On 5/26/26 10:25 PM, Taegu Ha wrote:
Global subprogram argument checking derives generic pointer sizes from BTF
and passes the resolved size to check_mem_reg() as a u32. The access-size
validation path then uses a signed int, and stack pointers negate the value
before calling check_helper_mem_access().
A BTF type such as int[0x3fffffff] resolves to 0xfffffffc bytes. On a stack
pointer, (int)mem_size becomes -4 and the negation validates only four
bytes. A caller can therefore pass a four-byte stack slot while the callee
is verified with a nearly 4GiB memory argument, allowing accesses outside
the caller object.
This was confirmed with a non-executing raw-BTF reproducer. On a
vulnerable kernel, the verifier accepted a program where the caller passed
a four-byte stack slot, while the callee argument was described by BTF as
int[0x3fffffff]. The verifier log showed:
R1=mem_or_null(id=1,sz=0xfffffffc)
r0 = *(u32 *)(r1 +4)
The program was only loaded to prove verifier acceptance and was not
attached or executed.
Reject sizes that cannot be represented by the signed verifier access-size
API before any conversion. Cast the non-stack case after the bound check to
make the conversion explicit, and add a verifier regression test for the
oversized BTF argument.
Fixes: 2cb27158adb3 ("bpf: poison dead stack slots")
Signed-off-by: Taegu Ha <[email protected]>
---
kernel/bpf/verifier.c | 7 ++++++-
.../bpf/progs/verifier_global_subprogs.c | 17 +++++++++++++++++
2 files changed, 23 insertions(+), 1 deletion(-)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 7fb88e1cd7c4..1007f204a1f5 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -7107,6 +7107,11 @@ static int check_mem_reg(struct bpf_verifier_env *env,
struct bpf_reg_state *reg
struct bpf_reg_state saved_reg;
int err;
+ if (mem_size > S32_MAX) {
+ verbose(env, "R%d memory size %u is too large\n", regno,
mem_size);
+ return -EACCES;
+ }
+
if (bpf_register_is_null(reg))
return 0;
@@ -7119,7 +7124,7 @@ static int check_mem_reg(struct bpf_verifier_env *env, struct bpf_reg_state *reg
mark_ptr_not_null_reg(reg);
}
- int size = base_type(reg->type) == PTR_TO_STACK ? -(int)mem_size : mem_size;
+ int size = base_type(reg->type) == PTR_TO_STACK ? -(int)mem_size :
(int)mem_size;
err = check_helper_mem_access(env, regno, size, BPF_READ, true, NULL);
err = err ?: check_helper_mem_access(env, regno, size, BPF_WRITE, true,
NULL);
diff --git a/tools/testing/selftests/bpf/progs/verifier_global_subprogs.c
b/tools/testing/selftests/bpf/progs/verifier_global_subprogs.c
index 1e08aff7532e..0ff8f85b4d46 100644
--- a/tools/testing/selftests/bpf/progs/verifier_global_subprogs.c
+++ b/tools/testing/selftests/bpf/progs/verifier_global_subprogs.c
@@ -151,6 +151,23 @@ int anon_user_mem_valid(void *ctx)
return subprog_user_anon_mem(&t);
}
+__noinline __weak int subprog_user_anon_mem_huge(int (*p)[0x3fffffff])
+{
+ return p ? (*p)[1] : 0;
+}
+
+SEC("?tracepoint")
+__failure __log_level(2)
+__msg("R1 memory size 4294967292 is too large")
+int anon_user_mem_huge_size_invalid(void *ctx)
+{
+ int (*p)[0x3fffffff];
+ int tiny = 42;
+
+ p = (void *)&tiny;
+ return subprog_user_anon_mem_huge(p) + tiny;
+}
Without verifier.c change, verification is successful.
The objdump:
0000000000000160 <subprog_user_anon_mem_huge>:
; {
44: b4 00 00 00 00 00 00 00 w0 = 0x0
; return p ? (*p)[1] : 0;
45: 15 01 01 00 00 00 00 00 if r1 == 0x0 goto +0x1 <L0>
46: 61 10 04 00 00 00 00 00 w0 = *(u32 *)(r1 + 0x4)
<L0>:
47: 95 00 00 00 00 00 00 00 exit
0000000000000040 <anon_user_mem_huge_size_invalid>:
; int tiny = 42;
8: b4 01 00 00 2a 00 00 00 w1 = 0x2a
9: 63 1a fc ff 00 00 00 00 *(u32 *)(r10 - 0x4) = w1
10: bf a1 00 00 00 00 00 00 r1 = r10
11: 07 01 00 00 fc ff ff ff r1 += -0x4
; return subprog_user_anon_mem_huge(p) + tiny;
12: 85 10 00 00 ff ff ff ff call -0x1
0000000000000060: R_BPF_64_32 subprog_user_anon_mem_huge
13: 61 a1 fc ff 00 00 00 00 w1 = *(u32 *)(r10 - 0x4)
14: 0c 01 00 00 00 00 00 00 w1 += w0
15: bc 10 00 00 00 00 00 00 w0 = w1
16: 95 00 00 00 00 00 00 00 exit
The big 0x3fffffff does not really matter.
+
__noinline __weak int subprog_nonnull_ptr_good(int *p1 __arg_nonnull, int *p2
__arg_nonnull)
{
return (*p1) * (*p2); /* good, no need for NULL checks */
