exec: Avoid pathological argc, envc, and bprm->p values

Make sure nothing goes wrong with the string counters or the bprm's
belief about the stack pointer. Add checks and matching self-tests.

Take special care for !CONFIG_MMU, since argmin is not exposed there.

For 32-bit validation, 32-bit UML was used:
$ tools/testing/kunit/kunit.py run \
	--make_options CROSS_COMPILE=i686-linux-gnu- \
	--make_options SUBARCH=i386 \
	exec

For !MMU validation, m68k was used:
$ tools/testing/kunit/kunit.py run \
	--arch m68k --make_option CROSS_COMPILE=m68k-linux-gnu- \
	exec

Link: https://lore.kernel.org/r/20240520021615.741800-2-keescook@chromium.org
Link: https://lore.kernel.org/r/20240621205046.4001362-2-kees@kernel.orgSigned-off-by: Kees Cook <kees@kernel.org>

fs/exec.c

@@ -490,6 +490,9 @@ static inline int bprm_set_stack_limit(struct linux_binprm *bprm,
 				       unsigned long limit)
 {
 #ifdef CONFIG_MMU
+	/* Avoid a pathological bprm->p. */
+	if (bprm->p < limit)
+		return -E2BIG;
 	bprm->argmin = bprm->p - limit;
 #endif
 	return 0;
@@ -531,6 +534,9 @@ static int bprm_stack_limits(struct linux_binprm *bprm)
 	 * of argument strings even with small stacks
 	 */
 	limit = max_t(unsigned long, limit, ARG_MAX);
+	/* Reject totally pathological counts. */
+	if (bprm->argc < 0 || bprm->envc < 0)
+		return -E2BIG;
 	/*
 	 * We must account for the size of all the argv and envp pointers to
 	 * the argv and envp strings, since they will also take up space in
@@ -544,7 +550,9 @@ static int bprm_stack_limits(struct linux_binprm *bprm)
 	 * argc can never be 0, to keep them from walking envp by accident.
 	 * See do_execveat_common().
 	 */
-	ptr_size = (max(bprm->argc, 1) + bprm->envc) * sizeof(void *);
+	if (check_add_overflow(max(bprm->argc, 1), bprm->envc, &ptr_size) ||
+	    check_mul_overflow(ptr_size, sizeof(void *), &ptr_size))
+		return -E2BIG;
 	if (limit <= ptr_size)
 		return -E2BIG;
 	limit -= ptr_size;

fs/exec_test.c

@@ -8,9 +8,34 @@ struct bprm_stack_limits_result {
 };
 
 static const struct bprm_stack_limits_result bprm_stack_limits_results[] = {
-	/* Giant values produce -E2BIG */
+	/* Negative argc/envc counts produce -E2BIG */
+	{ { .p = ULONG_MAX, .rlim_stack.rlim_cur = ULONG_MAX,
+	    .argc = INT_MIN, .envc = INT_MIN }, .expected_rc = -E2BIG },
+	{ { .p = ULONG_MAX, .rlim_stack.rlim_cur = ULONG_MAX,
+	    .argc = 5, .envc = -1 }, .expected_rc = -E2BIG },
+	{ { .p = ULONG_MAX, .rlim_stack.rlim_cur = ULONG_MAX,
+	    .argc = -1, .envc = 10 }, .expected_rc = -E2BIG },
+	/* The max value of argc or envc is MAX_ARG_STRINGS. */
 	{ { .p = ULONG_MAX, .rlim_stack.rlim_cur = ULONG_MAX,
 	    .argc = INT_MAX, .envc = INT_MAX }, .expected_rc = -E2BIG },
+	{ { .p = ULONG_MAX, .rlim_stack.rlim_cur = ULONG_MAX,
+	    .argc = MAX_ARG_STRINGS, .envc = MAX_ARG_STRINGS }, .expected_rc = -E2BIG },
+	{ { .p = ULONG_MAX, .rlim_stack.rlim_cur = ULONG_MAX,
+	    .argc = 0, .envc = MAX_ARG_STRINGS }, .expected_rc = -E2BIG },
+	{ { .p = ULONG_MAX, .rlim_stack.rlim_cur = ULONG_MAX,
+	    .argc = MAX_ARG_STRINGS, .envc = 0 }, .expected_rc = -E2BIG },
+	/*
+	 * On 32-bit system these argc and envc counts, while likely impossible
+	 * to represent within the associated TASK_SIZE, could overflow the
+	 * limit calculation, and bypass the ptr_size <= limit check.
+	 */
+	{ { .p = ULONG_MAX, .rlim_stack.rlim_cur = ULONG_MAX,
+	    .argc = 0x20000001, .envc = 0x20000001 }, .expected_rc = -E2BIG },
+#ifdef CONFIG_MMU
+	/* Make sure a pathological bprm->p doesn't cause an overflow. */
+	{ { .p = sizeof(void *), .rlim_stack.rlim_cur = ULONG_MAX,
+	    .argc = 10, .envc = 10 }, .expected_rc = -E2BIG },
+#endif
 	/*
 	 * 0 rlim_stack will get raised to ARG_MAX. With 1 string pointer,
 	 * we should see p - ARG_MAX + sizeof(void *).
@@ -88,6 +113,7 @@ static void exec_test_bprm_stack_limits(struct kunit *test)
 	/* Double-check the constants. */
 	KUNIT_EXPECT_EQ(test, _STK_LIM, SZ_8M);
 	KUNIT_EXPECT_EQ(test, ARG_MAX, 32 * SZ_4K);
+	KUNIT_EXPECT_EQ(test, MAX_ARG_STRINGS, 0x7FFFFFFF);
 
 	for (int i = 0; i < ARRAY_SIZE(bprm_stack_limits_results); i++) {
 		const struct bprm_stack_limits_result *result = &bprm_stack_limits_results[i];