[PATCH stable 5.4,5.5,5.6 1/4] bpf: fix buggy r0 retval refinement for tracing helpers

[PATCH stable 5.4,5.5,5.6 1/4] bpf: fix buggy r0 retval refinement for tracing helpers

[Daniel Borkmann]

[ no upstream commit ]

See the glory details in 100605035e15 ("bpf: Verifier, do_refine_retval_range
may clamp umin to 0 incorrectly") for why 849fa50662fb ("bpf/verifier: refine
retval R0 state for bpf_get_stack helper") is buggy. The whole series however
is not suitable for stable since it adds significant amount [0] of verifier
complexity in order to add 32bit subreg tracking. Something simpler is needed.

Unfortunately, reverting 849fa50662fb ("bpf/verifier: refine retval R0 state
for bpf_get_stack helper") or just cherry-picking 100605035e15 ("bpf: Verifier,
do_refine_retval_range may clamp umin to 0 incorrectly") is not an option since
it will break existing tracing programs badly (at least those that are using
bpf_get_stack() and bpf_probe_read_str() helpers). Not fixing it in stable is
also not an option since on 4.19 kernels an error will cause a soft-lockup due
to hitting dead-code sanitized branch since we don't hard-wire such branches
in old kernels yet. But even then for 5.x 849fa50662fb ("bpf/verifier: refine
retval R0 state for bpf_get_stack helper") would cause wrong bounds on the
verifier simluation when an error is hit.

In one of the earlier iterations of mentioned patch series for upstream there
was the concern that just using smax_value in do_refine_retval_range() would
nuke bounds by subsequent <<32 >>32 shifts before the comparison against 0 [1]
which eventually led to the 32bit subreg tracking in the first place. While I
initially went for implementing the idea [1] to pattern match the two shift
operations, it turned out to be more complex than actually needed, meaning, we
could simply treat do_refine_retval_range() similarly to how we branch off
verification for conditionals or under speculation, that is, pushing a new
reg state to the stack for later verification. This means, instead of verifying
the current path with the ret_reg in [S32MIN, msize_max_value] interval where
later bounds would get nuked, we split this into two: i) for the success case
where ret_reg can be in [0, msize_max_value], and ii) for the error case with
ret_reg known to be in interval [S32MIN, -1]. Latter will preserve the bounds
during these shift patterns and can match reg < 0 test. test_progs also succeed
with this approach.

  [0] https://lore.kernel.org/bpf/158507130343.15666.8018068546764556975.stgit@john-Precision-5820-Tower/
  [1] https://lore.kernel.org/bpf/158015334199.28573.4940395881683556537.stgit@john-XPS-13-9370/T/#m2e0ad1d5949131014748b6daa48a3495e7f0456d

Fixes: 849fa50662fb ("bpf/verifier: refine retval R0 state for bpf_get_stack helper")
Reported-by: Lorenzo Fontana <fontanalorenz@gmail.com>
Reported-by: Leonardo Di Donato <leodidonato@gmail.com>
Reported-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Tested-by: John Fastabend <john.fastabend@gmail.com>
---
 [ Lorenzo, Leonardo, I did check my local checkout of driver/bpf/probe.o,
   but please make sure to double check 4.19 with this patch here also from
   your side, so we can add a Tested-by from one of you before Greg takes
   it into stable. Thanks guys! ]

 kernel/bpf/verifier.c | 45 ++++++++++++++++++++++++++++++++-----------
 1 file changed, 34 insertions(+), 11 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 5080469094af..f6c27494c0f3 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -227,8 +227,7 @@ struct bpf_call_arg_meta {
 	bool pkt_access;
 	int regno;
 	int access_size;
-	s64 msize_smax_value;
-	u64 msize_umax_value;
+	u64 msize_max_value;
 	int ref_obj_id;
 	int func_id;
 	u32 btf_id;
@@ -3568,8 +3567,7 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 regno,
 		/* remember the mem_size which may be used later
 		 * to refine return values.
 		 */
-		meta->msize_smax_value = reg->smax_value;
-		meta->msize_umax_value = reg->umax_value;
+		meta->msize_max_value = reg->umax_value;
 
