selftests/bpf: Cover invariant violation case from syzbot

This patch adds a selftest for the change in the previous patch. The
selftest is derived from a syzbot reproducer from [1] (among the 22
reproducers on that page, only 4 still reproduced on latest bpf tree,
all being small variants of the same invariant violation).

The test case failure without the previous patch is shown below.

  0: R1=ctx() R10=fp0
  0: (85) call bpf_get_prandom_u32#7    ; R0=scalar()
  1: (bf) r5 = r0                       ; R0=scalar(id=1) R5=scalar(id=1)
  2: (57) r5 &= -4                      ; R5=scalar(smax=0x7ffffffffffffffc,umax=0xfffffffffffffffc,smax32=0x7ffffffc,umax32=0xfffffffc,var_off=(0x0; 0xfffffffffffffffc))
  3: (bf) r7 = r0                       ; R0=scalar(id=1) R7=scalar(id=1)
  4: (57) r7 &= 1                       ; R7=scalar(smin=smin32=0,smax=umax=smax32=umax32=1,var_off=(0x0; 0x1))
  5: (07) r7 += -43                     ; R7=scalar(smin=smin32=-43,smax=smax32=-42,umin=0xffffffffffffffd5,umax=0xffffffffffffffd6,umin32=0xffffffd5,umax32=0xffffffd6,var_off=(0xffffffffffffffd4; 0x3))
  6: (5e) if w5 != w7 goto pc+1
  verifier bug: REG INVARIANTS VIOLATION (false_reg1): range bounds violation u64=[0xffffffd5, 0xffffffffffffffd4] s64=[0x80000000ffffffd5, 0x7fffffffffffffd4] u32=[0xffffffd5, 0xffffffd4] s32=[0xffffffd5, 0xffffffd4] var_off=(0xffffffd4, 0xffffffff00000000)

R5 and R7 are prepared such that their tnums intersection results in a
known constant but that constant isn't within R7's u32 bounds.
is_branch_taken isn't able to detect this case today, so the verifier
walks the impossible fallthrough branch. After regs_refine_cond_op and
reg_bounds_sync refine R5 on the assumption that the branch is taken,
the impossibility becomes apparent and results in an invariant violation
for R5: umin32 is greater than umax32.

The previous patch fixes this by using regs_refine_cond_op and
reg_bounds_sync in is_branch_taken to detect the impossible branch. The
fallthrough branch is therefore correctly detected as dead code.

Link: https://syzkaller.appspot.com/bug?extid=c950cc277150935cc0b5 [1]
Signed-off-by: Paul Chaignon <paul.chaignon@gmail.com>
Acked-by: Mykyta Yatsenko <yatsenko@meta.com>
Link: https://lore.kernel.org/r/b1e22233a3206ead522f02eda27b9c5c991a0de9.1775142354.git.paul.chaignon@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 29 insertions, 0 deletions

diff --git a/tools/testing/selftests/bpf/progs/verifier_bounds.c b/tools/testing/selftests/bpf/progs/verifier_bounds.c
index bb20f0f06f05..df7c62a12318 100644
--- a/tools/testing/selftests/bpf/progs/verifier_bounds.c
+++ b/tools/testing/selftests/bpf/progs/verifier_bounds.c
@@ -2165,4 +2165,33 @@ l0_%=:	r0 = 0;						\
 	: __clobber_all);
 }
 
+/*
+ * Last jump can be detected as always taken because the intersection of R5 and
+ * R7 32bit tnums produces a constant that isn't within R7's s32 bounds.
+ */
+SEC("socket")
+__description("dead branch: tnums give impossible constant if equal")
+__success
+__flag(BPF_F_TEST_REG_INVARIANTS)
+__naked void tnums_equal_impossible_constant(void *ctx)
+{
+	asm volatile("						\
+	call %[bpf_get_prandom_u32];				\
+	r5 = r0;						\
+	/* Set r5's var_off32 to (0; 0xfffffffc) */		\
+	r5 &= 0xfffffffffffffffc;				\
+	r7 = r0;						\
+	/* Set r7's var_off32 to (0x0; 0x1) */			\
+	r7 &= 0x1;						\
+	/* Now, s32=[-43; -42], var_off32=(0xffffffd4; 0x3) */	\
+	r7 += -43;						\
+	/* On fallthrough, var_off32=-44, not in s32 */		\
+	if w5 != w7 goto +1;					\
+	r10 = 0;						\
+	exit;							\
+"	:
+	: __imm(bpf_get_prandom_u32)
+	: __clobber_all);
+}
+
 char _license[] SEC("license") = "GPL";