On 12/11/23 1:39 AM, Menglong Dong wrote:
Hello,
On Mon, Dec 11, 2023 at 1:09 PM Yonghong Song <yonghong.song@xxxxxxxxx> wrote:
On 12/10/23 5:00 AM, Menglong Dong wrote:
We can derive some new information for BPF_JNE in regs_refine_cond_op().
Take following code for example:
/* The type of "a" is u16 */
if (a > 0 && a < 100) {
/* the range of the register for a is [0, 99], not [1, 99],
* and will cause the following error:
*
* invalid zero-sized read
*
* as a can be 0.
*/
bpf_skb_store_bytes(skb, xx, xx, a, 0);
}
Could you have a C test to demonstrate this example?
Also, you should have a set of inline asm code (progs/verifier*.c)
to test various cases as in mark_reg32_not_equal() and
mark_reg_not_equal().
Yeah! I found that this part is tested in the test_progs/reg_bounds_crafted
too, and this commit failed that test case, which I should fix in the next
version.
In the code above, "a > 0" will be compiled to "jmp xxx if a == 0". In the
TRUE branch, the dst_reg will be marked as known to 0. However, in the
fallthrough(FALSE) branch, the dst_reg will not be handled, which makes
the [min, max] for a is [0, 99], not [1, 99].
For BPF_JNE, we can reduce the range of the dst reg if the src reg is a
const and is exactly the edge of the dst reg.
Signed-off-by: Menglong Dong <menglong8.dong@xxxxxxxxx>
---
kernel/bpf/verifier.c | 45 ++++++++++++++++++++++++++++++++++++++++++-
1 file changed, 44 insertions(+), 1 deletion(-)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 727a59e4a647..7b074ac93190 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -1764,6 +1764,40 @@ static void __mark_reg_const_zero(struct bpf_reg_state *reg)
reg->type = SCALAR_VALUE;
}
+#define CHECK_REG_MIN(value) \
+do { \
+ if ((value) == (typeof(value))imm) \
+ value++; \
+} while (0)
+
+#define CHECK_REG_MAX(value) \
+do { \
+ if ((value) == (typeof(value))imm) \
+ value--; \
+} while (0)
+
+static void mark_reg32_not_equal(struct bpf_reg_state *reg, u64 imm)
+{
What if reg->s32_min_value == imm and reg->s32_max_value == imm?
Has this been handled in previous verifier logic?
Will such a case happen? In current code path, the src reg is a const,
and the is_branch_taken() will return 0 or 1 if the
dst_reg->s32_min_value == dst_reg->s32_max_value.
Enn......maybe we can do more checking here in case that someone
calls this function in another place.
I double checked the source code as well. Indeed, 'reg' should
not be a constant as it has been handled in is_branch_taken()
properly. Ignore my comments above then. Thanks!
Thanks!
Menglong Dong
+ CHECK_REG_MIN(reg->s32_min_value);
+ CHECK_REG_MAX(reg->s32_max_value);
+ CHECK_REG_MIN(reg->u32_min_value);
+ CHECK_REG_MAX(reg->u32_max_value);
+}
+
+static void mark_reg_not_equal(struct bpf_reg_state *reg, u64 imm)
+{
+ CHECK_REG_MIN(reg->smin_value);
+ CHECK_REG_MAX(reg->smax_value);
+
+ CHECK_REG_MIN(reg->umin_value);
+ CHECK_REG_MAX(reg->umax_value);
+
+ CHECK_REG_MIN(reg->s32_min_value);
+ CHECK_REG_MAX(reg->s32_max_value);
+ CHECK_REG_MIN(reg->u32_min_value);
+ CHECK_REG_MAX(reg->u32_max_value);
+}
+
static void mark_reg_known_zero(struct bpf_verifier_env *env,
struct bpf_reg_state *regs, u32 regno)
{
@@ -14332,7 +14366,16 @@ static void regs_refine_cond_op(struct bpf_reg_state *reg1, struct bpf_reg_state
}
break;
case BPF_JNE:
- /* we don't derive any new information for inequality yet */
+ /* try to recompute the bound of reg1 if reg2 is a const and
+ * is exactly the edge of reg1.
+ */
+ if (is_reg_const(reg2, is_jmp32)) {
+ val = reg_const_value(reg2, is_jmp32);
+ if (is_jmp32)
+ mark_reg32_not_equal(reg1, val);
+ else
+ mark_reg_not_equal(reg1, val);
+ }
break;
case BPF_JSET:
if (!is_reg_const(reg2, is_jmp32))
