_default);
- size_access = bpf_ctx_off_adjust_machine(size_access);
-
-#ifdef __LITTLE_ENDIAN
- return (off & (size_default - 1)) == 0;
-#else
- return (off & (size_default - 1)) + size_access == size_default;
-#endif
-}
-
static inline bool
bpf_ctx_narrow_access_ok(u32 off, u32 size, u32 size_default)
{
- return bpf_ctx_narrow_align_ok(off, size, size_default) &&
- size <= size_default && (size & (size - 1)) == 0;
+ return size <= size_default && (size & (size - 1)) == 0;
}
#define bpf_classic_proglen(fprog) (fprog->len * sizeof(fprog->filter[0]))
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 8d0977980cfa..b5222aa61d54 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -5718,10 +5718,10 @@ static int convert_ctx_accesses(struct bpf_verifier_env *env)
int i, cnt, size, ctx_field_size, delta = 0;
const int insn_cnt = env->prog->len;
struct bpf_insn insn_buf[16], *insn;
+ u32 target_size, size_default, off;
struct bpf_prog *new_prog;
enum bpf_access_type type;
bool is_narrower_load;
- u32 target_size;
if (ops->gen_prologue || env->seen_direct_write) {
if (!ops->gen_prologue) {
@@ -5814,9 +5814,9 @@ static int convert_ctx_accesses(struct bpf_verifier_env *env)
* we will apply proper mask to the result.
*/
is_narrower_load = size < ctx_field_size;
+ size_default = bpf_ctx_off_adjust_machine(ctx_field_size);
+ off = insn->off;
if (is_narrower_load) {
- u32 size_default = bpf_ctx_off_adjust_machine(ctx_field_size);
- u32 off = insn->off;
u8 size_code;
if (type == BPF_WRITE) {
@@ -5844,12 +5844,23 @@ static int convert_ctx_accesses(struct bpf_verifier_env *env)
}
if (is_narrower_load && size < target_size) {
- if (ctx_field_size <= 4)
+ u8 shift = (off & (size_default - 1)) * 8;
+
+ if (ctx_field_size <= 4) {
+ if (shift)
+ insn_buf[cnt++] = BPF_ALU32_IMM(BPF_RSH,
+ insn->dst_reg,
+ shift);
insn_buf[cnt++] = BPF_ALU32_IMM(BPF_AND, insn->dst_reg,
(1 << size * 8) - 1);
- else
+ } else {
+ if (shift)
+ insn_buf[cnt++] = BPF_ALU64_IMM(BPF_RSH,
+ insn->dst_reg,
+ shift);
insn_buf[cnt++] = BPF_ALU64_IMM(BPF_AND, insn->dst_reg,
(1 << size * 8) - 1);
+ }
}
new_prog = bpf_patch_insn_data(env, i + delta, insn_buf, cnt);