 		/* The register is SCALAR_VALUE; the access check
 		 * happens using its boundaries.
@@ -4095,21 +4093,44 @@ static int prepare_func_exit(struct bpf_verifier_env *env, int *insn_idx)
 	return 0;
 }
 
-static void do_refine_retval_range(struct bpf_reg_state *regs, int ret_type,
-				   int func_id,
-				   struct bpf_call_arg_meta *meta)
+static int do_refine_retval_range(struct bpf_verifier_env *env,
+				  struct bpf_reg_state *regs, int ret_type,
+				  int func_id, struct bpf_call_arg_meta *meta)
 {
 	struct bpf_reg_state *ret_reg = &regs[BPF_REG_0];
+	struct bpf_reg_state tmp_reg = *ret_reg;
+	bool ret;
 
 	if (ret_type != RET_INTEGER ||
 	    (func_id != BPF_FUNC_get_stack &&
 	     func_id != BPF_FUNC_probe_read_str))
-		return;
+		return 0;
+
+	/* Error case where ret is in interval [S32MIN, -1]. */
+	ret_reg->smin_value = S32_MIN;
+	ret_reg->smax_value = -1;
 
-	ret_reg->smax_value = meta->msize_smax_value;
-	ret_reg->umax_value = meta->msize_umax_value;
 	__reg_deduce_bounds(ret_reg);
 	__reg_bound_offset(ret_reg);
+	__update_reg_bounds(ret_reg);
+
+	ret = push_stack(env, env->insn_idx + 1, env->insn_idx, false);
+	if (!ret)
+		return -EFAULT;
+
+	*ret_reg = tmp_reg;
+
+	/* Success case where ret is in range [0, msize_max_value]. */
+	ret_reg->smin_value = 0;
+	ret_reg->smax_value = meta->msize_max_value;
+	ret_reg->umin_value = ret_reg->smin_value;
+	ret_reg->umax_value = ret_reg->smax_value;
+
+	__reg_deduce_bounds(ret_reg);
+	__reg_bound_offset(ret_reg);
+	__update_reg_bounds(ret_reg);
+
+	return 0;
 }
 
 static int
@@ -4377,7 +4398,9 @@ static int check_helper_call(struct bpf_verifier_env *env, int func_id, int insn
 		regs[BPF_REG_0].ref_obj_id = id;
 	}
 
-	do_refine_retval_range(regs, fn->ret_type, func_id, &meta);
+	err = do_refine_retval_range(env, regs, fn->ret_type, func_id, &meta);
+	if (err)
+		return err;
 
 	err = check_map_func_compatibility(env, meta.map_ptr, func_id);
 	if (err)
-- 
2.20.1

[PATCH stable 5.4,5.5,5.6 2/4] bpf: Test_verifier, bpf_get_stack return value add <0

[Daniel Borkmann]

From: John Fastabend <john.fastabend@gmail.com>

[ upstream commit 9ac26e9973bac5716a2a542e32f380c84db2b88c ]

With current ALU32 subreg handling and retval refine fix from last
patches we see an expected failure in test_verifier. With verbose
verifier state being printed at each step for clarity we have the
following relavent lines [I omit register states that are not
necessarily useful to see failure cause],

