[PATCH bpf-next v3 0/6] Improvements for tracking scalars in the BPF verifier

[PATCH bpf-next v3 0/6] Improvements for tracking scalars in the BPF verifier

[Maxim Mikityanskiy]

From: Maxim Mikityanskiy <maxim@isovalent.com>

The goal of this series is to extend the verifier's capabilities of
tracking scalars when they are spilled to stack, especially when the
spill or fill is narrowing. It also contains a fix by Eduard for
infinite loop detection and a state pruning optimization by Eduard that
compensates for a verification complexity regression introduced by
tracking unbounded scalars. These improvements reduce the surface of
false rejections that I saw while working on Cilium codebase.

Patches 1-9 of the original series were previously applied in v2.

Patches 1-2 (Maxim): Support the case when boundary checks are first
performed after the register was spilled to the stack.

Patches 3-4 (Maxim): Support narrowing fills.

Patches 5-6 (Eduard): Optimization for state pruning in stacksafe() to
mitigate the verification complexity regression.

veristat -e file,prog,states -f '!states_diff<50' -f '!states_pct<10' -f '!states_a<10' -f '!states_b<10' -C ...

 * Without patch 5:

File                  Program   States (A)  States (B)  States    (DIFF)
--------------------  --------  ----------  ----------  ----------------
pyperf100.bpf.o       on_event        4878        6528   +1650 (+33.83%)
pyperf180.bpf.o       on_event        6936       11032   +4096 (+59.05%)
pyperf600.bpf.o       on_event       22271       39455  +17184 (+77.16%)
pyperf600_iter.bpf.o  on_event         400         490     +90 (+22.50%)
strobemeta.bpf.o      on_event        4895       14028  +9133 (+186.58%)

 * With patch 5:

File                     Program        States (A)  States (B)  States   (DIFF)
-----------------------  -------------  ----------  ----------  ---------------
bpf_xdp.o                tail_lb_ipv4         2770        2224   -546 (-19.71%)
pyperf100.bpf.o          on_event             4878        5848   +970 (+19.89%)
pyperf180.bpf.o          on_event             6936        8868  +1932 (+27.85%)
pyperf600.bpf.o          on_event            22271       29656  +7385 (+33.16%)
pyperf600_iter.bpf.o     on_event              400         450    +50 (+12.50%)
xdp_synproxy_kern.bpf.o  syncookie_tc          280         226    -54 (-19.29%)
xdp_synproxy_kern.bpf.o  syncookie_xdp         302         228    -74 (-24.50%)

v2 changes:

Fixed comments in patch 1, moved endianness checks to header files in
patch 12 where possible, added Eduard's ACKs.

v3 changes:

Maxim: Removed __is_scalar_unbounded altogether, addressed Andrii's
comments.

Eduard: Patch #5 (#14 in v2) changed significantly:
- Logical changes:
  - Handling of STACK_{MISC,ZERO} mix turned out to be incorrect:
    a mix of MISC and ZERO in old state is not equivalent to e.g.
    just MISC is current state, because verifier could have deduced
    zero scalars from ZERO slots in old state for some loads.
  - There is no reason to limit the change only to cases when
    old or current stack is a spill of unbounded scalar,
    it is valid to compare any 64-bit scalar spill with fake
    register impersonating MISC.
  - STACK_ZERO vs spilled zero case was dropped,
    after recent changes for zero handling by Andrii and Yonghong
    it is hard (impossible?) to conjure all ZERO slots for an spi.
    => the case does not make any difference in veristat results.
- Use global static variable for unbound_reg (Andrii)
- Code shuffling to remove duplication in stacksafe() (Andrii)

Eduard Zingerman (2):
  bpf: handle scalar spill vs all MISC in stacksafe()
  selftests/bpf: states pruning checks for scalar vs STACK_MISC

Maxim Mikityanskiy (4):
  bpf: Track spilled unbounded scalars
  selftests/bpf: Test tracking spilled unbounded scalars
  bpf: Preserve boundaries and track scalars on narrowing fill
  selftests/bpf: Add test cases for narrowing fill

 include/linux/bpf_verifier.h                  |   9 +
 kernel/bpf/verifier.c                         | 103 ++++--
 .../selftests/bpf/progs/verifier_spill_fill.c | 324 +++++++++++++++++-
 3 files changed, 404 insertions(+), 32 deletions(-)

-- 
2.43.0

[PATCH bpf-next v3 1/6] bpf: Track spilled unbounded scalars

[Maxim Mikityanskiy]

From: Maxim Mikityanskiy <maxim@isovalent.com>

Support the pattern where an unbounded scalar is spilled to the stack,
then boundary checks are performed on the src register, after which the
stack frame slot is refilled into a register.

