[PATCH 2/2] exec: Avoid pathological argc, envc, and bprm->p values

[Kees Cook]

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

For 32-bit validation, this was run under 32-bit UML:
$ tools/testing/kunit/kunit.py run --make_options SUBARCH=i386 exec

Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
Cc: Eric Biederman <ebiederm@xxxxxxxxxxxx>
Cc: Justin Stitt <justinstitt@xxxxxxxxxx>
Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Christian Brauner <brauner@xxxxxxxxxx>
Cc: Jan Kara <jack@xxxxxxx>
Cc: linux-fsdevel@xxxxxxxxxxxxxxx
Cc: linux-mm@xxxxxxxxx
---
 fs/exec.c      | 11 ++++++++++-
 fs/exec_test.c | 26 +++++++++++++++++++++++++-
 2 files changed, 35 insertions(+), 2 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index 1d45e1a2d620..5dcdd115739e 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -503,6 +503,9 @@ 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
@@ -516,11 +519,17 @@ 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;
 
+	/* Avoid a pathological bprm->p. */
+	if (bprm->p < limit)
+		return -E2BIG;
+
 	bprm->argmin = bprm->p - limit;
 	return 0;
 }
diff --git a/fs/exec_test.c b/fs/exec_test.c
index 32a90c6f47e7..f2d4a80c861d 100644
--- a/fs/exec_test.c
+++ b/fs/exec_test.c
@@ -8,9 +8,32 @@ 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 },
+	/* 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 },
 	/*
 	 * 0 rlim_stack will get raised to ARG_MAX. With 1 string pointer,
 	 * we should see p - ARG_MAX + sizeof(void *).
@@ -88,6 +111,7 @@ 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];
-- 
2.34.1