[PATCH v5 bpf-next 08/23] bpf: try harder to deduce register bounds from different numeric domains

Andrii Nakryiko
2023-10-27T18:16:47Z

There are cases (caught by subsequent reg_bounds tests in selftests/bpf)
where performing one round of __reg_deduce_bounds() doesn't propagate
all the information from, say, s32 to u32 bounds and than from newly
learned u32 bounds back to u64 and s64. So perform __reg_deduce_bounds()
twice to make sure such derivations are propagated fully after
reg_bounds_sync().

One such example is test `(s64)[0xffffffff00000001; 0] (u64)<
0xffffffff00000000` from selftest patch from this patch set. It demonstrates an
intricate dance of u64 -> s64 -> u64 -> u32 bounds adjustments, which requires
two rounds of __reg_deduce_bounds(). Here are corresponding refinement log from
selftest, showing evolution of knowledge.

REFINING (FALSE R1) (u64)SRC=[0xffffffff00000000; U64_MAX] (u64)DST_OLD=[0; U64_MAX] (u64)DST_NEW=[0xffffffff00000000; U64_MAX]
REFINING (FALSE R1) (u64)SRC=[0xffffffff00000000; U64_MAX] (s64)DST_OLD=[0xffffffff00000001; 0] (s64)DST_NEW=[0xffffffff00000001; -1]
REFINING (FALSE R1) (s64)SRC=[0xffffffff00000001; -1] (u64)DST_OLD=[0xffffffff00000000; U64_MAX] (u64)DST_NEW=[0xffffffff00000001; U64_MAX]
REFINING (FALSE R1) (u64)SRC=[0xffffffff00000001; U64_MAX] (u32)DST_OLD=[0; U32_MAX] (u32)DST_NEW=[1; U32_MAX]

R1 initially has smin/smax set to [0xffffffff00000001; -1], while umin/umax is
unknown. After (u64)< comparison, in FALSE branch we gain knowledge that
umin/umax is [0xffffffff00000000; U64_MAX]. That causes smin/smax to learn that
zero can't happen and upper bound is -1. Then smin/smax is adjusted from
umin/umax improving lower bound from 0xffffffff00000000 to 0xffffffff00000001.
And then eventually umin32/umax32 bounds are drived from umin/umax and become
[1; U32_MAX].

Selftest in the last patch is actually implementing a multi-round fixed-point
convergence logic, but so far all the tests are handled by two rounds of
reg_bounds_sync() on the verifier state, so we keep it simple for now.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
---
kernel/bpf/verifier.c | 1 +
1 file changed, 1 insertion(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 768247e3d667..6b0736c04ebe 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -2607,6 +2607,7 @@ static void reg_bounds_sync(struct bpf_reg_state *reg)
__update_reg_bounds(reg);
/* We might have learned something about the sign bit. */
__reg_deduce_bounds(reg);
+ __reg_deduce_bounds(reg);
/* We might have learned some bits from the bounds. */
__reg_bound_offset(reg);
/* Intersecting with the old var_off might have improved our bounds
--
2.34.1