Before this commit, the verifier didn't treat the src register and the
stack slot as related if the src register was an unbounded scalar. The
register state wasn't copied, the id wasn't preserved, and the stack
slot was marked as STACK_MISC. Subsequent boundary checks on the src
register wouldn't result in updating the boundaries of the spilled
variable on the stack.

After this commit, the verifier will preserve the bond between src and
dst even if src is unbounded, which permits to do boundary checks on src
and refill dst later, still remembering its boundaries. Such a pattern
is sometimes generated by clang when compiling complex long functions.

One test is adjusted to reflect that now unbounded scalars are tracked.

Signed-off-by: Maxim Mikityanskiy <maxim@isovalent.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
---
 kernel/bpf/verifier.c                            | 16 +---------------
 .../selftests/bpf/progs/verifier_spill_fill.c    |  6 +++---
 2 files changed, 4 insertions(+), 18 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index c5d68a9d8acc..f4b9fd0006d1 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -4380,20 +4380,6 @@ static u64 reg_const_value(struct bpf_reg_state *reg, bool subreg32)
 	return subreg32 ? tnum_subreg(reg->var_off).value : reg->var_off.value;
 }
 
-static bool __is_scalar_unbounded(struct bpf_reg_state *reg)
-{
-	return tnum_is_unknown(reg->var_off) &&
-	       reg->smin_value == S64_MIN && reg->smax_value == S64_MAX &&
-	       reg->umin_value == 0 && reg->umax_value == U64_MAX &&
-	       reg->s32_min_value == S32_MIN && reg->s32_max_value == S32_MAX &&
-	       reg->u32_min_value == 0 && reg->u32_max_value == U32_MAX;
-}
-
-static bool register_is_bounded(struct bpf_reg_state *reg)
-{
-	return reg->type == SCALAR_VALUE && !__is_scalar_unbounded(reg);
-}
-
 static bool __is_pointer_value(bool allow_ptr_leaks,
 			       const struct bpf_reg_state *reg)
 {
@@ -4504,7 +4490,7 @@ static int check_stack_write_fixed_off(struct bpf_verifier_env *env,
 		return err;
 
 	mark_stack_slot_scratched(env, spi);
-	if (reg && !(off % BPF_REG_SIZE) && register_is_bounded(reg) && env->bpf_capable) {
+	if (reg && !(off % BPF_REG_SIZE) && reg->type == SCALAR_VALUE && env->bpf_capable) {
 		bool reg_value_fits;
 
 		reg_value_fits = get_reg_width(reg) <= BITS_PER_BYTE * size;
diff --git a/tools/testing/selftests/bpf/progs/verifier_spill_fill.c b/tools/testing/selftests/bpf/progs/verifier_spill_fill.c
index 7013a9694163..317806451762 100644
--- a/tools/testing/selftests/bpf/progs/verifier_spill_fill.c
+++ b/tools/testing/selftests/bpf/progs/verifier_spill_fill.c
@@ -452,9 +452,9 @@ l0_%=:	r1 >>= 16;					\
 SEC("raw_tp")
 __log_level(2)
 __success
-__msg("fp-8=0m??mmmm")
-__msg("fp-16=00mm??mm")
-__msg("fp-24=00mm???m")
+__msg("fp-8=0m??scalar()")
+__msg("fp-16=00mm??scalar()")
+__msg("fp-24=00mm???scalar()")
 __naked void spill_subregs_preserve_stack_zero(void)
 {
 	asm volatile (
-- 
2.43.0

[PATCH bpf-next v3 2/6] selftests/bpf: Test tracking spilled unbounded scalars

[Maxim Mikityanskiy]

From: Maxim Mikityanskiy <maxim@isovalent.com>

The previous commit added tracking for unbounded scalars on spill. Add
the test case to check the new functionality.

Signed-off-by: Maxim Mikityanskiy <maxim@isovalent.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
---
 .../selftests/bpf/progs/verifier_spill_fill.c | 27 +++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/tools/testing/selftests/bpf/progs/verifier_spill_fill.c b/tools/testing/selftests/bpf/progs/verifier_spill_fill.c
index 317806451762..f9803005e1c0 100644
--- a/tools/testing/selftests/bpf/progs/verifier_spill_fill.c
+++ b/tools/testing/selftests/bpf/progs/verifier_spill_fill.c
@@ -940,4 +940,31 @@ l0_%=:	r0 = 0;						\
 	: __clobber_all);
 }
 
+SEC("xdp")
+__description("spill unbounded reg, then range check src")
+__success __retval(0)
+__naked void spill_unbounded(void)
+{
+	asm volatile ("					\
+	/* Produce an unbounded scalar. */		\
+	call %[bpf_get_prandom_u32];			\
+	/* Spill r0 to stack. */			\
+	*(u64*)(r10 - 8) = r0;				\
+	/* Boundary check on r0. */			\
+	if r0 > 16 goto l0_%=;				\
+	/* Fill r0 from stack. */			\
+	r0 = *(u64*)(r10 - 8);				\
+	/* Boundary check on r0 with predetermined result. */\
+	if r0 <= 16 goto l0_%=;				\
+	/* Dead branch: the verifier should prune it. Do an invalid memory\
+	 * access if the verifier follows it.		\
+	 */						\
+	r0 = *(u64*)(r9 + 0);				\
+l0_%=:	r0 = 0;						\
+	exit;						\
+"	:
+	: __imm(bpf_get_prandom_u32)
+	: __clobber_all);
+}
+
 char _license[] SEC("license") = "GPL";
-- 
2.43.0

