[PATCH v2] mm/madvise: reject invalid process_madvise() advice for zero-length vectors

fujunjie
2026-04-27T09:45:58Z

process_madvise() used to validate the advice while walking each
imported iovec. If the vector has zero total length, vector_madvise()
does not enter the loop and can return success without checking whether
the advice value is valid.

For a local mm, such as process_madvise(PIDFD_SELF, ...), the remote-only
process_madvise_remote_valid() check is skipped. As a result, an invalid
advice can be reported as success when the vector has zero total length.
This differs from madvise(), which rejects an invalid advice before
returning success for a zero-length range.

Validate the generic madvise behavior at the syscall-facing entry points
before any vector walk. In process_madvise(), do this before the
remote-only advice restriction so unsupported advice is rejected with the
same priority for local and remote mm. Then keep the per-range helper
focused on address/length validation, avoiding repeated behavior checks
for every iovec.

Valid zero-length requests remain no-ops and continue to return 0. Add a
selftest that covers invalid advice with a zero-length iovec and an empty
vector, while also checking that a valid zero-length request still
succeeds.

Fixes: 021781b01275 ("mm/madvise: unrestrict process_madvise() for current process")
Signed-off-by: fujunjie <fujunjie1@qq.com>
---
v2:
- Validate behavior at the syscall-facing entry points and leave the range
helper for address/length checks, avoiding repeated behavior checks in the
iovec loop.
- Put the generic process_madvise() behavior check before
process_madvise_remote_valid(), as suggested by David.
- Keep the zero-length selftest coverage from v1.

Testing:
Built bzImage and tools/testing/selftests/mm/process_madv. In QEMU, the
process_madv selftest reports 7/7 passed.

mm/madvise.c | 29 ++++++++++++++++-------------
tools/testing/selftests/mm/process_madv.c | 29 +++++++++++++++++++++++++++++
2 files changed, 45 insertions(+), 13 deletions(-)

diff --git a/mm/madvise.c b/mm/madvise.c
index 69708e953cf56..ce238dd96f158 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -1834,13 +1834,10 @@ static void madvise_finish_tlb(struct madvise_behavior *madv_behavior)
tlb_finish_mmu(madv_behavior->tlb);
}

-static bool is_valid_madvise(unsigned long start, size_t len_in, int behavior)
+static bool is_valid_madvise_range(unsigned long start, size_t len_in)
{
size_t len;

- if (!madvise_behavior_valid(behavior))
- return false;
-
if (!PAGE_ALIGNED(start))
return false;
len = PAGE_ALIGN(len_in);
@@ -1859,17 +1856,15 @@ static bool is_valid_madvise(unsigned long start, size_t len_in, int behavior)
* madvise_should_skip() - Return if the request is invalid or nothing.
* @start: Start address of madvise-requested address range.
* @len_in: Length of madvise-requested address range.
- * @behavior: Requested madvise behavior.
* @err: Pointer to store an error code from the check.
*
- * If the specified behaviour is invalid or nothing would occur, we skip the
- * operation. This function returns true in the cases, otherwise false. In
- * the former case we store an error on @err.
+ * If the specified range is invalid or nothing would occur, we skip the
+ * operation. This function returns true in these cases, otherwise false. In
+ * the former case we store an error in @err.
*/
-static bool madvise_should_skip(unsigned long start, size_t len_in,
- int behavior, int *err)
+static bool madvise_should_skip(unsigned long start, size_t len_in, int *err)
{
- if (!is_valid_madvise(start, len_in, behavior)) {
+ if (!is_valid_madvise_range(start, len_in)) {
*err = -EINVAL;
return true;
}
@@ -2013,7 +2008,10 @@ int do_madvise(struct mm_struct *mm, unsigned long start, size_t len_in, int beh
.tlb = &tlb,
};

- if (madvise_should_skip(start, len_in, behavior, &error))
+ if (!madvise_behavior_valid(behavior))
+ return -EINVAL;
+
+ if (madvise_should_skip(start, len_in, &error))
return error;
error = madvise_lock(&madv_behavior);
if (error)
@@ -2056,7 +2054,7 @@ static ssize_t vector_madvise(struct mm_struct *mm, struct iov_iter *iter,
size_t len_in = iter_iov_len(iter);
int error;

- if (madvise_should_skip(start, len_in, behavior, &error))
+ if (madvise_should_skip(start, len_in, &error))
ret = error;
else
ret = madvise_do_behavior(start, len_in, &madv_behavior);
@@ -2131,6 +2129,11 @@ SYSCALL_DEFINE5(process_madvise, int, pidfd, const struct iovec __user *, vec,
goto release_task;
}

+ if (!madvise_behavior_valid(behavior)) {
+ ret = -EINVAL;
+ goto release_mm;
+ }
+
/*
* We need only perform this check if we are attempting to manipulate a
* remote process's address space.
diff --git a/tools/testing/selftests/mm/process_madv.c b/tools/testing/selftests/mm/process_madv.c
index cd4610baf5d7d..9a7e2788fcc50 100644
--- a/tools/testing/selftests/mm/process_madv.c
+++ b/tools/testing/selftests/mm/process_madv.c
@@ -309,6 +309,35 @@ TEST_F(process_madvise, invalid_vlen)
ASSERT_EQ(munmap(map, pagesize), 0);
}

+/*
+ * Test that invalid advice is rejected even when the iovec has zero total
+ * length. A zero-length advice is a no-op for valid advice, but invalid
+ * advice should still fail with EINVAL.
+ */
+TEST_F(process_madvise, invalid_advice_zero_length)
+{
+ struct iovec vec = {
+ .iov_base = NULL,
+ .iov_len = 0,
+ };
+ int pidfd = self->pidfd;
+ ssize_t ret;
+
+ errno = 0;
+ ret = sys_process_madvise(pidfd, &vec, 1, -1, 0);
+ ASSERT_EQ(ret, -1);
+ ASSERT_EQ(errno, EINVAL);
+
+ errno = 0;
+ ret = sys_process_madvise(pidfd, &vec, 1, MADV_DONTNEED, 0);
+ ASSERT_EQ(ret, 0);
+
+ errno = 0;
+ ret = sys_process_madvise(pidfd, NULL, 0, -1, 0);
+ ASSERT_EQ(ret, -1);
+ ASSERT_EQ(errno, EINVAL);
+}
+
/*
* Test process_madvise() with an invalid flag value. Currently, only a flag
* value of 0 is supported. This test is reserved for the future, e.g., if
base-commit: 1b55f8358e35a67bf3969339ea7b86988af92f66
--
2.34.1