[PATCH 5.15 014/230] bpf: Fix incorrect verifier simulation around jmp32s jeq/jne

Greg Kroah-Hartman
2022-07-11T09:38:55Z

From: Daniel Borkmann <daniel@iogearbox.net>

commit a12ca6277eca6aeeccf66e840c23a2b520e24c8f upstream.

Kuee reported a quirk in the jmp32's jeq/jne simulation, namely that the
register value does not match expectations for the fall-through path. For
example:

Before fix:

0: R1=ctx(off=0,imm=0) R10=fp0
0: (b7) r2 = 0 ; R2_w=P0
1: (b7) r6 = 563 ; R6_w=P563
2: (87) r2 = -r2 ; R2_w=Pscalar()
3: (87) r2 = -r2 ; R2_w=Pscalar()
4: (4c) w2 |= w6 ; R2_w=Pscalar(umin=563,umax=4294967295,var_off=(0x233; 0xfffffdcc),s32_min=-2147483085) R6_w=P563
5: (56) if w2 != 0x8 goto pc+1 ; R2_w=P571 <--- [*]
6: (95) exit
R0 !read_ok

After fix:

0: R1=ctx(off=0,imm=0) R10=fp0
0: (b7) r2 = 0 ; R2_w=P0
1: (b7) r6 = 563 ; R6_w=P563
2: (87) r2 = -r2 ; R2_w=Pscalar()
3: (87) r2 = -r2 ; R2_w=Pscalar()
4: (4c) w2 |= w6 ; R2_w=Pscalar(umin=563,umax=4294967295,var_off=(0x233; 0xfffffdcc),s32_min=-2147483085) R6_w=P563
5: (56) if w2 != 0x8 goto pc+1 ; R2_w=P8 <--- [*]
6: (95) exit
R0 !read_ok

As can be seen on line 5 for the branch fall-through path in R2 [*] is that
given condition w2 != 0x8 is false, verifier should conclude that r2 = 8 as
upper 32 bit are known to be zero. However, verifier incorrectly concludes
that r2 = 571 which is far off.

The problem is it only marks false{true}_reg as known in the switch for JE/NE
case, but at the end of the function, it uses {false,true}_{64,32}off to
update {false,true}_reg->var_off and they still hold the prior value of
{false,true}_reg->var_off before it got marked as known. The subsequent
__reg_combine_32_into_64() then propagates this old var_off and derives new
bounds. The information between min/max bounds on {false,true}_reg from
setting the register to known const combined with the {false,true}_reg->var_off
based on the old information then derives wrong register data.

Fix it by detangling the BPF_JEQ/BPF_JNE cases and updating relevant
{false,true}_{64,32}off tnums along with the register marking to known
constant.

Fixes: 3f50f132d840 ("bpf: Verifier, do explicit ALU32 bounds tracking")
Reported-by: Kuee K1r0a <liulin063@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20220701124727.11153-1-daniel@iogearbox.net
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
kernel/bpf/verifier.c | 41 ++++++++++++++++++++++++-----------------
1 file changed, 24 insertions(+), 17 deletions(-)

--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -8591,26 +8591,33 @@ static void reg_set_min_max(struct bpf_r
return;

switch (opcode) {
+ /* JEQ/JNE comparison doesn't change the register equivalence.
+ *
+ * r1 = r2;
+ * if (r1 == 42) goto label;
+ * ...
+ * label: // here both r1 and r2 are known to be 42.
+ *
+ * Hence when marking register as known preserve it's ID.
+ */
case BPF_JEQ:
+ if (is_jmp32) {
+ __mark_reg32_known(true_reg, val32);
+ true_32off = tnum_subreg(true_reg->var_off);
+ } else {
+ ___mark_reg_known(true_reg, val);
+ true_64off = true_reg->var_off;
+ }
+ break;
case BPF_JNE:
- {
- struct bpf_reg_state *reg =
- opcode == BPF_JEQ ? true_reg : false_reg;
-
- /* JEQ/JNE comparison doesn't change the register equivalence.
- * r1 = r2;
- * if (r1 == 42) goto label;
- * ...
- * label: // here both r1 and r2 are known to be 42.
- *
- * Hence when marking register as known preserve it's ID.
- */
- if (is_jmp32)
- __mark_reg32_known(reg, val32);
- else
- ___mark_reg_known(reg, val);
+ if (is_jmp32) {
+ __mark_reg32_known(false_reg, val32);
+ false_32off = tnum_subreg(false_reg->var_off);
+ } else {
+ ___mark_reg_known(false_reg, val);
+ false_64off = false_reg->var_off;
+ }
break;
- }
case BPF_JSET:
if (is_jmp32) {
false_32off = tnum_and(false_32off, tnum_const(~val32));