[PATCH bpf-next v3 3/6] bpf: Preserve boundaries and track scalars on narrowing fill

[Maxim Mikityanskiy]

From: Maxim Mikityanskiy <maxim@isovalent.com>

When the width of a fill is smaller than the width of the preceding
spill, the information about scalar boundaries can still be preserved,
as long as it's coerced to the right width (done by coerce_reg_to_size).
Even further, if the actual value fits into the fill width, the ID can
be preserved as well for further tracking of equal scalars.

Implement the above improvements, which makes narrowing fills behave the
same as narrowing spills and MOVs between registers.

Two tests are adjusted to accommodate for endianness differences and to
take into account that it's now allowed to do a narrowing fill from the
least significant bits.

reg_bounds_sync is added to coerce_reg_to_size to correctly adjust
umin/umax boundaries after the var_off truncation, for example, a 64-bit
value 0xXXXXXXXX00000000, when read as a 32-bit, gets umin = 0, umax =
0xFFFFFFFF, var_off = (0x0; 0xffffffff00000000), which needs to be
synced down to umax = 0, otherwise reg_bounds_sanity_check doesn't pass.

Signed-off-by: Maxim Mikityanskiy <maxim@isovalent.com>
---
 include/linux/bpf_verifier.h                  |  9 +++++++
 kernel/bpf/verifier.c                         | 15 ++++++++---
 .../selftests/bpf/progs/verifier_spill_fill.c | 26 ++++++++++++++-----
 3 files changed, 39 insertions(+), 11 deletions(-)

diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
index 7f5816482a10..897be9dd6f12 100644
--- a/include/linux/bpf_verifier.h
+++ b/include/linux/bpf_verifier.h
@@ -918,6 +918,15 @@ static inline void mark_verifier_state_scratched(struct bpf_verifier_env *env)
 	env->scratched_stack_slots = ~0ULL;
 }
 
+static inline bool bpf_stack_narrow_access_ok(int off, int fill_size, int spill_size)
+{
+#ifdef __BIG_ENDIAN
+	off -= spill_size - fill_size;
+#endif
+
+	return !(off % BPF_REG_SIZE);
+}
+
 const char *reg_type_str(struct bpf_verifier_env *env, enum bpf_reg_type type);
 const char *dynptr_type_str(enum bpf_dynptr_type type);
 const char *iter_type_str(const struct btf *btf, u32 btf_id);
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index f4b9fd0006d1..81b08a0e9e2c 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -4778,7 +4778,8 @@ static int check_stack_read_fixed_off(struct bpf_verifier_env *env,
 			if (dst_regno < 0)
 				return 0;
 
-			if (!(off % BPF_REG_SIZE) && size == spill_size) {
+			if (size <= spill_size &&
+			    bpf_stack_narrow_access_ok(off, size, spill_size)) {
 				/* The earlier check_reg_arg() has decided the
 				 * subreg_def for this insn.  Save it first.
 				 */
@@ -4786,6 +4787,12 @@ static int check_stack_read_fixed_off(struct bpf_verifier_env *env,
 
 				copy_register_state(&state->regs[dst_regno], reg);
 				state->regs[dst_regno].subreg_def = subreg_def;
+
+				/* Break the relation on a narrowing fill.
+				 * coerce_reg_to_size will adjust the boundaries.
+				 */
+				if (get_reg_width(reg) > size * BITS_PER_BYTE)
+					state->regs[dst_regno].id = 0;
 			} else {
 				int spill_cnt = 0, zero_cnt = 0;
 
@@ -6061,10 +6068,10 @@ static void coerce_reg_to_size(struct bpf_reg_state *reg, int size)
 	 * values are also truncated so we push 64-bit bounds into
 	 * 32-bit bounds. Above were truncated < 32-bits already.
 	 */
-	if (size < 4) {
+	if (size < 4)
 		__mark_reg32_unbounded(reg);
-		reg_bounds_sync(reg);
-	}
+
+	reg_bounds_sync(reg);
 }
 
 static void set_sext64_default_val(struct bpf_reg_state *reg, int size)
diff --git a/tools/testing/selftests/bpf/progs/verifier_spill_fill.c b/tools/testing/selftests/bpf/progs/verifier_spill_fill.c
index f9803005e1c0..3e5d063ea7e8 100644
--- a/tools/testing/selftests/bpf/progs/verifier_spill_fill.c
+++ b/tools/testing/selftests/bpf/progs/verifier_spill_fill.c
@@ -217,7 +217,7 @@ __naked void uninit_u32_from_the_stack(void)
 
