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authorDaniel Borkmann <daniel@iogearbox.net>2019-12-19 22:19:50 +0100
committerAlexei Starovoitov <ast@kernel.org>2019-12-19 13:39:22 -0800
commitcc52d9140aa920d8d61c7f6de3fff5fea6692ea9 (patch)
tree9a2cd65e8f4dbe029fcb58884b4baad46352586f /kernel
parent1148f9adbe71415836a18a36c1b4ece999ab0973 (diff)
bpf: Fix record_func_key to perform backtracking on r3
While testing Cilium with /unreleased/ Linus' tree under BPF-based NodePort implementation, I noticed a strange BPF SNAT engine behavior from time to time. In some cases it would do the correct SNAT/DNAT service translation, but at a random point in time it would just stop and perform an unexpected translation after SYN, SYN/ACK and stack would send a RST back. While initially assuming that there is some sort of a race condition in BPF code, adding trace_printk()s for debugging purposes at some point seemed to have resolved the issue auto-magically. Digging deeper on this Heisenbug and reducing the trace_printk() calls to an absolute minimum, it turns out that a single call would suffice to trigger / not trigger the seen RST issue, even though the logic of the program itself remains unchanged. Turns out the single call changed verifier pruning behavior to get everything to work. Reconstructing a minimal test case, the incorrect JIT dump looked as follows: # bpftool p d j i 11346 0xffffffffc0cba96c: [...] 21: movzbq 0x30(%rdi),%rax 26: cmp $0xd,%rax 2a: je 0x000000000000003a 2c: xor %edx,%edx 2e: movabs $0xffff89cc74e85800,%rsi 38: jmp 0x0000000000000049 3a: mov $0x2,%edx 3f: movabs $0xffff89cc74e85800,%rsi 49: mov -0x224(%rbp),%eax 4f: cmp $0x20,%eax 52: ja 0x0000000000000062 54: add $0x1,%eax 57: mov %eax,-0x224(%rbp) 5d: jmpq 0xffffffffffff6911 62: mov $0x1,%eax [...] Hence, unexpectedly, JIT emitted a direct jump even though retpoline based one would have been needed since in line 2c and 3a we have different slot keys in BPF reg r3. Verifier log of the test case reveals what happened: 0: (b7) r0 = 14 1: (73) *(u8 *)(r1 +48) = r0 2: (71) r0 = *(u8 *)(r1 +48) 3: (15) if r0 == 0xd goto pc+4 R0_w=inv(id=0,umax_value=255,var_off=(0x0; 0xff)) R1=ctx(id=0,off=0,imm=0) R10=fp0 4: (b7) r3 = 0 5: (18) r2 = 0xffff89cc74d54a00 7: (05) goto pc+3 11: (85) call bpf_tail_call#12 12: (b7) r0 = 1 13: (95) exit from 3 to 8: R0_w=inv13 R1=ctx(id=0,off=0,imm=0) R10=fp0 8: (b7) r3 = 2 9: (18) r2 = 0xffff89cc74d54a00 11: safe processed 13 insns (limit 1000000) [...] Second branch is pruned by verifier since considered safe, but issue is that record_func_key() couldn't have seen the index in line 3a and therefore decided that emitting a direct jump at this location was okay. Fix this by reusing our backtracking logic for precise scalar verification in order to prevent pruning on the slot key. This means verifier will track content of r3 all the way backwards and only prune if both scalars were unknown in state equivalence check and therefore poisoned in the first place in record_func_key(). The range is [x,x] in record_func_key() case since the slot always would have to be constant immediate. Correct verification after fix: 0: (b7) r0 = 14 1: (73) *(u8 *)(r1 +48) = r0 2: (71) r0 = *(u8 *)(r1 +48) 3: (15) if r0 == 0xd goto pc+4 R0_w=invP(id=0,umax_value=255,var_off=(0x0; 0xff)) R1=ctx(id=0,off=0,imm=0) R10=fp0 4: (b7) r3 = 0 5: (18) r2 = 0x0 7: (05) goto pc+3 11: (85) call bpf_tail_call#12 12: (b7) r0 = 1 13: (95) exit from 3 to 8: R0_w=invP13 R1=ctx(id=0,off=0,imm=0) R10=fp0 8: (b7) r3 = 2 9: (18) r2 = 0x0 11: (85) call bpf_tail_call#12 12: (b7) r0 = 1 13: (95) exit processed 15 insns (limit 1000000) [...] And correct corresponding JIT dump: # bpftool p d j i 11 0xffffffffc0dc34c4: [...] 