101/p bpf_get_stack return R0 within range FAIL
Failed to load prog 'Success'!
[..]
14: (85) call bpf_get_stack#67
 R0_w=map_value(id=0,off=0,ks=8,vs=48,imm=0)
 R3_w=inv48
15:
 R0=inv(id=0,smax_value=48,var32_off=(0x0; 0xffffffff))
15: (b7) r1 = 0
16:
 R0=inv(id=0,smax_value=48,var32_off=(0x0; 0xffffffff))
 R1_w=inv0
16: (bf) r8 = r0
17:
 R0=inv(id=0,smax_value=48,var32_off=(0x0; 0xffffffff))
 R1_w=inv0
 R8_w=inv(id=0,smax_value=48,var32_off=(0x0; 0xffffffff))
17: (67) r8 <<= 32
18:
 R0=inv(id=0,smax_value=48,var32_off=(0x0; 0xffffffff))
 R1_w=inv0
 R8_w=inv(id=0,smax_value=9223372032559808512,
               umax_value=18446744069414584320,
               var_off=(0x0; 0xffffffff00000000),
               s32_min_value=0,
               s32_max_value=0,
               u32_max_value=0,
               var32_off=(0x0; 0x0))
18: (c7) r8 s>>= 32
19
 R0=inv(id=0,smax_value=48,var32_off=(0x0; 0xffffffff))
 R1_w=inv0
 R8_w=inv(id=0,smin_value=-2147483648,
               smax_value=2147483647,
               var32_off=(0x0; 0xffffffff))
19: (cd) if r1 s< r8 goto pc+16
 R0=inv(id=0,smax_value=48,var32_off=(0x0; 0xffffffff))
 R1_w=inv0
 R8_w=inv(id=0,smin_value=-2147483648,
               smax_value=0,
               var32_off=(0x0; 0xffffffff))
20:
 R0=inv(id=0,smax_value=48,var32_off=(0x0; 0xffffffff))
 R1_w=inv0
 R8_w=inv(id=0,smin_value=-2147483648,
               smax_value=0,
 R9=inv48
20: (1f) r9 -= r8
21: (bf) r2 = r7
22:
 R2_w=map_value(id=0,off=0,ks=8,vs=48,imm=0)
22: (0f) r2 += r8
value -2147483648 makes map_value pointer be out of bounds

After call bpf_get_stack() on line 14 and some moves we have at line 16
an r8 bound with max_value 48 but an unknown min value. This is to be
expected bpf_get_stack call can only return a max of the input size but
is free to return any negative error in the 32-bit register space. The
C helper is returning an int so will use lower 32-bits.

Lines 17 and 18 clear the top 32 bits with a left/right shift but use
ARSH so we still have worst case min bound before line 19 of -2147483648.
At this point the signed check 'r1 s< r8' meant to protect the addition
on line 22 where dst reg is a map_value pointer may very well return
true with a large negative number. Then the final line 22 will detect
this as an invalid operation and fail the program. What we want to do
is proceed only if r8 is positive non-error. So change 'r1 s< r8' to
'r1 s> r8' so that we jump if r8 is negative.

Next we will throw an error because we access past the end of the map
value. The map value size is 48 and sizeof(struct test_val) is 48 so
we walk off the end of the map value on the second call to
get bpf_get_stack(). Fix this by changing sizeof(struct test_val) to
24 by using 'sizeof(struct test_val) / 2'. After this everything passes
as expected.

Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/158560426019.10843.3285429543232025187.stgit@john-Precision-5820-Tower
---
 tools/testing/selftests/bpf/verifier/bpf_get_stack.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/tools/testing/selftests/bpf/verifier/bpf_get_stack.c b/tools/testing/selftests/bpf/verifier/bpf_get_stack.c
index f24d50f09dbe..69b048cf46d9 100644
--- a/tools/testing/selftests/bpf/verifier/bpf_get_stack.c
+++ b/tools/testing/selftests/bpf/verifier/bpf_get_stack.c
@@ -9,17 +9,17 @@
 	BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem),
 	BPF_JMP_IMM(BPF_JEQ, BPF_REG_0, 0, 28),
 	BPF_MOV64_REG(BPF_REG_7, BPF_REG_0),
-	BPF_MOV64_IMM(BPF_REG_9, sizeof(struct test_val)),
+	BPF_MOV64_IMM(BPF_REG_9, sizeof(struct test_val)/2),
 	BPF_MOV64_REG(BPF_REG_1, BPF_REG_6),
 	BPF_MOV64_REG(BPF_REG_2, BPF_REG_7),
-	BPF_MOV64_IMM(BPF_REG_3, sizeof(struct test_val)),
+	BPF_MOV64_IMM(BPF_REG_3, sizeof(struct test_val)/2),
 	BPF_MOV64_IMM(BPF_REG_4, 256),
 	BPF_EMIT_CALL(BPF_FUNC_get_stack),
 	BPF_MOV64_IMM(BPF_REG_1, 0),
 	BPF_MOV64_REG(BPF_REG_8, BPF_REG_0),
 	BPF_ALU64_IMM(BPF_LSH, BPF_REG_8, 32),
 	BPF_ALU64_IMM(BPF_ARSH, BPF_REG_8, 32),
-	BPF_JMP_REG(BPF_JSLT, BPF_REG_1, BPF_REG_8, 16),
+	BPF_JMP_REG(BPF_JSGT, BPF_REG_1, BPF_REG_8, 16),
 	BPF_ALU64_REG(BPF_SUB, BPF_REG_9, BPF_REG_8),
 	BPF_MOV64_REG(BPF_REG_2, BPF_REG_7),
 	BPF_ALU64_REG(BPF_ADD, BPF_REG_2, BPF_REG_8),
@@ -29,7 +29,7 @@
 	BPF_MOV64_REG(BPF_REG_3, BPF_REG_2),
 	BPF_ALU64_REG(BPF_ADD, BPF_REG_3, BPF_REG_1),
 	BPF_MOV64_REG(BPF_REG_1, BPF_REG_7),
-	BPF_MOV64_IMM(BPF_REG_5, sizeof(struct test_val)),
+	BPF_MOV64_IMM(BPF_REG_5, sizeof(struct test_val)/2),
 	BPF_ALU64_REG(BPF_ADD, BPF_REG_1, BPF_REG_5),
 	BPF_JMP_REG(BPF_JGE, BPF_REG_3, BPF_REG_1, 4),
 	BPF_MOV64_REG(BPF_REG_1, BPF_REG_6),
-- 
2.20.1