 SEC("tc")
 __description("Spill a u32 const scalar.  Refill as u16.  Offset to skb->data")
-__failure __msg("invalid access to packet")
+__success __retval(0)
 __naked void u16_offset_to_skb_data(void)
 {
 	asm volatile ("					\
@@ -225,13 +225,19 @@ __naked void u16_offset_to_skb_data(void)
 	r3 = *(u32*)(r1 + %[__sk_buff_data_end]);	\
 	w4 = 20;					\
 	*(u32*)(r10 - 8) = r4;				\
-	r4 = *(u16*)(r10 - 8);				\
+	"
+#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
+	"r4 = *(u16*)(r10 - 8);"
+#else
+	"r4 = *(u16*)(r10 - 6);"
+#endif
+	"						\
 	r0 = r2;					\
-	/* r0 += r4 R0=pkt R2=pkt R3=pkt_end R4=umax=65535 */\
+	/* r0 += r4 R0=pkt R2=pkt R3=pkt_end R4=20 */\
 	r0 += r4;					\
-	/* if (r0 > r3) R0=pkt,umax=65535 R2=pkt R3=pkt_end R4=umax=65535 */\
+	/* if (r0 > r3) R0=pkt,off=20 R2=pkt R3=pkt_end R4=20 */\
 	if r0 > r3 goto l0_%=;				\
-	/* r0 = *(u32 *)r2 R0=pkt,umax=65535 R2=pkt R3=pkt_end R4=20 */\
+	/* r0 = *(u32 *)r2 R0=pkt,off=20 R2=pkt R3=pkt_end R4=20 */\
 	r0 = *(u32*)(r2 + 0);				\
 l0_%=:	r0 = 0;						\
 	exit;						\
@@ -268,7 +274,7 @@ l0_%=:	r0 = 0;						\
 }
 
 SEC("tc")
-__description("Spill a u32 const scalar.  Refill as u16 from fp-6.  Offset to skb->data")
+__description("Spill a u32 const scalar.  Refill as u16 from MSB.  Offset to skb->data")
 __failure __msg("invalid access to packet")
 __naked void _6_offset_to_skb_data(void)
 {
@@ -277,7 +283,13 @@ __naked void _6_offset_to_skb_data(void)
 	r3 = *(u32*)(r1 + %[__sk_buff_data_end]);	\
 	w4 = 20;					\
 	*(u32*)(r10 - 8) = r4;				\
-	r4 = *(u16*)(r10 - 6);				\
+	"
+#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
+	"r4 = *(u16*)(r10 - 6);"
+#else
+	"r4 = *(u16*)(r10 - 8);"
+#endif
+	"						\
 	r0 = r2;					\
 	/* r0 += r4 R0=pkt R2=pkt R3=pkt_end R4=umax=65535 */\
 	r0 += r4;					\
-- 
2.43.0

[PATCH bpf-next v3 4/6] selftests/bpf: Add test cases for narrowing fill

[Maxim Mikityanskiy]

From: Maxim Mikityanskiy <maxim@isovalent.com>

The previous commit allowed to preserve boundaries and track IDs of
scalars on narrowing fills. Add test cases for that pattern.

Signed-off-by: Maxim Mikityanskiy <maxim@isovalent.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
---
 .../selftests/bpf/progs/verifier_spill_fill.c | 111 ++++++++++++++++++
 1 file changed, 111 insertions(+)

diff --git a/tools/testing/selftests/bpf/progs/verifier_spill_fill.c b/tools/testing/selftests/bpf/progs/verifier_spill_fill.c
index 3e5d063ea7e8..7f3b1319bd99 100644
--- a/tools/testing/selftests/bpf/progs/verifier_spill_fill.c
+++ b/tools/testing/selftests/bpf/progs/verifier_spill_fill.c
@@ -979,4 +979,115 @@ l0_%=:	r0 = 0;						\
 	: __clobber_all);
 }
 