21: movzbq 0x30(%rdi),%rax 26: cmp $0xd,%rax 2a: je 0x000000000000003a 2c: xor %edx,%edx 2e: movabs $0xffff9928b4c02200,%rsi 38: jmp 0x0000000000000049 3a: mov $0x2,%edx 3f: movabs $0xffff9928b4c02200,%rsi 49: cmp $0x4,%rdx 4d: jae 0x0000000000000093 4f: and $0x3,%edx 52: mov %edx,%edx 54: cmp %edx,0x24(%rsi) 57: jbe 0x0000000000000093 59: mov -0x224(%rbp),%eax 5f: cmp $0x20,%eax 62: ja 0x0000000000000093 64: add $0x1,%eax 67: mov %eax,-0x224(%rbp) 6d: mov 0x110(%rsi,%rdx,8),%rax 75: test %rax,%rax 78: je 0x0000000000000093 7a: mov 0x30(%rax),%rax 7e: add $0x19,%rax 82: callq 0x000000000000008e 87: pause 89: lfence 8c: jmp 0x0000000000000087 8e: mov %rax,(%rsp) 92: retq 93: mov $0x1,%eax [...] Also explicitly adding explicit env->allow_ptr_leaks to fixup_bpf_calls() since backtracking is enabled under former (direct jumps as well, but use different test). In case of only tracking different map pointers as in c93552c443eb ("bpf: properly enforce index mask to prevent out-of-bounds speculation"), pruning cannot make such short-cuts, neither if there are paths with scalar and non-scalar types as r3. mark_chain_precision() is only needed after we know that register_is_const(). If it was not the case, we already poison the key on first path and non-const key in later paths are not matching the scalar range in regsafe() either. Cilium NodePort testing passes fine as well now. Note, released kernels not affected. Fixes: d2e4c1e6c294 ("bpf: Constant map key tracking for prog array pokes") Signed-off-by: Daniel Borkmann <daniel@iogearbox.net> Signed-off-by: Alexei Starovoitov <ast@kernel.org> Link: https://lore.kernel.org/bpf/ac43ffdeb7386c5bd688761ed266f3722bb39823.1576789878.git.daniel@iogearbox.net
Diffstat (limited to 'kernel')
-rw-r--r--kernel/bpf/verifier.c8
1 files changed, 7 insertions, 1 deletions
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 6ef71429d997..4983940cbdca 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -4134,6 +4134,7 @@ record_func_key(struct bpf_verifier_env *env, struct bpf_call_arg_meta *meta,
struct bpf_map *map = meta->map_ptr;
struct tnum range;
u64 val;
+ int err;
if (func_id != BPF_FUNC_tail_call)
return 0;
@@ -4150,6 +4151,10 @@ record_func_key(struct bpf_verifier_env *env, struct bpf_call_arg_meta *meta,
return 0;
}
+ err = mark_chain_precision(env, BPF_REG_3);
+ if (err)
+ return err;
+
val = reg->var_off.value;
if (bpf_map_key_unseen(aux))
bpf_map_key_store(aux, val);
@@ -9272,7 +9277,8 @@ static int fixup_bpf_calls(struct bpf_verifier_env *env)
insn->code = BPF_JMP | BPF_TAIL_CALL;
aux = &env->insn_aux_data[i + delta];
- if (prog->jit_requested && !expect_blinding &&
+ if (env->allow_ptr_leaks && !expect_blinding &&
+ prog->jit_requested &&
!bpf_map_key_poisoned(aux) &&
!bpf_map_ptr_poisoned(aux) &&
!bpf_map_ptr_unpriv(aux)) {

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