[PATCH stable 5.4,5.5,5.6 3/4] bpf: Test_progs, add test to catch retval refine error handling

[Daniel Borkmann]

From: John Fastabend <john.fastabend@gmail.com>

[ upstream commit d2db08c7a14e0b5eed6132baf258b80622e041a9 ]

Before this series the verifier would clamp return bounds of
bpf_get_stack() to [0, X] and this led the verifier to believe
that a JMP_JSLT 0 would be false and so would prune that path.

The result is anything hidden behind that JSLT would be unverified.
Add a test to catch this case by hiding an goto pc-1 behind the
check which will cause an infinite loop if not rejected.

Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/158560423908.10843.11783152347709008373.stgit@john-Precision-5820-Tower
---
 .../bpf/prog_tests/get_stack_raw_tp.c         |  5 ++++
 .../bpf/progs/test_get_stack_rawtp_err.c      | 26 +++++++++++++++++++
 2 files changed, 31 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/progs/test_get_stack_rawtp_err.c

diff --git a/tools/testing/selftests/bpf/prog_tests/get_stack_raw_tp.c b/tools/testing/selftests/bpf/prog_tests/get_stack_raw_tp.c
index eba9a970703b..925722217edf 100644
--- a/tools/testing/selftests/bpf/prog_tests/get_stack_raw_tp.c
+++ b/tools/testing/selftests/bpf/prog_tests/get_stack_raw_tp.c
@@ -82,6 +82,7 @@ static void get_stack_print_output(void *ctx, int cpu, void *data, __u32 size)
 void test_get_stack_raw_tp(void)
 {
 	const char *file = "./test_get_stack_rawtp.o";
+	const char *file_err = "./test_get_stack_rawtp_err.o";
 	const char *prog_name = "raw_tracepoint/sys_enter";
 	int i, err, prog_fd, exp_cnt = MAX_CNT_RAWTP;
 	struct perf_buffer_opts pb_opts = {};
@@ -93,6 +94,10 @@ void test_get_stack_raw_tp(void)
 	struct bpf_map *map;
 	cpu_set_t cpu_set;
 
+	err = bpf_prog_load(file_err, BPF_PROG_TYPE_RAW_TRACEPOINT, &obj, &prog_fd);
+	if (CHECK(err >= 0, "prog_load raw tp", "err %d errno %d\n", err, errno))
+		return;
+
 	err = bpf_prog_load(file, BPF_PROG_TYPE_RAW_TRACEPOINT, &obj, &prog_fd);
 	if (CHECK(err, "prog_load raw tp", "err %d errno %d\n", err, errno))
 		return;
diff --git a/tools/testing/selftests/bpf/progs/test_get_stack_rawtp_err.c b/tools/testing/selftests/bpf/progs/test_get_stack_rawtp_err.c
new file mode 100644
index 000000000000..8941a41c2a55
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/test_get_stack_rawtp_err.c
@@ -0,0 +1,26 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+
+#define MAX_STACK_RAWTP 10
+
+SEC("raw_tracepoint/sys_enter")
+int bpf_prog2(void *ctx)
+{
+	__u64 stack[MAX_STACK_RAWTP];
+	int error;
+
+	/* set all the flags which should return -EINVAL */
+	error = bpf_get_stack(ctx, stack, 0, -1);
+	if (error < 0)
+		goto loop;
+
+	return error;
+loop:
+	while (1) {
+		error++;
+	}
+}
+
+char _license[] SEC("license") = "GPL";
-- 
2.20.1