+SEC("xdp")
+__description("32-bit fill after 64-bit spill")
+__success __retval(0)
+__naked void fill_32bit_after_spill_64bit(void)
+{
+	asm volatile("					\
+	/* Randomize the upper 32 bits. */		\
+	call %[bpf_get_prandom_u32];			\
+	r0 <<= 32;					\
+	/* 64-bit spill r0 to stack. */			\
+	*(u64*)(r10 - 8) = r0;				\
+	/* 32-bit fill r0 from stack. */		\
+	"
+#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
+	"r0 = *(u32*)(r10 - 8);"
+#else
+	"r0 = *(u32*)(r10 - 4);"
+#endif
+	"						\
+	/* Boundary check on r0 with predetermined result. */\
+	if r0 == 0 goto l0_%=;				\
+	/* Dead branch: the verifier should prune it. Do an invalid memory\
+	 * access if the verifier follows it.		\
+	 */						\
+	r0 = *(u64*)(r9 + 0);				\
+l0_%=:	exit;						\
+"	:
+	: __imm(bpf_get_prandom_u32)
+	: __clobber_all);
+}
+
+SEC("xdp")
+__description("32-bit fill after 64-bit spill of 32-bit value should preserve ID")
+__success __retval(0)
+__naked void fill_32bit_after_spill_64bit_preserve_id(void)
+{
+	asm volatile ("					\
+	/* Randomize the lower 32 bits. */		\
+	call %[bpf_get_prandom_u32];			\
+	w0 &= 0xffffffff;				\
+	/* 64-bit spill r0 to stack - should assign an ID. */\
+	*(u64*)(r10 - 8) = r0;				\
+	/* 32-bit fill r1 from stack - should preserve the ID. */\
+	"
+#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
+	"r1 = *(u32*)(r10 - 8);"
+#else
+	"r1 = *(u32*)(r10 - 4);"
+#endif
+	"						\
+	/* Compare r1 with another register to trigger find_equal_scalars. */\
+	r2 = 0;						\
+	if r1 != r2 goto l0_%=;				\
+	/* The result of this comparison is predefined. */\
+	if r0 == r2 goto l0_%=;				\
+	/* Dead branch: the verifier should prune it. Do an invalid memory\
+	 * access if the verifier follows it.		\
+	 */						\
+	r0 = *(u64*)(r9 + 0);				\
+	exit;						\
+l0_%=:	r0 = 0;						\
+	exit;						\
+"	:
+	: __imm(bpf_get_prandom_u32)
+	: __clobber_all);
+}
+
+SEC("xdp")
+__description("32-bit fill after 64-bit spill should clear ID")
+__failure __msg("math between ctx pointer and 4294967295 is not allowed")
+__naked void fill_32bit_after_spill_64bit_clear_id(void)
+{
+	asm volatile ("					\
+	r6 = r1;					\
+	/* Roll one bit to force the verifier to track both branches. */\
+	call %[bpf_get_prandom_u32];			\
+	r0 &= 0x8;					\
+	/* Put a large number into r1. */		\
+	r1 = 0xffffffff;				\
+	r1 <<= 32;					\
+	r1 += r0;					\
+	/* 64-bit spill r1 to stack - should assign an ID. */\
+	*(u64*)(r10 - 8) = r1;				\
+	/* 32-bit fill r2 from stack - should clear the ID. */\
+	"
+#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
+	"r2 = *(u32*)(r10 - 8);"
+#else
+	"r2 = *(u32*)(r10 - 4);"
+#endif
+	"						\
+	/* Compare r2 with another register to trigger find_equal_scalars.\
+	 * Having one random bit is important here, otherwise the verifier cuts\
+	 * the corners. If the ID was mistakenly preserved on fill, this would\
+	 * cause the verifier to think that r1 is also equal to zero in one of\
+	 * the branches, and equal to eight on the other branch.\
+	 */						\
+	r3 = 0;						\
+	if r2 != r3 goto l0_%=;				\
+l0_%=:	r1 >>= 32;					\
+	/* The verifier shouldn't propagate r2's range to r1, so it should\
+	 * still remember r1 = 0xffffffff and reject the below.\
+	 */						\
+	r6 += r1;					\
+	r0 = *(u32*)(r6 + 0);				\
+	exit;						\
+"	:
+	: __imm(bpf_get_prandom_u32)
+	: __clobber_all);
+}
+
 char _license[] SEC("license") = "GPL";
-- 
2.43.0

[PATCH bpf-next v3 5/6] bpf: handle scalar spill vs all MISC in stacksafe()

[Maxim Mikityanskiy]

From: Eduard Zingerman <eddyz87@gmail.com>

When check_stack_read_fixed_off() reads value from an spi
all stack slots of which are set to STACK_{MISC,INVALID},
the destination register is set to unbound SCALAR_VALUE.

Exploit this fact by allowing stacksafe() to use a fake
unbound scalar register to compare 'mmmm mmmm' stack value
in old state vs spilled 64-bit scalar in current state
and vice versa.

