4.20-stable review patch. If anyone has any objections, please let me know.
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[ Upstream commit 46f53a65d2de3e1591636c22b626b09d8684fd71 ]
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 Signed-off-by: Sasha Levin sashal@kernel.org --- include/linux/filter.h | 16 +--------------- kernel/bpf/verifier.c | 21 ++++++++++++++++----- 2 files changed, 17 insertions(+), 20 deletions(-)
diff --git a/include/linux/filter.h b/include/linux/filter.h index a8b9d90a8042..25a556589ae8 100644 --- a/include/linux/filter.h +++ b/include/linux/filter.h @@ -675,24 +675,10 @@ 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 51ba84d4d34a..a81f52b2c92e 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -5789,10 +5789,10 @@ 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) { @@ -5885,9 +5885,9 @@ 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) { @@ -5915,12 +5915,23 @@ 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);