[PATCH stable 5.4,5.5,5.6 4/4] bpf, test_verifier: switch bpf_get_stack's 0 s> r8 test

[Daniel Borkmann]

[ no upstream commit ]

Switch the comparison, so that is_branch_taken() will recognize that below
branch is never taken:

  [...]
  17: [...] R1_w=inv0 [...] R8_w=inv(id=0,smin_value=-2147483648,smax_value=-1,umin_value=18446744071562067968,var_off=(0xffffffff80000000; 0x7fffffff)) [...]
  17: (67) r8 <<= 32
  18: [...] R8_w=inv(id=0,smax_value=-4294967296,umin_value=9223372036854775808,umax_value=18446744069414584320,var_off=(0x8000000000000000; 0x7fffffff00000000)) [...]
  18: (c7) r8 s>>= 32
  19: [...] R8_w=inv(id=0,smin_value=-2147483648,smax_value=-1,umin_value=18446744071562067968,var_off=(0xffffffff80000000; 0x7fffffff)) [...]
  19: (6d) if r1 s> r8 goto pc+16
  [...] R1_w=inv0 [...] R8_w=inv(id=0,smin_value=-2147483648,smax_value=-1,umin_value=18446744071562067968,var_off=(0xffffffff80000000; 0x7fffffff)) [...]
  [...]

Currently we check for is_branch_taken() only if either K is source, or source
is a scalar value that is const. For upstream it would be good to extend this
properly to check whether dst is const and src not.

For the sake of the test_verifier, it is probably not needed here:

  # ./test_verifier 101
  #101/p bpf_get_stack return R0 within range OK
  Summary: 1 PASSED, 0 SKIPPED, 0 FAILED

I haven't seen this issue in test_progs* though, they are passing fine:

  # ./test_progs-no_alu32 -t get_stack
  Switching to flavor 'no_alu32' subdirectory...
  #20 get_stack_raw_tp:OK
  Summary: 1/0 PASSED, 0 SKIPPED, 0 FAILED

  # ./test_progs -t get_stack
  #20 get_stack_raw_tp:OK
  Summary: 1/0 PASSED, 0 SKIPPED, 0 FAILED

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: John Fastabend <john.fastabend@gmail.com>
---
 tools/testing/selftests/bpf/verifier/bpf_get_stack.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/testing/selftests/bpf/verifier/bpf_get_stack.c b/tools/testing/selftests/bpf/verifier/bpf_get_stack.c
index 69b048cf46d9..371926771db5 100644
--- a/tools/testing/selftests/bpf/verifier/bpf_get_stack.c
+++ b/tools/testing/selftests/bpf/verifier/bpf_get_stack.c
@@ -19,7 +19,7 @@
 	BPF_MOV64_REG(BPF_REG_8, BPF_REG_0),
 	BPF_ALU64_IMM(BPF_LSH, BPF_REG_8, 32),
 	BPF_ALU64_IMM(BPF_ARSH, BPF_REG_8, 32),
-	BPF_JMP_REG(BPF_JSGT, BPF_REG_1, BPF_REG_8, 16),
+	BPF_JMP_REG(BPF_JSLT, BPF_REG_8, BPF_REG_1, 16),
 	BPF_ALU64_REG(BPF_SUB, BPF_REG_9, BPF_REG_8),
 	BPF_MOV64_REG(BPF_REG_2, BPF_REG_7),
 	BPF_ALU64_REG(BPF_ADD, BPF_REG_2, BPF_REG_8),
-- 
2.20.1