[PATCH] bpf: selftest for late caller stack size increase

[Jann Horn]

This checks that it is not possible to bypass the total stack size check in
update_stack_depth() by calling a function that uses a large amount of
stack memory *before* using a large amount of stack memory in the caller.

Currently, the first added testcase causes a rejection as expected, but
the second testcase is (AFAICS incorrectly) accepted:

[...]
#483/p calls: stack overflow using two frames (post-call access) FAIL
Unexpected success to load!
0: (85) call pc+2
caller:
 R10=fp0,call_-1
callee:
 frame1: R1=ctx(id=0,off=0,imm=0) R10=fp0,call_0
3: (72) *(u8 *)(r10 -300) = 0
4: (b7) r0 = 0
5: (95) exit
returning from callee:
 frame1: R0_w=inv0 R1=ctx(id=0,off=0,imm=0) R10=fp0,call_0
to caller at 1:
 R0_w=inv0 R10=fp0,call_-1

from 5 to 1: R0=inv0 R10=fp0,call_-1
1: (72) *(u8 *)(r10 -300) = 0
2: (95) exit
processed 6 insns, stack depth 300+300
[...]
Summary: 704 PASSED, 1 FAILED

AFAICS the JIT-generated code for the second testcase shows that this
really causes the stack pointer to be decremented by 300+300:

first function:
00000000  55                push rbp
00000001  4889E5            mov rbp,rsp
00000004  4881EC58010000    sub rsp,0x158
0000000B  4883ED28          sub rbp,byte +0x28
[...]
00000025  E89AB3AFE5        call 0xffffffffe5afb3c4
0000002A  C685D4FEFFFF00    mov byte [rbp-0x12c],0x0
[...]
00000041  4883C528          add rbp,byte +0x28
00000045  C9                leave
00000046  C3                ret

second function:
00000000  55                push rbp
00000001  4889E5            mov rbp,rsp
00000004  4881EC58010000    sub rsp,0x158
0000000B  4883ED28          sub rbp,byte +0x28
[...]
00000025  C685D4FEFFFF00    mov byte [rbp-0x12c],0x0
[...]
0000003E  4883C528          add rbp,byte +0x28
00000042  C9                leave
00000043  C3                ret

Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
---
 tools/testing/selftests/bpf/test_verifier.c | 34 +++++++++++++++++++++++++++++
 1 file changed, 34 insertions(+)

diff --git a/tools/testing/selftests/bpf/test_verifier.c b/tools/testing/selftests/bpf/test_verifier.c
index 3bacff0d6f91..71fb0be81b78 100644
--- a/tools/testing/selftests/bpf/test_verifier.c
+++ b/tools/testing/selftests/bpf/test_verifier.c
@@ -8729,6 +8729,40 @@ static struct bpf_test tests[] = {
 		.prog_type = BPF_PROG_TYPE_XDP,
 		.result = ACCEPT,
 	},
+	{
+		"calls: stack overflow using two frames (pre-call access)",
+		.insns = {
+			/* prog 1 */
+			BPF_ST_MEM(BPF_B, BPF_REG_10, -300, 0),
+			BPF_RAW_INSN(BPF_JMP|BPF_CALL, 0, 1, 0, 1),
+			BPF_EXIT_INSN(),
+
+			/* prog 2 */
+			BPF_ST_MEM(BPF_B, BPF_REG_10, -300, 0),
+			BPF_MOV64_IMM(BPF_REG_0, 0),
+			BPF_EXIT_INSN(),
+		},
+		.prog_type = BPF_PROG_TYPE_XDP,
+		.errstr = "combined stack size",
+		.result = REJECT,
+	},
+	{
+		"calls: stack overflow using two frames (post-call access)",
+		.insns = {
+			/* prog 1 */
+			BPF_RAW_INSN(BPF_JMP|BPF_CALL, 0, 1, 0, 2),
+			BPF_ST_MEM(BPF_B, BPF_REG_10, -300, 0),
+			BPF_EXIT_INSN(),
+
+			/* prog 2 */
+			BPF_ST_MEM(BPF_B, BPF_REG_10, -300, 0),
+			BPF_MOV64_IMM(BPF_REG_0, 0),
+			BPF_EXIT_INSN(),
+		},
+		.prog_type = BPF_PROG_TYPE_XDP,
+		.errstr = "combined stack size",
+		.result = REJECT,
+	},
 	{
 		"calls: spill into caller stack frame",
 		.insns = {
-- 
2.15.1.620.gb9897f4670-goog