Veristat results after this patch show some gains:

./veristat -C -e file,prog,states -f 'states_pct>10'  not-opt after
File                     Program                States   (DIFF)
-----------------------  ---------------------  ---------------
bpf_overlay.o            tail_rev_nodeport_lb4    -45 (-15.85%)
bpf_xdp.o                tail_lb_ipv4            -541 (-19.57%)
pyperf100.bpf.o          on_event                -680 (-10.42%)
pyperf180.bpf.o          on_event               -2164 (-19.62%)
pyperf600.bpf.o          on_event               -9799 (-24.84%)
strobemeta.bpf.o         on_event               -9157 (-65.28%)
xdp_synproxy_kern.bpf.o  syncookie_tc             -54 (-19.29%)
xdp_synproxy_kern.bpf.o  syncookie_xdp            -74 (-24.50%)

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
---
 kernel/bpf/verifier.c | 72 +++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 69 insertions(+), 3 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 81b08a0e9e2c..6a174c4849ba 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -1155,6 +1155,12 @@ static bool is_spilled_scalar_reg(const struct bpf_stack_state *stack)
 	       stack->spilled_ptr.type == SCALAR_VALUE;
 }
 
+static bool is_spilled_scalar_reg64(const struct bpf_stack_state *stack)
+{
+	return stack->slot_type[0] == STACK_SPILL &&
+	       stack->spilled_ptr.type == SCALAR_VALUE;
+}
+
 /* Mark stack slot as STACK_MISC, unless it is already STACK_INVALID, in which
  * case they are equivalent, or it's STACK_ZERO, in which case we preserve
  * more precise STACK_ZERO.
@@ -2264,8 +2270,7 @@ static void __reg_assign_32_into_64(struct bpf_reg_state *reg)
 }
 
 /* Mark a register as having a completely unknown (scalar) value. */
-static void __mark_reg_unknown(const struct bpf_verifier_env *env,
-			       struct bpf_reg_state *reg)
+static void __mark_reg_unknown_imprecise(struct bpf_reg_state *reg)
 {
 	/*
 	 * Clear type, off, and union(map_ptr, range) and
@@ -2277,10 +2282,20 @@ static void __mark_reg_unknown(const struct bpf_verifier_env *env,
 	reg->ref_obj_id = 0;
 	reg->var_off = tnum_unknown;
 	reg->frameno = 0;
-	reg->precise = !env->bpf_capable;
+	reg->precise = false;
 	__mark_reg_unbounded(reg);
 }
 
+/* Mark a register as having a completely unknown (scalar) value,
+ * initialize .precise as true when not bpf capable.
+ */
+static void __mark_reg_unknown(const struct bpf_verifier_env *env,
+			       struct bpf_reg_state *reg)
+{
+	__mark_reg_unknown_imprecise(reg);
+	reg->precise = !env->bpf_capable;
+}
+
 static void mark_reg_unknown(struct bpf_verifier_env *env,
 			     struct bpf_reg_state *regs, u32 regno)
 {
@@ -16474,6 +16489,43 @@ static bool regsafe(struct bpf_verifier_env *env, struct bpf_reg_state *rold,
 	}
 }
 
+static struct bpf_reg_state unbound_reg;
+
+static __init int unbound_reg_init(void)
+{
+	__mark_reg_unknown_imprecise(&unbound_reg);
+	unbound_reg.live |= REG_LIVE_READ;
+	return 0;
+}
+late_initcall(unbound_reg_init);
+
+static bool is_stack_all_misc(struct bpf_verifier_env *env,
+			      struct bpf_stack_state *stack)
+{
+	u32 i;
+
+	for (i = 0; i < ARRAY_SIZE(stack->slot_type); ++i) {
+		if ((stack->slot_type[i] == STACK_MISC) ||
+		    (stack->slot_type[i] == STACK_INVALID && env->allow_uninit_stack))
+			continue;
+		return false;
+	}
+
+	return true;
+}
+
+static struct bpf_reg_state *scalar_reg_for_stack(struct bpf_verifier_env *env,
+						  struct bpf_stack_state *stack)
+{
+	if (is_spilled_scalar_reg64(stack))
+		return &stack->spilled_ptr;
+
+	if (is_stack_all_misc(env, stack))
+		return &unbound_reg;
+
+	return NULL;
+}
+
 static bool stacksafe(struct bpf_verifier_env *env, struct bpf_func_state *old,
 		      struct bpf_func_state *cur, struct bpf_idmap *idmap, bool exact)
 {
@@ -16512,6 +16564,20 @@ static bool stacksafe(struct bpf_verifier_env *env, struct bpf_func_state *old,
 		if (i >= cur->allocated_stack)
 			return false;
 
+		/* 64-bit scalar spill vs all slots MISC and vice versa.
+		 * Load from all slots MISC produces unbound scalar.
+		 * Construct a fake register for such stack and call
+		 * regsafe() to ensure scalar ids are compared.
+		 */
+		old_reg = scalar_reg_for_stack(env, &old->stack[spi]);
+		cur_reg = scalar_reg_for_stack(env, &cur->stack[spi]);
+		if (old_reg && cur_reg) {
+			if (!regsafe(env, old_reg, cur_reg, idmap, exact))
+				return false;
+			i += BPF_REG_SIZE - 1;
+			continue;
+		}
+
 		/* if old state was safe with misc data in the stack
 		 * it will be safe with zero-initialized stack.
 		 * The opposite is not true
-- 
2.43.0

