Merge branch 'bpf-register-bounds-range-vs-range-support'

Andrii Nakryiko says:

====================
BPF register bounds range vs range support

This patch set is a continuation of work started in [0]. It adds a big set of
manual, auto-generated, and now also random test cases validating BPF
verifier's register bounds tracking and deduction logic.

First few patches generalize verifier's logic to handle conditional jumps and
corresponding range adjustments in case when two non-const registers are
compared to each other. Patch #1 generalizes reg_set_min_max() portion, while
patch #2 does the same for is_branch_taken() part of the overall solution.

Patch #3 improves equality and inequality for cases when BPF program code
mixes 64-bit and 32-bit uses of the same register. Depending on specific
sequence, it's possible to get to the point where u64/s64 bounds will be very
generic (e.g., after signed 32-bit comparison), while we still keep pretty
tight u32/s32 bounds. If in such state we proceed with 32-bit equality or
inequality comparison, reg_set_min_max() might have to deal with adjusting s32
bounds for two registers that don't overlap, which breaks reg_set_min_max().
This doesn't manifest in <range> vs <const> cases, because if that happens
reg_set_min_max() in effect will force s32 bounds to be a new "impossible"
constant (from original smin32/smax32 bounds point of view). Things get tricky
when we have <range> vs <range> adjustments, so instead of trying to somehow
make sense out of such situations, it's best to detect such impossible
situations and prune the branch that can't be taken in is_branch_taken()
logic.  This equality/inequality was the only such category of situations with
auto-generated tests added later in the patch set.

But when we start mixing arithmetic operations in different numeric domains
and conditionals, things get even hairier. So, patch #4 adds sanity checking
logic after all ALU/ALU64, JMP/JMP32, and LDX operations. By default, instead
of failing verification, we conservatively reset range bounds to unknown
values, reporting violation in verifier log (if verbose logs are requested).
But to aid development, detection, and debugging, we also introduce a new test
flag, BPF_F_TEST_SANITY_STRICT, which triggers verification failure on range
sanity violation.

Patch #11 sets BPF_F_TEST_SANITY_STRICT by default for test_progs and
test_verifier. Patch #12 adds support for controlling this in veristat for
testing with production BPF object files.

Getting back to BPF verifier, patches #5 and #6 complete verifier's range
tracking logic clean up. See respective patches for details.

With kernel-side taken care of, we move to testing. We start with building
a tester that validates existing <range> vs <scalar> verifier logic for range
bounds. Patch #7 implements an initial version of such a tester. We guard
millions of generated tests behind SLOW_TESTS=1 envvar requirement, but also
have a relatively small number of tricky cases that came up during development
and debugging of this work. Those will be executed as part of a normal
test_progs run.

Patch #8 simulates more nuanced JEQ/JNE logic we added to verifier in patch #3.
Patch #9 adds <range> vs <range> "slow tests".

Patch #10 is a completely new one, it adds a bunch of randomly generated cases
to be run normally, without SLOW_TESTS=1 guard. This should help to get
a bunch of cover, and hopefully find some remaining latent problems if
verifier proactively as part of normal BPF CI runs.

Finally, a tiny test which was, amazingly, an initial motivation for this
whole work, is added in lucky patch #13, demonstrating how verifier is now
smart enough to track actual number of elements in the array and won't require
additional checks on loop iteration variable inside the bpf_for() open-coded
iterator loop.

  [0] https://patchwork.kernel.org/project/netdevbpf/list/?series=798308&state=*

v1->v2:
  - use x < y => y > x property to minimize reg_set_min_max (Eduard);
  - fix for JEQ/JNE logic in reg_bounds.c (Eduard);
  - split BPF_JSET and !BPF_JSET cases handling (Shung-Hsi);
  - adjustments to reg_bounds.c to make it easier to follow (Alexei);
  - added acks (Eduard, Shung-Hsi).
====================

Link: https://lore.kernel.org/r/20231112010609.848406-1-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: Alexei Starovoitov

include/linux/bpf_verifier.h

@@ -602,6 +602,7 @@ struct bpf_verifier_env {
 	int stack_size;			/* number of states to be processed */
 	bool strict_alignment;		/* perform strict pointer alignment checks */
 	bool test_state_freq;		/* test verifier with different pruning frequency */
+	bool test_sanity_strict;	/* fail verification on sanity violations */
 	struct bpf_verifier_state *cur_state; /* current verifier state */
 	struct bpf_verifier_state_list **explored_states; /* search pruning optimization */
 	struct bpf_verifier_state_list *free_list;

include/linux/tnum.h

@@ -106,6 +106,10 @@ int tnum_sbin(char *str, size_t size, struct tnum a);
 struct tnum tnum_subreg(struct tnum a);
 /* Returns the tnum with the lower 32-bit subreg cleared */
 struct tnum tnum_clear_subreg(struct tnum a);
+/* Returns the tnum with the lower 32-bit subreg in *reg* set to the lower
+ * 32-bit subreg in *subreg*
+ */
+struct tnum tnum_with_subreg(struct tnum reg, struct tnum subreg);
 /* Returns the tnum with the lower 32-bit subreg set to value */
 struct tnum tnum_const_subreg(struct tnum a, u32 value);
 /* Returns true if 32-bit subreg @a is a known constant*/

include/uapi/linux/bpf.h

@@ -1200,6 +1200,9 @@ enum bpf_perf_event_type {
  */
 #define BPF_F_XDP_DEV_BOUND_ONLY	(1U << 6)
 
+/* The verifier internal test flag. Behavior is undefined */
+#define BPF_F_TEST_SANITY_STRICT	(1U << 7)
+
 /* link_create.kprobe_multi.flags used in LINK_CREATE command for
  * BPF_TRACE_KPROBE_MULTI attach type to create return probe.
  */

kernel/bpf/syscall.c

@@ -2573,7 +2573,8 @@ static int bpf_prog_load(union bpf_attr *attr, bpfptr_t uattr, u32 uattr_size)
 				 BPF_F_SLEEPABLE |
 				 BPF_F_TEST_RND_HI32 |
 				 BPF_F_XDP_HAS_FRAGS |
-				 BPF_F_XDP_DEV_BOUND_ONLY))
+				 BPF_F_XDP_DEV_BOUND_ONLY |
+				 BPF_F_TEST_SANITY_STRICT))
 		return -EINVAL;
 
 	if (!IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS) &&

kernel/bpf/tnum.c

@@ -208,7 +208,12 @@ struct tnum tnum_clear_subreg(struct tnum a)
 	return tnum_lshift(tnum_rshift(a, 32), 32);
 }
 
+struct tnum tnum_with_subreg(struct tnum reg, struct tnum subreg)
+{
+	return tnum_or(tnum_clear_subreg(reg), tnum_subreg(subreg));
+}
+
 struct tnum tnum_const_subreg(struct tnum a, u32 value)
 {
-	return tnum_or(tnum_clear_subreg(a), tnum_const(value));
+	return tnum_with_subreg(a, tnum_const(value));
 }