[PATCH bpf-next v3 6/6] selftests/bpf: states pruning checks for scalar vs STACK_MISC

[Maxim Mikityanskiy]

From: Eduard Zingerman <eddyz87@gmail.com>

Check that stacksafe() compares spilled scalars with STACK_MISC.
The following combinations are explored:
- old spill of imprecise scalar is equivalent to cur STACK_{MISC,INVALID}
  (plus error in unpriv mode);
- old spill of precise scalar is not equivalent to cur STACK_MISC;
- old STACK_MISC is equivalent to cur scalar;
- old STACK_MISC is not equivalent to cur non-scalar.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
---
 .../selftests/bpf/progs/verifier_spill_fill.c | 154 ++++++++++++++++++
 1 file changed, 154 insertions(+)

diff --git a/tools/testing/selftests/bpf/progs/verifier_spill_fill.c b/tools/testing/selftests/bpf/progs/verifier_spill_fill.c
index 7f3b1319bd99..85e48069c9e6 100644
--- a/tools/testing/selftests/bpf/progs/verifier_spill_fill.c
+++ b/tools/testing/selftests/bpf/progs/verifier_spill_fill.c
@@ -1090,4 +1090,158 @@ l0_%=:	r1 >>= 32;					\
 	: __clobber_all);
 }
 
+/* stacksafe(): check if stack spill of an imprecise scalar in old state
+ * is considered equivalent to STACK_{MISC,INVALID} in cur state.
+ */
+SEC("socket")
+__success __log_level(2)
+__msg("8: (79) r1 = *(u64 *)(r10 -8)")
+__msg("8: safe")
+__msg("processed 11 insns")
+/* STACK_INVALID should prevent verifier in unpriv mode from
+ * considering states equivalent and force an error on second
+ * verification path (entry - label 1 - label 2).
+ */
+__failure_unpriv
+__msg_unpriv("8: (79) r1 = *(u64 *)(r10 -8)")
+__msg_unpriv("9: (95) exit")
+__msg_unpriv("8: (79) r1 = *(u64 *)(r10 -8)")
+__msg_unpriv("invalid read from stack off -8+2 size 8")
+__flag(BPF_F_TEST_STATE_FREQ)
+__naked void old_imprecise_scalar_vs_cur_stack_misc(void)
+{
+	asm volatile(
+	/* get a random value for branching */
+	"call %[bpf_ktime_get_ns];"
+	"if r0 == 0 goto 1f;"
+	/* conjure scalar at fp-8 */
+	"r0 = 42;"
+	"*(u64*)(r10 - 8) = r0;"
+	"goto 2f;"
+"1:"
+	/* conjure STACK_{MISC,INVALID} at fp-8 */
+	"call %[bpf_ktime_get_ns];"
+	"*(u16*)(r10 - 8) = r0;"
+	"*(u16*)(r10 - 4) = r0;"
+"2:"
+	/* read fp-8, should be considered safe on second visit */
+	"r1 = *(u64*)(r10 - 8);"
+	"exit;"
+	:
+	: __imm(bpf_ktime_get_ns)
+	: __clobber_all);
+}
+
+/* stacksafe(): check that stack spill of a precise scalar in old state
+ * is not considered equivalent to STACK_MISC in cur state.
+ */
+SEC("socket")
+__success __log_level(2)
+/* verifier should visit 'if r1 == 0x2a ...' two times:
+ * - once for path entry - label 2;
+ * - once for path entry - label 1 - label 2.
+ */
+__msg("if r1 == 0x2a goto pc+0")
+__msg("if r1 == 0x2a goto pc+0")
+__msg("processed 15 insns")
+__flag(BPF_F_TEST_STATE_FREQ)
+__naked void old_precise_scalar_vs_cur_stack_misc(void)
+{
+	asm volatile(
+	/* get a random value for branching */
+	"call %[bpf_ktime_get_ns];"
+	"if r0 == 0 goto 1f;"
+	/* conjure scalar at fp-8 */
+	"r0 = 42;"
+	"*(u64*)(r10 - 8) = r0;"
+	"goto 2f;"
+"1:"
+	/* conjure STACK_MISC at fp-8 */
+	"call %[bpf_ktime_get_ns];"
+	"*(u64*)(r10 - 8) = r0;"
+	"*(u32*)(r10 - 4) = r0;"
+"2:"
+	/* read fp-8, should not be considered safe on second visit */
+	"r1 = *(u64*)(r10 - 8);"
+	/* use r1 in precise context */
+	"if r1 == 42 goto +0;"
+	"exit;"
+	:
+	: __imm(bpf_ktime_get_ns)
+	: __clobber_all);
+}
+
+/* stacksafe(): check if STACK_MISC in old state is considered
+ * equivalent to stack spill of a scalar in cur state.
+ */
+SEC("socket")
+__success  __log_level(2)
+__msg("8: (79) r0 = *(u64 *)(r10 -8)")
+__msg("8: safe")
+__msg("processed 11 insns")
+__flag(BPF_F_TEST_STATE_FREQ)
+__naked void old_stack_misc_vs_cur_scalar(void)
+{
+	asm volatile(
+	/* get a random value for branching */
+	"call %[bpf_ktime_get_ns];"
+	"if r0 == 0 goto 1f;"
+	/* conjure STACK_{MISC,INVALID} at fp-8 */
+	"call %[bpf_ktime_get_ns];"
+	"*(u16*)(r10 - 8) = r0;"
+	"*(u16*)(r10 - 4) = r0;"
+	"goto 2f;"
+"1:"
+	/* conjure scalar at fp-8 */
+	"r0 = 42;"
+	"*(u64*)(r10 - 8) = r0;"
+"2:"
+	/* read fp-8, should be considered safe on second visit */
+	"r0 = *(u64*)(r10 - 8);"
+	"exit;"
+	:
+	: __imm(bpf_ktime_get_ns)
+	: __clobber_all);
+}
+
+/* stacksafe(): check that STACK_MISC in old state is not considered
+ * equivalent to stack spill of a non-scalar in cur state.
+ */
+SEC("socket")
+__success  __log_level(2)
+/* verifier should process exit instructions twice:
+ * - once for path entry - label 2;
+ * - once for path entry - label 1 - label 2.
+ */
+__msg("r1 = *(u64 *)(r10 -8)")
+__msg("exit")
+__msg("r1 = *(u64 *)(r10 -8)")
+__msg("exit")
+__msg("processed 11 insns")
+__flag(BPF_F_TEST_STATE_FREQ)
+__naked void old_stack_misc_vs_cur_ctx_ptr(void)
+{
+	asm volatile(
+	/* remember context pointer in r9 */
+	"r9 = r1;"
+	/* get a random value for branching */
+	"call %[bpf_ktime_get_ns];"
+	"if r0 == 0 goto 1f;"
+	/* conjure STACK_MISC at fp-8 */
+	"call %[bpf_ktime_get_ns];"
+	"*(u64*)(r10 - 8) = r0;"
+	"*(u32*)(r10 - 4) = r0;"
+	"goto 2f;"
+"1:"
+	/* conjure context pointer in fp-8 */
+	"*(u64*)(r10 - 8) = r9;"
+"2:"
+	/* read fp-8, should not be considered safe on second visit */
+	"r1 = *(u64*)(r10 - 8);"
+	"exit;"
+	:
+	: __imm(bpf_ktime_get_ns)
+	: __clobber_all);
+}
+
 char _license[] SEC("license") = "GPL";
-- 
2.43.0

Re: [PATCH bpf-next v3 0/6] Improvements for tracking scalars in the BPF verifier

[patchwork-bot+netdevbpf]

Hello:

This series was applied to bpf/bpf-next.git (master)
by Andrii Nakryiko <andrii@kernel.org>:

On Sat, 27 Jan 2024 19:52:31 +0200 you wrote:
> From: Maxim Mikityanskiy <maxim@isovalent.com>
> 
> The goal of this series is to extend the verifier's capabilities of
> tracking scalars when they are spilled to stack, especially when the
> spill or fill is narrowing. It also contains a fix by Eduard for
> infinite loop detection and a state pruning optimization by Eduard that
> compensates for a verification complexity regression introduced by
> tracking unbounded scalars. These improvements reduce the surface of
> false rejections that I saw while working on Cilium codebase.
> 
> [...]

Here is the summary with links:
  - [bpf-next,v3,1/6] bpf: Track spilled unbounded scalars
    https://git.kernel.org/bpf/bpf-next/c/e67ddd9b1cff
  - [bpf-next,v3,2/6] selftests/bpf: Test tracking spilled unbounded scalars
    https://git.kernel.org/bpf/bpf-next/c/6be503cec6c9
  - [bpf-next,v3,3/6] bpf: Preserve boundaries and track scalars on narrowing fill
    https://git.kernel.org/bpf/bpf-next/c/c1e6148cb4f8
  - [bpf-next,v3,4/6] selftests/bpf: Add test cases for narrowing fill
    https://git.kernel.org/bpf/bpf-next/c/067313a85c6f
  - [bpf-next,v3,5/6] bpf: handle scalar spill vs all MISC in stacksafe()
    https://git.kernel.org/bpf/bpf-next/c/6efbde200bf3
  - [bpf-next,v3,6/6] selftests/bpf: states pruning checks for scalar vs STACK_MISC
    https://git.kernel.org/bpf/bpf-next/c/73a28d9d000e

You are awesome, thank you!
-- 
Deet-doot-dot, I am a bot.
https://korg.docs.kernel.org/patchwork/pwbot.html